Hypothesis explaining the Properties of Light

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An Hypothesis Explaining the Properties of Light

Isaac Newton

SOURCE: An Hypothesis explaining the Properties of Light! printe" in #ho$as %irch! #he History

of the Royal Society &Lon"on! '()(*! +ol, -! ./(0-1),

AU#HOR: Isaac Newton

HAN2: Printe"

RE3ISION HIS#OR4#agge" transcription 5y Lin"a Cross '- 6e5r7ary0'( 8arch .11-,Chec9e" against original 5y Re5e9ah Higgitt ')0-' 7ly .11-,

;./(<2ece$5er =, #here was pro"7ce" a $an7script of 8r, NE>#ON! to7ching his theory of light an"colors! containing partly an hypothesis to explain the properties of light "isco7rse" of 5y hi$ in hisfor$er papers! partly the principal ph?no$ena of the +ario7s colo7rs exhi5ite" 5y thin plates or 5755les! estee$e" 5y hi$ to 5e of a $ore "iffic7lt consi"eration@ yet to "epen" also on the sai"properties of light,Of the hypothesis only the first part was rea"! gi+ing an acco7nt of refraction! reflection!transparency! an" opacity@ the secon" part explaining colo7rs 5eing referre" to the next $eeting,

#he first was as followsh :

Sir!

I ha+e sent yo7 the papers I $entione"! 5y OHN S#ILES, Upon re+iewing the$! I fin" so$e things

so o5sc7re! as $ight ha+e "eser+e" a f7rther explication 5y sche$es@ an" so$e other things! Ig7ess! will not 5e new to yo7! tho7gh al$ost all was new to $e when I wrote the$, %7t as they are!I hope yo7 will accept of the$! tho7gh not worth the a$ple than9s yo7 sent, I re$e$5er! in so$e"isco7rse with 8r, HOOE! I happene" to say! that I tho7ght light was reflecte"! not 5y the parts of glass! water! air! or other sensi5le 5o"ies@ 57t 5y the sa$e confine or s7perficies of the ?thereal$e"i7$s! which refracts it! the rays fin"ing so$e "iffic7lty to get thro7gh it in passing o7t of the"enser into the rarer $e"i7$! an" a greater "iffic7lty in passing o7t of the rarer into the "enser@ an"so 5eing either refracte" or reflecte" 5y that s7perficies! as the circ7$stances they happene" to 5ein at their inci"ence $a9e the$ a5le or 7na5le to get thro7gh it, An"! for confir$ation of this! I sai"f7rther! that I tho7ght the reflection of light! as its ten"ing o7t of glass into air! wo7l" not 5e"i$inishe" or wea9ene" 5y "rawing away the air in an air0p7$p! as it o7ght to 5e! if they were theparts of air that reflecte": an" a""e"! that I ha" not trie" this experi$ent! 57t tho7gh he was not7nacB7ainte" with notions of this 9in", #o which he replie"! that the notion was not new! an" hewo7l" the first opport7nity try the experi$ent I propo7n"e", %7t 7pon re+iewing the papers I sen"yo7! I fo7n" it there set "own for trie"@ which $a9es $e recollect! that a5o7t the ti$e I was writingthese papers! I ha" occasionally o5ser+e" in an air0p7$p here at Christs College! that I co7l" notpercei+e the reflection of the insi"e of the glass "i$inishe" in "rawing o7t the air, #his I tho7ght fitto $ention! least $y for$er forgetf7lness! thro7gh ha+ing long lai" asi"e $y tho7ghts on thesethings! sho7l" $a9e $e see$ to ha+e set "own for certain what I ne+er trie",

;./<

Sir! I ha" for$erly p7rpose" ne+er to write any hypothesis of light an" colo7rs! fearing it $ight 5e a$eans to engage $e in +ain "isp7tes: 57t I hope a "eclare" resol7tion to answer nothing! that loo9s

li9e a contro+ersy! 7nless possi5ly at $y own ti$e 7pon so$e 5y0occasion! $ay "efen" $e fro$that fear, An" therefore consi"ering! that s7ch an hypothesis wo7l" $7ch ill7strate the papers Ipro$ise" to sen" yo7@ an" ha+ing a little ti$e this last wee9 to spare! I ha+e not scr7ple" to



whether it shall 5e tho7ght pro5a5le or i$pro5a5le! so it "o 57t ren"er the papers I sen" yo7! an"others sent for$erly! $ore intelligi5le, 4o7 $ay see! 5y the scratching an" interlining! it was "one inhaste@ an" I ha+e not ha" ti$e to get it transcri5e"! which $a9es $e say I reser+e a li5erty of a""ing it@ an" "esire! that yo7 wo7l" ret7rn those an" the other papers when yo7 ha+e "one withthe$, I "o75t there is too $7ch to 5e rea" at one ti$e! 57t yo7 will soon 9now how to or"er that, Atthe en" of the hypothesis yo7 will see a paragraph to 5e inserte" as is there "irecte": I sho7l" ha+ea""e" another or two! 57t I ha" not ti$e! 57t s7ch as it is! I hope yo7 will accept it,

Sir! I a$! Dc,

Is, NE>#ON, An Hypothesis explaining the Properties of Light! "isco7rse" of in $y se+eral Papers,

Sir!

In $y answer to 8r, HOOE! yo7 $ay re$e$5er! I ha" occasion to say so$ething of hypotheses!where I ga+e a reason! why all allowa5le hypotheses in their gen7ine constit7tion sho7l" 5econfor$a5le to $y theories@ an" sai" of 8r, HOOES hypothesis! that I too9 the $ost free an"nat7ral application of it to ph?no$ena to 5e thisi : that the agitate" parts of 5o"ies! accor"ing totheir se+eral sies! fig7re! an" $otions! "o excite +i5rations in the ?ther of +ario7s "epths or

5ignesses! which 5eing pro$isc7o7sly propagate" thro7gh that $e"i7$ to o7r eyes! effect in 7s asensation of light of a white colo7r@ 57t! if 5y any $eans those of 7neB7al 5ignesses 5e separate"fro$ one another! the largest 5eget a sensation of a re" colo7r@ the least! or shortest! of a "eep+iolet@ an" the inter$e"iate ones! of inter$e"iate colo7rs: $7ch after the $anner that 5o"ies!accor"ing to their se+eral sies! shapes! an" $otions! excite +i5rations in the air of +ario7s5ignesses! which! accor"ing to those 5ignesses! $a9e se+eral tones in so7n"! Dc, I was gla" to7n"erstan"! as I apprehen"! fro$ 8r, HOOES "isco7rse at $y last 5eing at one of yo7r asse$5lies! that he ha" change" his for$er notion of all colo7rs 5eing co$po7n"e" of only twooriginal ones! $a"e 5y the two si"es of an o5liB7e p7lse@ an" acco$$o"ate" his hypothesis to this$y s7ggestion of colo7rs! li9e so7n"s! 5eing +ario7s! accor"ing to the +ario7s 5igness of thep7lses, 6or this I ta9e to 5e a $ore pla7si5le hypothesis than any other "escri5e" 5y for$er a7thors! 5eca7se I see not how the colo7rs of thin transparent plates or s9ins can 5e han"so$ely

explaine"! witho7t ha+ing reco7rse to ?thereal p7lses: 57t yet I ;./=< li9e another hypothesis5etter! which I ha" occasion to hint so$ething of in the sa$e letter in these wor"s9 :

#he hypothesis of lights 5eing a 5o"y! ha" I propo7n"e" it! has a $7ch greater affinity with theo5Fectors own hypothesis! than he see$s to 5e aware of@ the +i5rations of the ?ther 5eing as7sef7l an" necessary in this as in his, 6or! ass7$ing the rays of light to 5e s$all 5o"ies e$itte"e+ery way fro$ shining s75stances! those! when they i$pinge on any refracting or reflectings7perficies! $7st as necessarily excite +i5rations in the ?ther! as stones "o in water when throwninto it, An"! s7pposing these +i5rations to 5e of se+eral "epths or thic9nesses! accor"ingly as theyare excite" 5y the sai" corp7sc7lar rays of +ario7s sies an" +elocities@ of what 7se they will 5e for explicating the $anner of reflexion an" refraction@ the pro"7ction of heat 5y the s7n05ea$s@ thee$ission of light fro$ 57rning! p7trifying! or other s75stances! whose parts are +ehe$entlyagitate"@ the ph?no$ena of thin transparent plates! an" 5755les! an" of all nat7ral 5o"ies@ the$anner of +ision! an" the "ifference of colo7rs@ as also their har$ony an" "iscor"@ I shall lea+e totheir consi"eration! who $ay thin9 it worth their en"ea+o7r to apply this hypothesis to the sol7tion of ph?no$ena,

>ere I to ass7$e an hypothesis! it sho7l" 5e this! if propo7n"e" $ore generally! so as not to"eter$ine what light is! farther than that it is so$ething or other capa5le of exciting +i5rations in the?ther: for th7s it will 5eco$e so genreal an" co$prehensi+e of other hypotheses! as to lea+e littleroo$ for new ones to 5e in+ente", An" therefore! 5eca7se I ha+e o5ser+e" the hea"s of so$egreat +irt7osos to r7n $7ch 7pon hypotheses! as if $y "isco7rses wante" an hypothesis to explainthe$ 5y! an" fo7n"! that so$e! when I co7l" not $a9e the$ ta9e $y $eaning! when I spa9e of thenat7re of light an" colo7rs a5stracte"ly! ha+e rea"ily apprehen"e" it! when I ill7strate" $y"isco7rse 5y an hypothesis@ for this reason I ha+e here tho7ght fit to sen" yo7 a "escription of the

circ7$stances of this hypothesis as $7ch ten"ing to the ill7stration of the papers I herewith sen"yo7, An" tho7gh I shall not ass7$e either this or any other hypothesis! not thin9ing it necessary toconcern $yself! whether the properties of light! "isco+ere" 5y $e! 5e explaine" 5y this! or 8r,



so$eti$es! to a+oi" circ7$loc7tion! an" to represent it $ore con+eniently! spea9 of it! as if Iass7$e" it! an" propo7n"e" it to 5e 5elie+e", #his I tho7gh fit to express! that no $an $ayconfo7n" this with $y other "isco7rses! or $eas7re the certainty of one 5y the other! or thin9 $eo5lige" to answer o5Fections against this script: for I "esire to "ecline 5eing in+ol+e" in s7chtro75leso$e an" insignificant "isp7tes,

%7t to procee" to the hypothesis: 6irst! it is to 5e s7ppose" therein! that there is an ?thereal

$e"i7$ $7ch of the sa$e constit7tion with air! 57t far rarer! s75tler! an" $ore strongly elastic, Of the existence of this $e"i7$ the $otion of a pen"7l7$ in a glass exha7ste" of air al$ost as B7ic9lyas in ;.)1< the open air! is no inconsi"era5le arg7$ent, %7t it is not to 5e s7ppose"! that this$e"i7$ is one 7nifor$ $atter! 57t co$po7n"e"! partly of the $ain phleg$atic 5o"y of ?ther! partlyof other +ario7s ?thereal spirits! $7ch after the $anner! that air is co$po7n"e" of the phleg$atic5o"y of air inter$ixe" with +ario7s +apo7rs an" exhalations: for the electric an" $agnetic effl7+ia!an" gra+itating principle! see$ to arg7e s7ch +ariety, Perhaps the whole fra$e of nat7re $ay 5enothing 57t +ario7s context7res of so$e certain ?thereal spirits! or +apo7rs! con"ense" as it were5y precipitation! $7ch after the $anner! that +apo7rs are con"ense" into water! or exhalations intogrosser s75stances! tho7gh not so easliy con"ensi5le@ an" after con"ensation wro7ght into +ario7sfor$s@ at first 5y the i$$e"iate han" of the Creator@ an" e+er since 5y the power of nat7re@ which!5y +irt7e of the co$$an"! increase an" $7ltiply! 5eco$e a co$plete i$itator of the copies set her 5y the protoplast, #h7s perhaps $ay all things 5e originate" fro$ ?ther,

At least! the elastic effl7+ia see$ to instr7ct 7s! that there is so$ething of an ?thereal nat7recon"ense" in 5o"ies, I ha+e so$eti$es lai" 7pon a ta5le a ro7n" piece of glass a5o7t two inches5roa" set in a 5rass ring! so that the glass $ight 5e a5o7t one eighth or one sixth of an inch fro$the ta5le! an" the air 5etween the$ inclose" on all si"es 5y the ring! after the $anner as if I ha"whel$e" a little sie+e 7pon the ta5le@ an" then r755ing a pretty while the glass 5ris9ly with so$ero7gh an" ra9ing st7ff! till so$e +ery little frag$ents of +ery thin paper! lai" on the ta5le 7n"er theglass! 5egan to 5e attracte" an" $o+e ni$5ly to an" fro@ after I ha" "one r755ing the glass! thepapers wo7l" contin7e a pretty while in +ario7s $otions@ so$eti$es leaping 7p to the glass an"resting there a while@ then leaping "own an" resting there@ then leaping 7p! an" perhaps "own an"7p again! an" this so$eti$es in lines see$ing perpen"ic7lar to the ta5le@ so$eti$es in o5liB7eones@ so$eti$es also they wo7l" leap 7p in one arch an" "own in another! "i+ers ti$es together!

witho7t sensi5ly resting 5etween@ so$eti$es s9ip in a 5ow fro$ one part of the glass to another witho7t to7ching the ta5le! an" so$eti$es hang 5y a corner! an" t7rn often a5o7t +ery ni$5ly! as if they ha" 5een carrie" a5o7t in the $i"st of a whirlwin"! an" 5e otherwise +ario7sly $o+e"! e+erypaper with a "i+erse $otion, An" 7pon sli"ing $y finger on the 7pper si"e of the glass! tho7ghneither the glass! nor inclose" air 5elow! were $o+e" there5y! yet wo7l" the papers! as they h7ng7n"er the glass! recei+e so$e new $otion! inclining this way or that way! accor"ingly as I $o+e"$y finger, Now! whence all these irreg7lar $otions sho7l" spring! I cannot i$agine! 7nless fro$so$e 9in" of s75til $atter lying con"ense" in the glass! an" rarefie" 5y r755ing! as water is rarefie"into +apo7r 5y heat! an" in that refraction "iff7se" thro7gh the space ro7n" the glass to a great"istance! an" $a"e to $o+e an" circ7late +ario7sly! an" accor"ingly to act7ate the papers till itret7rn into the glass again! an" 5e recon"ense" there, An" as this con"ense" $atter 5y rarefactioninto an ?thereal win" &for 5y its easy penetrating an" circ7lating thro7gh glass I estee$ it ?thereal*$ay ca7se these o"" $otions! an" 5y con"ensing again $ay ca7se electrical attraction with itsret7rning to the glass to s7ccee" in the place of what is there contin7ally recon"ense"@ so $ay thegra+itating attraction of the ;.)'< earth 5e ca7se" 5y the contin7al con"ensation of so$e other s7ch li9e ?thereal spirit! not of the $ain 5o"y of phleg$atic ?ther! 57t of so$ething +ery thinly an"s75tilly "if7se" thro7gh it! perhaps of an 7nct7o7s or g7$$y! tenacio7s! an" springy nat7re! an"5earing $7ch the sa$e relation to ?ther! which the +ital aereal spirit! reB7isite for the conser+ationof fla$e an" +ital $otions! "oes to air, 6or! if s7ch an ?thereal spirit $ay 5e con"ense" infer$enting or 57rning 5o"ies! or otherwise coag7late" in the pores of the earth an" water into so$e9in" of h7$i" acti+e $atter! for the contin7al 7ses of nat7re! a"hering to the si"es of those pores!after the $anner that +apo7rs con"ense on the si"es of a +essel@ the +ast 5o"y of the earth! which$ay 5e e+ery where to the +ery center in perpet7al wor9ing! $ay contin7ally con"ense so $7ch of this spirit! as to ca7se it fro$ a5o+e to "escen" with great celerity for a s7pply@ in which "escent it$ay 5ear "own with it the 5o"ies it per+a"es with force proportional to the s7perficies of all their

parts it acts 7pon@ nat7re $a9ing a circ7lation 5y the slow ascent of as $7ch $atter o7t of the5owels of the earth in an aereal for$! which! for a ti$e! constit7es the at$osphere@ 57t 5eingcontin7ally 57oye" 7p 5y the new air@ exhalations an" +apo7rs rising 7n"erneath! at length &so$e



there perhaps in ti$e relents! an" is atten7ate" into its first principle: for nat7re is a perpet7alwor9er! generating fl7i"s o7t of soli"s! an" soli"s o7t of fl7i"s! fixe" things o7t of +olatile! an"+olatile o7t of fixe"! s75til o7t of gross an" gross o7t of s75til@ so$e things to ascen"! an" $a9e the7pper terrestrial F7ices! ri+ers! an" the at$osphere@ an" 5y conseB7ence! others to "escen" for areB7ital to the for$er, An"! as the earth! so perhaps $ay the s7n i$5i5e this spirit copio7sly! toconser+e his shining! an" 9eep the planets fro$ rece"ing f7rther fro$ hi$, An" they! that will! $ayalso s7ppose! that this spirit affor"s or carries with it thither the solary fewel an" $aterial principle

of light: an" that the +ast ?thereal spaces 5etween 7s an" the stars are for a s7fficient repositoryfor this foo" of the s7n an" planets, %7t this of the constit7tion of ?thereal nat7res 5y the 5y,

In the secon" place! it is to 5e s7ppose"! that the ?ther is a +i5rating $e"i7$ li9e air! only the+i5rations far $ore swift an" $in7te@ those of air! $a"e 5y a $ans or"inary +oice! s7ccee"ing oneanother at $ore than half a foot or a foot "istance@ 57t those of ?ther at a less "istance than theh7n"re" tho7san"th part of an inch, An"! as in air the +i5rations are so$e larger than others! 57tyet all eB7ally swift &for in a ring of 5ells the so7n" of e+ery tone is hear" at two or three $iles"istance! in the sa$e or"er that the 5ells are str7c9@* so! I s7ppose! the ?thereal +i5rations "iffer in5igness! 57t not in swiftness, Now! these +i5rations! 5esi"e their 7se in reflexion an" refraction!$ay 5e s7ppose" the chief $eans! 5y which the parts of fer$enting or p7trifying s75stances! fl7i"liB7ors! or $elte"! 57rning! or other hot 5o"ies! contin7e in $otion! are sha9en as7n"er li9e a ship5y wa+es! an" "issipate" into +apo7rs! exhalations! or s$o9e! an" light loose" or excite" in those

5o"ies! an" conseB7ently 5y which a 5o"y 5eco$es a 57rning coal! an" s$o9e! fla$e@ an"! Is7ppose! fla$e is nothing 57t the particles of s$o9e t7rne" 5y the access of light an" heat to57rning coals! little an" inn7$era5le,

;.).<

#hir"ly! as the air can per+a"e the 5ores of s$all glass pipes! 57t yet not so easily as if they werewi"er@ an" therefore stan"s at a greater "egree of rarity than in the free aereal spaces! an" at so$7ch a greater "egree of rarity as the pipe is s$aller! as is 9nown 5y the rising of water in s7chpipes to a $7ch greater hight than the s7rface of the stagnating water! into which they are "ippe"@so I s7ppose ?ther! tho7gh it per+a"es the pores of crystal! glass! water! an" other nat7ral 5o"ies!

yet it stan"s at a greater "egree of rarity in those pores! than in the free ?thereal spaces! an" at so$7ch a greater "egree of rarity! as the pores of the 5o"y are s$aller, >hence it $ay 5e! that thespirit of wine! for instance! tho7gh a lighter 5o"y! yet ha+ing s75tiler parts! an" conseB7entlys$aller pores! than water! is the $ore strongly refracting liB7or, #his also $ay 5e the principalca7se of the cohesion of the parts of soli"s an" fl7i"s! of the springiness of glass! an" 5o"ies!whose parts sli"e not one 7pon another in 5en"ing! an" of the stan"ing of the $erc7ry in the#orricellian experi$ent! so$eti$es to the top of the glass! tho7gh a $7ch greater hight than twenty0nine inches, 6or the "enser ?ther! which s7rro7n"s these 5o"ies! $7st cro7" an" press their partstogether! $7ch after the $anner that air s7rro7n"ing two $ar5les presses the$ together! if there 5elittle or no air 5etween the$, 4ea! an" that p7ling pro5le$@ %y what $eans the $7scles arecontracte" an" "ilate" to ca7se ani$al $otion! $ay recei+e greater light fro$ hence than fro$ any$eans $en ha+e hitherto 5een thin9ing on, 6or! if there 5e any power in $an to con"ense an""ilate at will the ?ther! that per+a"es the $7scle! that con"ensation or "ilation $7st +ary theco$pression of the $7scle! $a"e 5y the a$5ient ?ther! an" ca7se it to swell or shrin9 accor"ingly,6or tho7gh co$$on water will scarce shrin9 5y co$pression! an" swell 5y relaxation! yet &so far as$y o5ser+ation reaches* spirit of wine an" oil will@ an" 8r, %O4LES experi$ent of a ta"poleshrin9ing +ery $7ch 5y har" co$pressing the water! in which it swa$! is an arg7$ent! that ani$al F7ices "o the sa$e, An" as for their +ario7s pression 5y the a$5ient ?ther! it is plain! that that $7st5e $ore or less accor"ingly as there is $ore or less ?ther within! to s7stain an" co7nterpoise thepress7re of that witho7t, If 5oth ?thers were eB7ally "ense! the $7scle wo7l" 5e at li5erty! as if presse" 5y neither: if there were no ?ther within! the a$5ient wo7l" co$press it with the wholeforce of its spring, If the ?ther within were twice as $7ch "ilate" as that witho7t! so as to ha+e 57thalf as $7ch springiness! the a$5ient wo7l" ha+e half the force of its springiness co7nterpoise"there5y! an" exercise 57t the other half 7pon the $7scle@ an" so in all other cases the a$5ientco$presses the $7scle 5y the excess of the force of its springiness a5o+e that of the springiness of

the incl7"e", #o +ary the co$pression of the $7scle therefore! an" so to swell an" shrin9 it! therenee"s nothing 57t to change the consistence of the incl7"e" ?ther@ an" a +ery little change $ays7ffice! if the spring of ?ther 5e s7ppose" +ery strong! as I ta9e it to 5e $any "egrees stronger



Now for the changing the consistence of the ?ther@ so$e $ay 5e rea"y to grant! that the so7l $ayha+e an i$$e"iate power o+er the whole ?ther in any part of the 5o"y! to swell or shrin9 it at will:57t then how "epen"s the ;.)-< $7sc7lar $otion on the ner+esG Others therefore $ay 5e $oreapt to thin9 it "one 5y so$e certain ?thereal spirit incl7"e" within the "7ra $ater! which the so7l$ay ha+e power to contract or "ilate at will in any $7scle! an" so ca7se it to flow thither thro7gh thener+es, %7t still there is a "iffic7lty! why this force of the so7l 7pon it "oes not ta9e off the power of

its springiness! where5y it sho7l" s7stain! $ore or less! the force of the o7twar" ?ther, A thir"s7pposition $ay 5e! that the so7l has a power to inspire any $7scle with this spirit! 5y i$pelling itthither thro7gh the ner+es, %7t this too has its "iffic7lties! for it reB7ires a forci5le inten"ing thespring of the ?ther in the $7scles! 5y press7re exerte" fro$ the parts of the 5rain: an" it is har" toconcei+e! how so great force can 5e exercise" a$i"st so ten"er $atter as the 5rain is, An" 5esi"es!why "oes not this ?thereal spirit! 5eing s75til eno7gh! an" 7rge" with so great force! go awaythro7gh the "7ra $ater an" s9ins of the $7scle@ or at least so $7ch of the other ?ther to go o7t to$a9e way for this! which is cro7"e" inG #o ta9e away these "iffic7lties is a "igression@ 57t seeingthe s75Fect is a "eser+ing one! I shall not stic9 to tell yo7 how I thin9 it $ay 5e "one,

6irst then! I s7ppose! there is s7ch a spirit@ that is! that the ani$al spirits are neither li9e the liB7or!+apo7r! or gas of spirit of wine@ 57t of an ?thereal nat7re! s75til eno7gh to per+a"e the ani$al F7ices! as freely as the electric! or perhaps $agnetic! effl7+ia "o glass, An" to 9now! how the coats

of the 5rain! ner+es! an" $7scles! $ay 5eco$e a con+enient +essel to hol" so s75til a spirit! yo7$ay consi"er! how liB7ors an" spirits are "ispose" to per+a"e or not per+a"e things on other acco7nts than their s75tilty, >ater an" oil per+a"e woo" an" stone! which B7ic9sil+er "oes not@ an"B7ic9sil+er $etals! which water an" oil "o not: water an" aci" spirits per+a"e salts! which oil an"spirit of wine "o not@ an" oil an" spirit of wine per+a"e s7lph7r! which water an" aci" spirits "o not,So so$e fl7i"s! as oil an" water! tho7gh their parts are in free"o$ eno7gh to $ix with one another!yet 5y so$e secret principle of 7nsocia5leness they 9eep as7n"er@ an" so$e! that are socia5le!$ay 5eco$e 7nsocia5le! 5y a""ing a thir" thing to one of the$! as water to spirit of wine! 5y"issol+ing salt of tartar in it, #he li9e 7nsocia5leness $ay 5e in ?thereal nat7res! as perhaps5etween the ?thers in the +ortices of the s7n an" planets@ an" the reson! why air stan"s rarer in the5oxes of s$all glass0pipes! an" ?ther in the pores of 5o"ies! than elsewhere! $ay 5e! not want of s75tilty! 57t socia5leness, An" on this gro7n"! if the ?thereal +ital spirit in a $an 5e +ery socia5le

to the $arrow an" F7ices! an" 7nsocia5le to the coats of the 5rain! ner+es! an" $7scles! or anything lo"ge" in the pores of those we s7ppose no great +iolence "one to it to sB7eee it o7t@ an"that it $ay not 5e altogether so s75til as the $ain 5o"y of ?ther! tho7gh s75til eno7gh to per+a"erea"ily the ani$al F7ices! an" that! as any of it is spent! it is contin7ally s7pplie" 5y new spirit fro$the heart,

In the next place! for 9nowing how this spirit $ay 5e 7se" for ani$al $otion! yo7 $ay consi"er! howso$e things 7nsocia5le are $a"e socia5le 5y the ;.)/< $e"iation of a thir", >ater! which will not"isol+e copper! will "o it! if the copper 5e $elte" with s7lph7r: aB7a fortis! which will not per+a"egol"! will "o it 5y a""ition of a little sal ar$oniac! or spirit of salt: lea" will not $ix in $elting withcopper! 57t if a little tin or anti$ony 5e a""e"! they $ix rea"ily! an" part again of their own accor"!if the anti$ony 5e waste" 5y throwing saltpeter or otherwise: an" so lea" $elte" with sil+er B7ic9lyper+a"es an" liB7efies the sil+er in a $7ch less heat than is reB7site to $elt the sil+er alone@ 57t! if they 5e 9ept in the test till that little s75stance! that reconcile" the$! 5e waste" or altere"! they partagain of their own accor", An"! in li9e $anner! the ?thereal ani$al spirit in a $an $ay 5e a$e"iator 5etween the co$$on ?ther an" the $7sc7lar F7ices! to $a9e the$ $ix $ore freely@ an"so! 5y sen"ing a little of this spirit into any $7scle! tho7gh so little as to ca7se no sensi5le tensionof the $7scle 5y its own force@ yet! 5y ren"ering the F7ices $ore socia5le to the co$$on external?ther! it $ay ca7se that ?ther to per+a"e the $7scle of its own accor" in a $o$ent $ore freelyan" copio7sly than it wo7l" otherwise "o! an" to rece"e again as freely! so soon as this $e"iator of socia5leness is retracte", >hence! accor"ing to what I sai" a5o+e! will procee" the swelling or shrin9ing of the $7scle! an" conseB7ently the ani$al $otion "epen"ing thereon,

#h7s $ay therefore the so7l! 5y "eter$ining this ?thereal ani$al spirit or win" into this or thatner+e! perhaps with as $7ch ease as air is $o+e" in open spaces! ca7se all the $otions we see in

ani$als: for the $a9ing which $otions strong! it is not necessary! that we sho7l" s7ppose the ?ther within the $7scle +ery $7ch con"ense" or rarifie" 5y this $eans! 57t only that its spring is so +erygreat! that a little alteration of its "ensity shall ca7se a great alteration in the press7re! An" what is



spirit is not sent thither! as into other $7scles! 57t contin7ally generate" there 5y the fer$entationof the F7ices! with which its flesh is replenishe"! an" as it is generate"! let o7t 5y starts into the 5rainthro7gh so$e con+enient "7ct7s to perfor$ those $otions in other $7scles 5y i$pression! which it"i" in the heart 5y its generation, 6or I see not! why the fer$ent in the heart $ay not raise as s75tila spirit o7t of its F7ices! to ca7se these $otions! as r755ing "oes o7t of a glass! to ca7se electricattraction! or 57rning o7t of fewel! to penetrate glass! as 8r, %O4LE has shewn! an" calcine 5ycorrosion $etals $elte" therein,

Hitherto I ha+e 5een conte$plating the nat7re of ?ther an" ?thereal s75stances 5y their effectsan" 7ses@ an" now I co$e to Foin therewith the consi"eration of light,

In the fo7rth place therefore! I s7ppose light is neither ?ther! nor its +i5rating $otion! 57t so$ethingof a "ifferent 9in" propagate" fro$ l7ci" 5o"ies, #hey! that will! $ay s7ppose it an aggregate of +ario7s peripatetic B7alities, Others $ay s7ppose it $7ltit7"es of 7ni$agina5le s$all an" swiftcorp7scles of +ario7s sies! springing fro$ shining 5o"ies at great "istances one after another@ 57tyet witho7t any sensi5le inter+al of ti$e! an" contin7ally 7rge" forwar" 5y a ;.))< principle of $otion! which in the 5eginning accelerates the$! till the resistence of the ?thereal $e"i7$ eB7althe force of that principle! $7ch after the $anner that 5o"ies let fall in water are accelerate" till theresistance of the water eB7als the force of gra+ity, o"! who ga+e ani$als self0$otion 5eyon" o7r 7n"erstan"ing! is! witho7t "o75t! a5le to i$plant other principles of $otion in 5o"ies! which we $ay

7n"erstan" as little, So$e wo7l" rea"ily grant this $ay 5e a spirit7al one@ yet a $echanical one$ight 5e shewn! "i" not I thin9 it 5etter to pass it 5y, %7t they! that li9e not this! $ay s7ppose lightany other corporeal e$anation! or any i$p7lse or $otion of any other $e"i7$ or ?thereal spirit"iff7se" thro7gh the $ain 5o"y of ?ther! or what else they can i$agine proper for this p7rpose, #oa+oi" "isp7te! an" $a9e this hypothesis general! let e+ery $an here ta9e his fancy: only! whate+er light 5e! I s7ppose! it consists of rays "iffering fro$ one another in contingent circ7$stances! as5igness! for$! or +igo7r@ li9e as the san"s on the shore! the wa+es of the sea! the faces of $en! an"all other nat7ral things of the sa$e 9in" "iffer@ it 5eing al$ost i$possi5le for any sort of things to 5efo7n" witho7t so$e contingent +ariety, An" f7rther! I wo7l" s7ppose it "i+erse! fro$ the +i5rationsof the ?ther! 5eca7se &5esi"es! that were it these +i5rations! it o7ght always to +erge copio7sly incroo9e" lines into the "ar9 or B7iescent $e"i7$! "estroying all sha"ows@ an" to co$ply rea"ily withany croo9e" pores or passages! as so7n"s "o!* I see not how any s7perficies &as the si"e of a

glasspris$! on which the rays within are inci"ent at an angle of a5o+e forty "egrees* can 5e totallyopa9e, 6or the +i5rations 5eating against the refracting confine of the rarer an" "enser ?ther $7stnee"s $a9e that pliant s7perficies 7n"7late! an" those 7n"7lations will stir 7p an" propagate+i5rations on the other si"e, An" f7rther! how light! inci"ent on +ery thin s9ins or plates of anytransparent 5o"y! sho7l"! for $any s7ccessi+e thic9nesses of the plate in arith$etical progression!5e alternately reflecte" an" trans$itte"! as I fin" it is! p7les $e as $7ch, 6or! tho7gh thearith$etical progression of those thic9nesses! which reflect an" trans$it the rays alternately!arg7es! that it "epen"s 7pon the n7$5er of +i5rations 5etween the two s7perficies of the plate!whether the ray shall 5e reflecte" or trans$itte": yet I cannot see! how the n7$5er sho7l" +ary thecase! 5e it greater or less! whole or 5ro9en! 7nless light 5e s7ppose" so$ething else than these+i5rations, So$ething in"ee" I co7l" fancy towar"s helping the two last "iffic7lties! 57t nothingwhich I see not ins7fficient,

6ifthly! it is to 5e s7ppose"! that light an" ?ther $7t7ally act 7pon one another! ?ther in refractinglight! an" light in war$ing ?ther@ an" that the "ensest ?ther acts $ost strongly, >hen a raytherefore $o+es thro7gh ?ther of 7ne+en "ensity! I s7ppose it $ost presse"! 7rge"! or acte" 7pon5y the $e"i7$ on that si"e towar"s the "enser ?ther! an" recei+es a contin7al i$p7lse or ply fro$that si"e to rece"e towar"s the rarer! an" so is accelerate"! if it $o+e that way! or retar"e"! if thecontrary, On this gro7n"! if a ray $o+e o5liB7ely thro7gh s7ch an 7ne+enly "ense $e"i7$ &that is!o5liB7ely to those i$aginary s7perficies! which r7n thro7gh the eB7ally "ense parts of the $e"i7$!an" $ay 5e calle" the refracting s7perficies* it $7st 5e inc7r+e"! as it ;.)< is fo7n" to 5e! 5yo5ser+ation in water' ! whose lower parts were $a"e gra"7ally $ore salt! an" so $ore "ense thanthe 7pper, An" this $ay 5e the gro7n" of all refraction an" reflexion: for as the rarer air within as$all glass0pipe! an" the "enser witho7t! are not "isting7ishe" 5y a $eer $athe$atical s7perficies!57t ha+e air 5etween the$! at the orifice of the pipe! r7nning thro7gh all inter$e"iate "egrees of

"ensity: so I s7ppose the refracting s7perficies of ?ther! 5etween 7neB7ally "ense $e"i7$s! to 5enot a $athe$atical one@ 57t of so$e 5rea"th! the ?ther therein! at the orifices of the pores of thesoli" 5o"y! 5eing of all inter$e"iate "egrees of "ensity 5etween the rarer an" "enser ?thereal



while it is passing the physical s7perficies, Now! if the $otion of the ray 5e s7ppose" in thispassage to 5e increase" or "i$inishe" in a certain proportion! accor"ing to the "ifference of the"ensities of the ?thereal $e"i7$s! an" the a""ition or "etraction of the $otion 5e rec9one" in theperpen"ic7lar fro$ the refracting s7perficies! as it o7ght to 5e! the sines of inci"ence an" refractionwill 5e proportional accor"ing to what 2ES CAR#ES has "e$onstrate",

#he ray therefore! in passing o7t of the rarer $e"i7$ into the "enser! inclines contin7ally $ore an"

$ore towar"s parallelis$ with the refracting s7perficies@ an" if the "iffering "ensities of the$e"i7$s 5e not so great! nor the inci"ence of the ray so o5liB7e! as to $a9e it parallel to thats7perficies 5efore it gets thro7gh! then it goes thro7gh an" is refracte"@ 57t if! thro7gh the aforesai"ca7ses! the ray 5eco$e parallel to that s7perficies 5efore it can get thro7gh! then it $7st t7rn 5ac9an" 5e reflecte", #h7s! for instance! $ay 5e o5ser+e" in a triang7lar glass0pris$ O E 6! that therays A n! that sen" o7t of the glass into the air! "o! 5y inclining the$ $ore an" $ore to therefracting s7perficies! e$erge $ore an" $ore o5liB7ely till they 5e infinitely o5liB7e@ that is! in a$anner parallel to the s7perficies! which happens when the angle of inci"ence is a5o7t forty"egrees@ an" then! if they 5e a little $ore incline" are all reflecte"! as at A 3 J! 5eco$ing! Is7ppose! parallel to the s7perficies 5efore they can get thro7gh it, Let A % 2 C represent the rarer $e"i7$@ E 6 H the "enser! C 2 6 E the space 5etween the$! or refracting physical s7perficies! inwhich the ?ther is of all inter$e"iate "egrees of "ensity! fro$ the rarest ?ther at C 2! to the"ensest! at E 6@ A $ n L a ray! A $ its inci"ent part! $ n its inc7r+ation 5y the refracting s7perficies!

an" n L its e$ergent part, Now! if the ray A $ 5e so $7ch inc7r+e" as to 5eco$e at its e$ergencen! as nearly as $ay 5e! parallel to C 2! it is plain! that if that ray ha" 5een inci"ent a little $oreo5liB7ely! ;.)(< it $7st ha+e 5eco$e parallel to C 2! 5efore it ha" arri+e" at E 6!the f7rther si"e of the refracting s7perficies@ an" so co7l" ha+e got no nearer to E 6! 57t $7st ha+e t7rne" 5ac9 5yf7rther inc7r+ation! an" 5een reflecte"! as it is represente" at A K 3 J, An" the li9e wo7l" ha+ehappene"! if the "ensity of the ?ther ha" f7rther increase" fro$ E 6 to P @ so that P H $ight5e a "enser $e"i7$ than E 6 H was s7ppose"@ for then the ray! in passing fro$ $ to n! 5eing so$7ch inc7r+e"! as at n to 5eco$e parallel to C 2 or P ! it is i$possi5le it sho7l" e+er get nearer toP ! 57t $7st at n 5egin 5y f7rther inc7r+ation to t7rn 5ac9! an" so 5e reflecte", An" 5eca7se! if arefracte" ray! as n L! 5e $a"e inci"ent! the inci"ent! A $! shall 5eco$e the refracte"@ an" therefore!if the ray A K 3! after it is arri+e" at 3! where I s7ppose it parallel to the refracting s7perficies!sho7l" 5e refracte" perpen"ic7larly 5ac9! it wo7l" ret7rn 5ac9 in the line of inci"ence 3 K A,

#herefore going forwar"! it $7st go forwar" in s7ch another line! 3 M J! 5oth cases 5eing ali9e! an"so 5e reflecte" at an angle! eB7al to that of inci"ence,

#his $ay 5e the ca7se an" $anner of reflection! when light ten"s fro$ the rarer towar"s the "enser ?ther: 57t to 9now! how it sho7l" 5e reflecte"! when it stan"s fro$ the "enser towar"s the rarer!yo7 are f7rther to consi"er! how fl7i"s near their s7perficies are less pliant an" yiel"ing than in their $ore inwar" parts@ an"! if for$e" into thin plates! or shells! they 5eco$e $7ch $ore stiff an"tenacio7s than otherwise, #h7s! things! which rea"ily fall in water! if let fall 7pon a 5755le of water!they "o not easily 5rea9 thro7gh it! 57t are apt to sli"e "own 5y the si"es of it! if they 5e not too 5igan" hea+y, So! if two well polishe" con+ex glasses! gro7n" on +ery large spheres! 5e lai" one 7ponanother! the air 5etween the$ easily rece"es! till they al$ost to7ch@ 57t then 5egins to resist so$7ch! that the weight of the 7pper glass is too little to 5ring the$ together so as to $a9e the 5lac9!$entione" in the other papers I sen" yo7! appear in the $i"st of the rings of colo7rs: an"! if theglasses 5e plain! tho7gh no 5roa"er than a two0pence! a $an with his whole strength is not a5le topress all the air o7t fro$ 5etween the$! so as to $a9e the$ f7lly to7ch, 4o7 $ay o5ser+e also! thatinsects will wal9 7pon water witho7t wetting their feet! an" the water 5earing the$ 7p@ also $otesfalling 7pon water will often lie long 7pon it witho7t 5eing wette": an" so! I s7ppose! ?ther in theconfine of two $e"i7$s is less pliant an" yiel"ing than in other places! an" so $7ch the less pliant5y how $7ch the $e"i7$s "iffer in "ensity: so that in passing o7t of "enser ?ther into rarer! whenthere re$ains 57t a +ery little of the "enser ?ther to 5e past thro7gh! a ray fin"s $ore than or"inary"iffic7lty to get thro7gh@ an" so great "iffic7lty! where the $e"i7$s are of +ery "iffering "ensity! asto 5e reflecte" 5y inc7r+ation! after the $anner "escri5e" a5o+e@ the parts of ?ther on that si"e!where they are less pliant an" yiel"ing! acting 7pon the ray $7ch after the $anner that they wo7l""o were they "enser there than on the other si"e: for the resistance of the $e"i7$ o7ght to ha+ethe sa$e effect on the ray! fro$ what ca7se soe+er it arises, An" this! I s7ppose! $ay 5e the ca7se

of the reflection of B7ic9sil+er! an" other $etalline 5o"ies, It $7st also conc7r to increase thereflecti+e +irt7e of the s7perficies! when rays ten" o7t of the rarer $e"i7$ into the ;.)< "enser:an"! in that case therefore! the reflection ha+ing a "o75le ca7se! o7ght to 5e stronger than in the



nee" not 5e consi"ere"! 5eca7se so $7ch as the ray is there5y 5ent in passing to the $osttenacio7s an" rigi" part of the s7perficies! so $7ch it is there5y 7n5ent again in passing on fro$thence thro7gh the next parts gra"7ally less tenacio7s,

#h7s $ay rays 5e refracte" 5y so$e s7perficies! an" reflecte" 5y others! 5e the $e"i7$ they ten"into! "enser or rarer, %7t it re$ains f7rther to 5e explaine"! how rays ali9e inci"ent on the sa$es7perficies &s7ppose of crystal! glass! or water* $ay 5e at the sa$e ti$e so$e refracte"! others

reflecte", An" for explaining this! I s7ppose! that the rays! when they i$pinge on the rigi" resisting?thereal s7perficies! as they are acte" 7pon 5y it! so they react 7pon it an" ca7se +i5rations in it!as stones thrown into water "o in its s7rface@ an" that these +i5rations are propagate" e+ery wayinto 5oth the rarer an" the "enser $e"i7$s@ as the +i5rations of air! which ca7se so7n"! are fro$ astro9e! 57t yet contin7e strongest where they 5egan! an" alternately contract an" "ilate the ?ther inthat physical s7perficies, 6or it is plain 5y the heat! which light pro"7ces in 5o"ies! that it is a5le top7t their parts in $otion! an" $7ch $ore to heat an" p7t in $otion the $ore ten"er ?ther@ an" it is$ore pro5a5le! that it co$$7nicates $otion to the gross parts of 5o"ies 5y the $e"iation of ?ther than i$$e"iately@ as for instance! in the inwar" parts of B7ic9sil+er! tin! sil+er! an" other +ery opa9e5o"ies! 5y generating +i5rations! that r7n thro7gh the$! than 5y stri9ing the o7twar" parts only!witho7t entering the 5o"y, #he shoc9 of e+ery single ray $ay generate $any tho7san" +i5rations!an" 5y sen"ing the$ all o+er the 5o"y! $o+e all the parts! an" that perhaps with $ore $otion thanit co7l" $o+e one single part 5y an i$$e"iate stro9e@ for the +i5rations! 5y sha9ing each particle

5ac9war" an" forwar"! $ay e+ery ti$e increase its $otion! as a ringer "oes a 5ell 5y often p7llingit! an" so at length $o+e the particles to a +ery great "egree of agitation! which neither the si$pleshoc9 of a ray! nor any other $otion in the ?ther! 5esi"es a +i5rating one co7l" "o, #h7s in air sh7t7p in a +essel! the $otion of its parts ca7se" 5y heat! how +iolent soe+er! is 7na5le to $o+e the5o"ies h7ng in it! with either a tre$5ling or progressi+e $otion: 57t if air 5e p7t into a +i5rating$otion 5y 5eating a "r7$ or two! it sha9es glass0win"ows! the whole 5o"y of a $an! an" other $assy things! especially those of a congr7o7s tone: yea I ha+e o5ser+e" it $anifestly sha9e 7n"er $y feet a cellare" free0stone floor of a large hall! so as! I 5elie+e! the i$$e"iate stro9e of fi+eh7n"re" "r7$stic9s co7l" not ha+e "one! 7nless perhaps B7ic9y s7ccee"ing one another at eB7alinter+als of ti$e, thereal +i5rations are therefore the 5est $eans 5y which s7ch a s75tile agent aslight can sha9e the gross particles of soli" 5o"ies to heat the$: an" so s7pposing that light!i$pinging on a refracting or reflecting ?thereal s7perficies! p7ts it into a +i5rating $otion! that

physical s7perficies 5eing 5y the perpet7al app7lse of rays always 9ept in a +i5rating $otion! an"the ?ther therein contin7ally expan"e" an" co$presse" 5y t7rns@ if a ray of light i$pinge 7pon it!while it is $7ch co$presse"! I s7ppose it is then too "ense an" stiff to let the ray ;.)=< passthro7gh! an" so reflects it@ 57t the rays! that i$pinge on it at other ti$es! when it is either expan"e"5y the inter+al of two +i5rations! or not too $7ch co$presse" an" con"ense"! go thro7gh an" arerefracte",

#hese $ay 5e the ca7ses of refractions an" reflections in all cases@ 57t! for 7n"erstan"ing how theyco$e to 5e so reg7lar! it is f7rther to 5e consi"ere"! that in a heap of san"! altho7gh the s7rface 5er7gge"! yet if water 5e po7re" on it to fill its pores! the water! so soon as its pores are fille"! wille+enly o+ersprea" the s7rface! an" so $7ch the $ore e+enly! as the san" is finer: so! altho7gh thes7rface of all 5o"ies! e+en the $ost polishe"! 5e r7gge"! as I concei+e! yet where that r7gge"nessis not too gross an" coarse! the refracting ?thereal s7perficies $ay e+enly o+ersprea" it, Inpolishing glass or $etal! it is not to 5e i$agine"! that san"! p7tty! or other fretting pow"ers! sho7l"wear the s7rface so reg7larly! as to $a9e the front of e+ery particle exactly plain! an" all thoseplains loo9 the sa$e way! as they o7ght to "o in well polishe" 5o"ies! were reflection perfor$e" 5ytheir parts: 57t that those fretting pow"ers sho7l" wear the 5o"ies first to a coarse r7gge"ness!s7ch is sensi5le! an" then to a finer an" finer r7gge"ness! till it 5e so fine that the ?thereals7perficies e+enly o+ersprea"s it! an" so $a9es the 5o"y p7t on the appearance of a polish! is a+ery nat7ral an" intelligi5le s7pposition, So in fl7i"s! it is not well to 5e concei+e"! that the s7rfacesof their parts sho7l" 5e all plain! an" the plains of the s7perficial parts always 9ept loo9ing all thesa$e way! notwithstan"ing that they are in perpet7al $otion, An" yet witho7t these twos7ppositions! the s7perficies of fl7i"s co7l" not 5e so reg7larly reflexi+e as they are! were thereflexion "one 5y the parts the$sel+es! an" not 5y an ?thereal s7perficies e+enly o+ersprea"ingthe fl7i",

67rther! concerning the reg7lar $otion of light! it $ight 5e s7specte"! whether the +ario7s +i5rationsof the fl7i"! thro7gh which it passes! $ay not $7ch "ist7r5 it: 57t that s7spicion! I s7ppose! will



the hin"er$ost part! 5y a contrary action! $7st soon set it straight again,

Lastly! 5eca7se witho7t "o75t there are! in e+ery transparent 5o"y! pores of +ario7s sies! an" Isai"! that ?ther stan"s at the greatest rarity in the s$allest pores@ hence the ?ther in e+ery poresho7l" 5e of a "iffering rarity! an" so light 5e refracte" in its passage o7t of e+ery pore into the next!which wo7l" ca7se a great conf7sion! an" spoil the 5o"ys transparency, %7t consi"ering that the?ther! in all "ense 5o"ies! is agitate" 5y contin7al +i5rations! an" these +i5rations cannot 5e

perfor$e" witho7t forcing the parts of ?ther forwar" an" 5ac9war"! fro$ one pore to another! 5y a9in" of tre$or! so that the ?ther! which one $o$ent is in a greater pore! is the next $o$ent force"into a less@ an" on the contrary! this $7st e+enly sprea" the ?ther into all the pores not excee"ingso$e certain 5igness! s7ppose the 5rea"th of a +i5ration! an" so $a9e it of an e+en "ensitythro7gho7t the transparent 5o"y! agreea5le to the $i""le sort of pores, %7t where the pores excee"a certain 5igness! I s7ppose the ?ther s7its its "ensity to the 5igness of the pore! or to the $e"i7$within it@ an" so 5eing of a "i+erse "ensity fro$ the ?ther that s7rro7n"s it! refracts ;.1< or reflects light in its s7perficies! an" so $a9e the 5o"y! where $any s7ch interstices are! appear opa9e,

So$e of the $e$5ers ta9ing partic7lar notice! a$ong other things! of an experi$ent $entione" inthis hypothesis! "esire"! that it $ight 5e trie"@ +i, that ha+ing lai" 7pon a ta5le a ro7n" piece of glass! a5o7t two inches 5roa"! in a 5rass ring@ so that the glass $ight 5e one thir" part of an inch

fro$ the ta5le@ an" then r755ing the glass 5ris9ly! till so$e little frag$ents of paper lai" on the ta5le7n"er the glass 5egan to 5e attracte"! an" $o+e ni$5ly to an" fro@ after he ha" "one r755ing theglass! the papers wo7l" contin7e a pretty while in +ario7s $otions! so$eti$es leaping 7p to theglass! an" resting there a while! then leaping "own! an" resting there! an" then leaping 7p an""own again! an" this so$eti$es in lines see$ing perpen"ic7lar to the ta5le! so$eti$es in o5liB7eones@ so$eti$es also leaping 7p in one arch! an" leaping "own in another "i+ers ti$es together!witho7t sensi5ly resting 5etween@ so$eti$es s9ipping in a 5ow fro$ one part of the glass toanother! witho7t to7ching the ta5le! an" so$eti$es hanging 5y a corner! an" t7rning often a5o7t+ery ni$5ly! as if they ha" 5een carrie" a5o7t in the $i""le of a whirlwin"@ an" 5eing otherwise+ario7sly $o+e"! e+ery paper with a "ifferent $otion, An" 7pon sli"ing his finger 7pon the 7pper si"e of the glass! tho7gh neither the glass not the inclose" air 5elow were $o+e" there5y! yet wo7l"the papers! as they h7ng 7n"er the glass! recei+e so$e new $otion! inclining this or that way!

accor"ing as he $o+e" his finger,

#his experi$ent 8r, NE>#ON propose" to 5e +arie" with a larger glass place" farther fro$ theta5le! an" to $a9e 7se of 5its of leaf gol" instea" of papers@ thin9ing! that this wo7l" s7ccee" $7ch5etter! so as perhaps to $a9e the leaf gol" rise an" fall in spiral lines! or whirl for a ti$e in the air!witho7t to7ching either the ta5le or glass,

It was or"ere"! that this experi$ent sho7l" 5e trie" at the next $eeting@ an" 8r, HOOE pro$ise"to prepare it for that $eeting,

8r, OL2EN%UR was "esire" to enB7ire 5y letter of 8r, NE>#ON! whether he wo7l" consent! thata copy $ight 5e ta9en of his papers! for the 5etter consi"eration of their contents,

8r, OL2EN%UR presente" fro$ 8r, 8AR#4N! the printer to the Society! 8r, >ILLUH%4SOrnithologia! printe" at Lon"on! '(! in fol,

2ece$5er ', 8r, NE>#ONS experi$ent of glass r755e" to ca7se +ario7s $otions in 5its of paper 7n"erneath! was trie"! 57t "i" not s7ccee" in those circ7$stances! with which it was trie", #his trialwas $a"e 7pon the rea"ing of a letter of his own to 8r, OL2EN%UR! "ate" at Ca$5ri"ge! '/th2ece$5er! '()$ ! in which he gi+es so$e $ore partic7lar "irections a5o7t that experi$ent,

#he letter was as follows:

;.'<

#he notice yo7 ga+e $e of the Royal Societys inten"ing to see the experi$ent of glass r755e"! toca7se +ario7s $otions in 5its of paper 7n"erneath! p7t $e 7pon recollecting $yself a little f7rther



was as near again on the ta5le as it was when the other e"ge was lai" "ownwar"! an" that thepapers playe" 5est when the glass was nearest to the ta5le@ I 5egan to s7spect! that I ha" set "owna greater "istance of the glass fro$ the ta5le than I sho7l" ha+e "one@ for in setting "own thatexperi$ent! I tr7ste" to the i"ea I ha" of the 5igness of the hoop! in which I $ight easily 5e$ista9en! ha+ing not seen it of a long ti$e, An" this s7spicion was increase" 5y trying theexperi$ent with an o5Fect glass of a telescope! place" a5o7t the thir" part of an inch fro$ the ta5le@for I co7l" not see the papers play any thing near so well as I ha" seen the$ for$erly, >here7pon I

loo9e" for the ol" hoop with its glass! an" at length fo7n" the hoop! the glass 5eing gone@ 57t 5y thehoop I percei+e"! that! when one e"ge was t7rne" "own! the glass was al$ost the thir" part of aninch fro$ the ta5le! an" when the other e"ge was "own! which $a"e the papers play so well! theglass was scarce the eighth part of an inch fro$ the ta5le, #his I tho7ght fit to signify to yo7! that! if the experi$ent s7ccee" not well at the "istance I set "own! it $ay 5e trie" at a less "istance! an"that yo7 $ay alter $y paper! an" write in it the eighth part of an inch instea" of '. or '- of an inch,#he 5its of paper o7ght to 5e +ery little! an" of thin paper@ perhaps little 5its of the wings of a fly! or other light s75stances! $ay "o 5etter than paper, So$e of the $otions! as that of hanging 5y acorner an" twirling a5o7t! an" that of leaping fro$ one part of the glass to another! witho7t to7chingthe ta5le! happen 57t sel"o$@ 57t it $a"e $e ta9e the $ore notice of the$,

Pray present $y h7$5le ser+ice to 8r, %O4LE! when yo7 see hi$! an" than9s for the fa+o7r of thecon+erse I ha" with hi$ at Spring, 8y conceit of trepaning the co$$on ?ther! as he was please"

to express it! $a9es $e 5egin to ha+e the 5etter tho7ghts on that he was please" to entertain with as$ile, I a$ apt to thin9! that when he has a set of experi$ents to try in his air0p7$p! he will $a9ethat one! to see how the co$pression or relaxation of a $7scle will shrin9 or swell! soften or har"en!lengthen or shorten it,

As for registring the two "isco7rses! yo7 $ay "o it@ only I "esire yo7 wo7l" s7spen" till $y nextletter! in which I inten" to set "own so$ething to 5e altere"! an" so$ething to 5e a""e" in thehypothesis,

It was or"ere"! that 8r, OL2EN%UR sho7l" again write to 8r, NE>#ON! an" acB7aint hi$ withthe want of s7ccess of his experi$ent! an" "esire hi$ to sen" his own apparat7s! with which heha" $a"e it: as also to enB7ire! whether he ha" sec7re" the papers 5eing $o+e" fro$ the air! that

$ight so$ewhere steal in,

Here7pon the seB7el of his hypothesis! the first part of which was rea" at the prece"ing $eetings!was rea" to the en",

;..<

#h7s $7ch of refraction! reflection! transparency! an" opacity@ an" now to explain colo7rs@ Is7ppose! that as 5o"ies of +ario7s sies! "ensities! or sensations! "o 5y perc7ssion or other actionexcite so7n"s of +ario7s tones! an" conseB7ently +i5rations in the air of +ario7s 5igness@ so whenthe rays of light! 5y i$pinging on the stiff refracting s7perficies! excite +i5rations in the ?ther! thoserays! whate+er they 5e! as they happen to "iffer in $agnit7"e! strength or +igo7r! excite +i5rationsof +ario7s 5igness@ the 5iggest! strongest! or $ost potent rays! the largest +i5rations@ an" othersshorter! accor"ing to their 5igness! strength! or power: an" therefore the en"s of the capilla$enta of the optic ner+e! which pa+e or face the retina! 5eing s7ch refracting s7perficies! when the raysi$pinge 7pon the$! they $7st there excite these +i5rations! which +i5rations &li9e those of so7n" ina tr7n9 or tr7$pet* will r7n along the aB7eo7s pores or crystalline pith of the capilla$enta thro7ghthe optic ner+es into the sensori7$ &which light itself cannot "o* an" there! I s7ppose! affect thesense with +ario7s colo7rs! accor"ing to their 5igness an" $ixt7re@ the 5iggest with the strongestcolo7rs! re"s an" yellows@ the least with the wea9est! 5l7es an" +iolets@ the $i""le with green! an"a conf7sion of all with white! $7ch after the $anner! that in the sense of hearing! nat7re $a9es 7seof aereal +i5rations of se+eral 5ignesses to generate so7n"s of "i+ers tones@ for the analogy of nat7re is to 5e o5ser+e", An" f7rther! as the har$ony an" "iscor" of so7n"s procee" fro$ theproportions of the aereal +i5rations! so $ay the har$ony of so$e colo7rs! as of gol"en an" 5l7e!

an" the "iscor" of others! as of re" an" 5l7e! procee" fro$ the proportions of the ?thereal, An"possi5ly colo7r $ay 5e "isting7ishe" into its principal "egrees! re"! orange! yellow! green! 5l7e!in"igo! an" "eep +iolet! on the sa$e gro7n"! that so7n" within an eighth is gra"7ate" into tones,



7pon a paper a5o7t two an" twenty foot "istant fro$ the pris$! I "esire" a frien" to "raw with apencil lines cross the i$age! or pillar of colo7rs! where e+ery one of the se+en aforena$e" colo7rswas $ost f7ll an" 5ris9! an" also where he F7"ge" the tr7est confines of the$ to 5e! whilst I hel" thepaper so! that the sai" i$age $ight fall within a certain co$pass $ar9e" on it, An" this I "i"! partly5eca7se $y own eyes are not +ery critical in "isting7ishing colo7rs! partly 5eca7se another! towho$ I ha" not co$$7nicate" $y tho7ghts a5o7t this $atter! co7l" ha+e nothing 57t his eyes to"eter$ine his fancy in $a9ing those $ar9s, #his o5ser+ation we repeate" "i+ers ti$es! 5oth in the

sa$e an" "i+ers "ays! to see how the $ar9s on se+eral papers wo7l" agree@ an" co$paring theo5ser+ations! tho7gh the F7st confines of the colo7rs are har" to 5e assigne"! 5eca7se they passinto one another 5y insensi5le gra"ation@ yet the "ifferences of the o5ser+ations were 57t little!especially towar"s the re" en"! an" ta9ing $eans 5etween those "ifferences! that were! the lengthof the i$age &rec9one" not 5y the "istance of the +erges of the se$icirc7lar en"s! 57t 5y the"istance of the centres of those se$icircles! or length of the strait si"es as it o7ght to 5e* was"i+i"e" in a5o7t the sa$e proportion that a string is! 5etween the en" an" the $i""le! to so7n" thetones in the eighth, 4o7 will 7n"erstan" $e 5est 5y +iewing the annexe" fig7re! in which A % an" C2 represent the strait si"es! a5o7t ten inches long! A % C an" % # 2 the se$icirc7lar en"s! an";.-< 4 the centres of those se$icicles! Q the length of a $7sical string "o75le to 4! an""i+i"e" 5etween an" 4! so as to so7n" the tones expresse" at the si"e &that is H the half! an" I the thir" part! 4 the fifth part! 4 8 the eighth part! an" E the ninth part of 4* an" theinter+als 5etween these "i+isions express the spaces which the colo7rs written there too9 7p! e+ery

colo7r 5eing the $ost 5ris9ly specific in the $i""le of those spaces,

Now for the ca7se of these an" s7ch li9e colo7rs $a"e 5y refraction! the 5iggest or strongest rays$7st penetrate the refracting s7perficies $ore freely an" easily than the wea9er! an" so 5e lesst7rne" awry 5y it! that is! less refracte"@ which is as $7ch as to say! the rays! which $a9e re"! areleast refrangi5le! those! which $a9e 5l7e an" +iolet! $ost refrangi5le! an" others otherwiserefrangi5le accor"ing to their colo7r: whence! if the rays! which co$e pro$isc7o7sly fro$ the s7n!5e refracte" 5y a pris$! as in the aforesai" experi$ent! these of se+eral sorts 5eing +ario7slyrefracte"! $7st go to se+eral places on an opposite paper or wall! an" so parte"! exhi5it e+ery onetheir own colo7rs! which they co7l" not "o while 5len"e" together, An"! 5eca7se refraction onlyse+ers the$! an" changes not the 5igness or strength of the ray! thence it is! that after they areonce well se+ere"! refraction cannot $a9e any f7rther changes in their colo7r,

On this gro7n" $ay all the ph?no$ena of refractions 5e 7n"erstoo": 57t to explain the colo7rs$a"e 5y reflections! I $7st f7rther s7ppose! that! tho7gh light 5e 7ni$agina5ly swift! yet the?thereal +i5rations! excite" 5y a ray! $o+e faster than the ray itself! an" so o+erta9e an" o7tr7n itone after another, An" this! I s7ppose! they will thin9 an allowa5le s7pposition! who ha+e 5eenincline" to s7spect! that these +i5rations the$sel+es $ight 5e light, %7t to $a9e it the $oreallowa5le! it is possi5le light itself $ay not 5e so swift! as so$e are apt to thin9@ for! notwithstan"ingany arg7$ent! that I 9now yet to the contrary! it $ay 5e an ho7r or two! if not $ore! in $o+ing fro$the s7n to 7s, #his celerity of the +i5rations therefore s7ppose"! if light 5e inci"ent on a thin s9in or plate of any transparent 5o"y! the wa+es! excite" 5y its passage thro7gh the first s7perficies!o+erta9ing it one after another! till it arri+e at the secon" s7perficies! will ca7se it to 5e therereflecte" or refracte" accor"ingly as the con"ense" or expan"e" part of the wa+e o+erta9es itthere, If the plate 5e of s7ch a thic9ness! that the con"ense" part of the first wa+e o+erta9e the rayat the secon" s7perficies! it $7st 5e reflecte" there@ if "o75le that thic9ness! that the followingrarifie" part of the wa+e! that is! the space 5etween that an" the next ;./< wa+e! o+erta9e it! thereit $7st 5e trans$itte"@ if triple the thic9ness! that the con"ense" part of the secon" wa+e o+erta9eit! there it $7st 5e reflecte"! an" so where the plate is fi+e! se+en! or nine ti$es that thic9ness! it$7st 5e reflecte" 5y reason of the thir"! fo7rth! or fifth wa+e! o+erta9ing it at the secon" s7perficies@57t when it is fo7r! six! or eight ti$es that thic9ness! so that the ray $ay 5e o+erta9en there 5y the"ilate" inter+al of those wa+es! it shall 5e trans$itte"! an" so on@ the secon" s7perficies 5eing$a"e a5le or 7na5le to reflect accor"ingly as it is con"ense" or expan"e" 5y the wa+es, 6or instance! let A H represent the s7perficies of a spherically con+ex glass lai" 7pon a plain glass AI R! an" A I R H the thin plane0conca+e plate of air 5etween the$! an" % C! 2 E! 6 ! H I! Dc,thic9nesses of that plate! or "istances of the glasses in the arith$etical progression of the n7$5ers', ., -, /, Dc, whereof % C is the "istance! at which the ray is o+erta9en 5y the $ost con"ense" part

of the first wa+e: I say! the rays inci"ent at %! 6! ! an" O! o7ght to 5e reflecte" at C! ! L! an" P!an" those inci"ent at 2! H! 8! an" ! o7ght to 5e trans$itte" at E! I! N! an" R@ an" this! 5eca7sethe ray % C arri+es at the s7perficies A C! when it is con"ense"! 5y the first wa+e that o+erta9es it@



H I! when rarifie" 5y the inter+al of the secon" an" thir"@ an" so on for an in"eter$inate n7$5er of s7ccessions@ an" at A! the center or contact of the glasses! the light $7st 5e trans$itte"! 5eca7sethere the ?thereal $e"i7$s in 5oth glasses are contin7e" as if 57t one 7nifor$ $e"i7$, >hence!if the glasses in this post7re 5e loo9e" 7pon! there o7ght to appear at A! the contact of the glasses!a 5lac9 spot! an" a5o7t that $any concentric circles of light an" "ar9ness! the sB7ares of whosese$i"ia$eters are to sense an" arith$etical progression, 4et all the rays! witho7t exception! o7ghtnot to 5e th7s reflecte" or trans$itte": for so$eti$es a ray $ay 5e o+erta9en at the secon"

s7perficies! 5y the +i5rations raise" 5y another collateral or i$$e"iately s7ccee"ing ray@ which+i5ration! 5eing as strong or stronger than its own! $ay ca7se it to 5e reflecte" or trans$itte" whenits own +i5ration alone wo7l" "o the contrary, An" hence so$e little light will 5e reflecte" fro$ the5lac9 rings! which $a9es ;.)< the$ rather 5lac9 than totally "ar9@ an" so$e trans$itte" at thel7ci" rings! which $a9es the 5lac9 rings! appearing on the other si"e of the glasses! not so 5lac9 asthey wo7l" otherwise 5e, An" so at the central 5lac9 spot! where the glasses "o not a5sol7telyto7ch! a little light will 5e reflecte"! which $a9es the spot "ar9est in the $i""le! an" only 5lac9 atthe +erges, 6or th7s I ha+e o5ser+e" it to 5e! 5y tying +ery har" together two glass pris$s! whichwere acci"entally &one of the$ at least* a +ery little con+ex! an" +iewing 5y "i+ers lights this 5lac9spot at their contact, If a white paper was place" at a litle "istance 5ehin" a can"le! an" the can"lean" paper +iewe" alternately 5y reflection fro$ the spot! the +erges of the spot! which loo9e" 5y thelight of the paper as 5lac9 as the $i""le part! appeare" 5y the stronger light of the can"le l7ci"eno7gh! so as to $a9e the spot see$ less than 5efore@ 57t in the $i""le part contin7e" as

a5sol7tely 5lac9 in one case as in the other! so$e spec9s an" strea9s in it only excepte"! where Is7ppose the glasses! thro7gh so$e 7ne+enness in the polish! "i" not f7lly to7ch, #he sa$e I ha+eo5ser+e" 5y +iewing the spot 5y the li9e reflection of the s7n an" clo7"s alternately,

%7t to ret7rn to the l7ci" an" 5lac9 rings! those rings o7ght always to appear after the $anner "escri5e"! were light 7nifor$, An" after that $anner! when two contig7o7s glasses A an" A Rha+e 5een ill7strate"! in a "ar9 roo$! 5y light of any 7nifor$ colo7r $a"e 5y a pris$! I ha+e seenthe l7ci" circles appear to a5o7t twenty in n7$5er! with $any "ar9 ones 5etween the$! the colo7r of the l7ci" ones 5eing that of the light! with which the glasses were ill7strate", An" if the glasseswere hel" 5etween the eye an" pris$atic colo7rs! cast on a sheet of white0paper! or if any pris$aticcolo7r was "irectly traFecte" thro7gh the glasses to a sheet of paper place" a little way 5ehin"!there wo7l" appear s7ch other rings of colo7r an" "ar9ness &in the first case 5etween the glasses!

in the secon"! on the paper* oppositely correspon"ing to those! which appeare" 5y reflection: I$ean! that! whereas 5y reflecte" light there appeare" a 5lac9 spot in the $i""le! an" then acolo7re" circle@ on the contrary! 5y trans$itte" light there appeare" a colo7re" spot in the $i""le!an" then a 5lac9 circle! an" so on@ the "ia$eters of the colo7re" circles! $a"e 5y trans$ission!eB7alling the "ia$eters of the 5lac9 ones $a"e 5y reflection,

#h7s! I say! the rings "o an" o7ght to appear when $a"e 5y 7nifor$ light@ 57t in co$po7n" light itis otherwise, 6or the rays! which exhi5it re" an" yellow! exciting! as I sai"! larger p7lses in the?ther than those! which $a9e 5l7e an" +iolet! an" conseB7ently $a9ing 5igger circles in a certainproportion! as I ha+e $anifestly fo7n" they "o! 5y ill7$inating the glasses s7ccessi+ely 5y theaforesai" colo7rs of pris$ in a well "ar9ene" roo$! witho7t changing the position of $y eye or of the glasses@ hence the circles! $a"e 5y ill7strating the glasses with white light! o7ght not to appear 5lac9 an" white 5y t7rns! as the circles $a"e 5y ill7strating the glasses@ for instance! with re" light!appear re" an" 5lac9@ 57t the colo7rs! which co$po7n" the white light! $7st "isplay the$sel+es 5y5eing reflecte"! the 5l7e an" +iolet nearer to the center than the re" an" yellow! where5y e+eryl7ci" circle $7st 5eco$e +iolet in the inwar" +erge! re" in the o7twar"! an" of inter$e"iate colo7rsin the inter$e"iate ;.< parts! an" 5e $a"e 5roa"er than 5efore! sprea"ing the colo7rs 5othways into those spaces! which I call the 5lac9 rings! an" which wo7l" here appear 5lac9! were there"! yellow! 5l7e! an" +iolet! which $a9e the +erge of the rings! ta9en o7t of the inci"ent white light!which ill7strates the glasses! an" the green only left to $a9e the l7ci" rings, S7ppose C %! 2! L 6!P 8! R N! S ! represent B7a"rants of the circles $a"e in a "ar9 roo$ 5y the +ery "eepestpris$atic re" alone@ an" 4 ! T! J ! M K! V W! X Y! the B7a"rants of li9e circles $a"e also in a "ar9roo$! 5y the +ery "eepest pris$atic +iolet alone: an" then! if the glasses 5e ill7$inate" 5y open "aylight! in which all sorts of rays are 5len"e"! it is $anifest! that the first l7ci" ring will 5e 4 % C@ thesecon" T 2 ! the thir"! J 6 L! the fo7rth M K 8 P! the fifth V W N R! the sixth X Y S! Dc, in all

which the "eepest +iolet $7st 5e reflecte" at the inwar" e"ges represente" 5y the pric9e" lines!where it wo7l" 5e reflecte" were it alone! an" the "eepest re" at the o7twar" e"ges represente" 5ythe 5lac9 lines! where it wo7l" 5e reflecte"! were it alone@ an" all inter$e"iate colo7rs at those



the$ in a "ar9 roo$! parte" fro$ all other colo7rs 5y the refraction of a pris$, An" 5eca7se thesB7ares of the se$i"ia$eters of the o7twar" +erges AC! A ! A L! Dc, as also of A 4! A ! A J! Dc,the se$i"ia$eters of the inwar" are in arith$etical progression of the n7$5ers '! -! )! (! =! ''! Dc,an" the sB7ares of the inwar" are to the sB7ares of the o7twar" &A 4= to A C=! A = to A =! A J=to A L=! Dc,* as = to '/! &as I ha+e fo7n" 5y $eas7ring the$ caref7lly an" often! an" co$paring theo5ser+ations:* therefore the o7twar" re" +erge of the secon" ring! an" inwar" +iolet one of the thir"!shall 5or"er 7pon one another &as yo7 $ay 9now 5y co$p7tation! an" see the$ represente" in the

fig7re* an" the li9e e"ges of the thir" an" fo7rth rings shall interfere! an" those of the fo7rth an" fifthinterfere $ore! an" so on, 4ea! the colo7rs of e+ery ring $7st sprea" the$sel+es so$ething $ore5oth ways than is here represente"! 5eca7se the B7a"rantal arcs here "escri5e" represent not the+erges! 57t the $i""le of the rings $a"e in a "ar9 roo$ 5y the extre$e +iolet an" re"@ the +ioletfalling on 5oth si"es the pric9e" arches! an" re" on 5oth si"es the 5lac9 line arches, An" hence itis! that these rings or circ7its of colo7rs s7ccee" one another contin7ally! witho7t any ;.(<inte+ening 5lac9! an" that the colo7rs are p7re only in the three or fo7r first rings! an" theninter+ening an" $ixing $ore an" $ore! "il7te one another so $7ch! that after eight of nine ringsthey are no $ore to 5e "isting7ishe"! 57t see$ to constit7te an e+en whiteness@ whereas! whenthey were $a"e in a "ar9 roo$ 5y one of the pris$atic colo7rs alone! I ha+e! as I sai"! seen a5o+etwenty of the$! an" witho7t "o75t co7l" ha+e seen the$ to a greater n7$5er! ha" I ta9en the painsto $a9e the pri$atic colo7rs $ore 7nco$po7n"e", 6or 5y 7nfol"ing these rings fro$ one another!5y certain refractions expresse" in the others papers I sen" yo7! I ha+e! e+en in "ay0light!

"isco+ere" the$ to a5o+e an h7n"re"@ an" perhaps they wo7l" ha+e appeare" inn7$era5le! ha"the light or colo7r ill7strating the glasses 5een a5sol7tely 7nco$po7n"e"! an" the p7pil of $y eye57t a $athe$atical point@ so that the rays! which ca$e fro$ the sa$e point of the glass $ight ha+egone into $y eye at the sa$e o5liB7ity to the glass,

>hat has 5een hitherto sai" of the rings! is to 5e 7n"erstoo" of their appearance to an 7n$o+e"eye: 57t if yo7 +ary the position of the eye! the $ore o5liB7ely yo7 loo9 on the glass! the larger therings appear, An" of this the reason $ay 5e! partly that an o5liB7e ray is longer in passing thro7ghthe first s7perficies! an" so there is $ore ti$e 5etween the wa+ing forwar" an" 5ac9war" of thats7perficies! an" conseB7ently a larger wa+e generate"! an" partly! that the wa+e in creeping along5etween the two s7perficies $ay 5e i$pe"e" an" retar"e" 5y the rigi"ness of those s7perficies!5o7n"ing it at either en"! an" so not o+erta9e the ray so soon as a wa+e! that $o+es

perpen"ic7larly cross,

#he 5igness of the circles $a"e 5y e+ery colo7r! an" at all o5liB7ities of the eye to the glass! an"the thic9ness of the air! or inter+als of the glasses! where each circle is $a"e! yo7 will fin"expresse" in the other papers I sen" yo7@ where also I ha+e $ore at large "escri5e"! how $7chthese rings interfere! or sprea" into one another@ what colo7rs appear in e+ery ring! where they are$ost li+ely! where an" how $7ch "il7te" 5y $ixing with the colo7rs of other rings@ an" how thecontrary colo7rs appear on the 5ac9 si"e of the glasses 5y the trans$itte" light! the glassestrans$itting light of one colo7r at the sa$e place! where they reflect that of another, Nor nee" I a""any thing f7rther of the colo7rs of other thinly plate" $e"i7$s! as of water 5etween the aforesai"glasses! or for$e" into 5755les! an" so enco$passe" with air! or of glass 5lown into +ery thin5755les at a la$p f7rnace! Dc, the case 5eing the sa$e in all these! excepting that! where thethic9ness of the plate is not reg7lar! the rings will not 5e so@ that in plates of "enser transparent5o"ies! the rings are $a"e at a less thic9ness of the plate &the +i5rations! I s7ppose! 5eing shorter in rarer ?ther than in "enser* an" that in a "enser plate! s7rro7n"e" with a rarer 5o"y! the colo7rsare $ore +i+i" than in the rarer s7rro7n"e" with the "enser@ as! for instance! $ore +i+i" in a plate of glass s7rro7n"e" with air! than in a plate of air s7rro7n"e" with glass@ of which the reason is! thatthe reflection of the secon" s7perficies! which ca7ses the colo7rs! is! as was sai" a5o+e! stronger inthe for0 ;.< $er case than in the latter: for which reason also the colo7rs are $ost +i+i"! whenthe "ifference of the "ensity of the $e"i7$ is greatest,

Of the colo7rs of nat7ral 5o"ies also I ha+e sai" eno7gh in those papers! shewing how the +ario7ssies of the transparent particles! of which they consist! is s7fficient to pro"7ce the$ all! thoseparticles reflecting or trans$itting this or that sort of rays! accor"ing to their thic9ness! li9e theaforesai" plates! as if they were frag$ents thereof, 6or! I s7ppose! if a plate of an e+en thic9ness!

an" conseB7ently of an 7nifor$ colo7r! were 5ro9en into frag$ents of the sa$e thic9ness with theplate! a heap of those frag$ents wo7l" 5e a pow"er $7ch of the sa$e colo7r with the plates, An"so! if the parts 5e of the thic9ness of the water in the 5lac9 spot at the top of a 5755le "escri5e" in



of which 5lac9ness! I s7ppose! that the particles of that sie 5eing "ispose" to reflect al$ost no lighto7twar"! 57t to refract it contin7ally in its passage fro$ e+ery part to the next@ 5y this $7ltit7"e of refractions! the rays are 9ept so long straggling to an" fro within the 5o"y! till at last al$ost alli$pinge on the soli" parts of the 5o"y! an" so are stoppe" an" stifle"@ those parts ha+ing nos7fficient elasticity! or other "isposition to ret7rn ni$5ly eno7gh the s$art shoc9 of the ray 5ac97pon it,

I sho7l" here concl7"e! 57t that there is another strange ph?no$enon of colo7rs! which $ay"eser+e to 5e ta9en notice of, 8r, HOOE! yo7 $ay re$e$5er! was spea9ing of an o"" straying of light! ca7se" in its passage near the e"ge of a raor! 9nife! or other opa9e 5o"y in a "ar9 roo$@ therays! which pass +ery near the e"ge! 5eing there5y $a"e to stray at all angles into the sha"ow of the 9nife,

#o this Sir >ILLIA8 PE##4! then presi"ent! ret7rne" a +ery pertinent B7ery! >hether that strayingwas in c7r+e linesG an" that $a"e $e! ha+ing hear" 8r, HOOE so$e "ays 5efore co$pare it tothe straying of so7n" into the B7iescent $e"i7$! say! that I too9 it to 5e only a new 9in" of refraction! ca7se" perhaps 5y the external ?thers 5eginning to grow rarer a little 5efore it ca$e atthe opa9e 5o"y! than it was in free spaces@ the "enser ?ther witho7t the 5o"y! an" the rarer withinit! 5eing ter$inate" not in a $athe$atical s7perficies! 57t passing into one another thro7gh allinter$e"iate "egrees of "ensity: whence the rays! that pass so near the 5o"y! as to co$e within

that co$pass! where the o7twar" ?ther 5egins to grow rarer! $7st 5e refracte" 5y the 7ne+en"enseness thereof! an" 5len"e" inwar"s towar" the rarer $e"i7$ of the 5o"y, #o this 8r, HOOEwas then please" to answer! that tho7gh it sho7l" 5e 57t a new 9in" of refraction! yet it was a newone, >hat to $a9e of this 7nexpecte" reply! I 9new not@ ha+ing no other tho7ghts! 57t that a new9in" of refraction $ight 5e as no5le an in+ention as any thing else a5o7t light@ 57t it $a"e $eafterwar"s! I 9now not 7pon what occasion! happen to say! a$ong so$e that were present to whatpasse" 5efore! that I tho7ght I ha" seen the experi$ent 5efore in so$e Italian a7thor, An" thea7thor is HONORA#US 6A%ER! in his "ialog7e 2e L7$ine! who ha" it fro$ RI8AL2O@ ;.=<who$ I $ention! 5eca7se I a$ to "escri5e so$ething f7rther o7t of hi$! which yo7 will apprehen"5y this fig7re: s7pppose the s7n shine thro7gh the little hole H into a "ar9 roo$ 7pon the paper P! an" with a we"ge 8 N O intercept all 57t a little of that 5ea$! an" yo7 will see 7pon the paper six rows of colo7rs! R! S! #! 3! ! 4! an" 5eyon" the$ a +ery faint light sprea"ing either way! s7ch

as rays 5ro9en! li9e H N Q! $7st $a9e, #he a7thor "escri5es it $ore largely in "i+ers sche$es, Iha+e ti$e only to hint the s7$ of what he says,

Now for the 5rea9ing of the ray H N Q! s7ppose! in the next fig7re 8 N O 5e the soli" we"ge! A % Cthe inwar" 5o7n" of the 7nifor$ rarer ?ther within! 5eween which 5o7n"s the ?ther r7ns thro7ghall the inter$e"iate "egrees@ an" it is $anifest! that! if a ray co$e 5etween % an" N! it $7st in itspassage there 5en" fro$ the "enser $e"i7$ towar"s C! an" that so $7ch the $ore! 5y how $7chit co$es nearer N, 67rther! for the three rows of colo7rs 3 4! those $ay perhaps procee" fro$the n7$5er of +i5rations &whether one! two! or three* which o+erta9e the ray in its passage fro$ !till it 5e a5o7t the $i"0way 5etween an" H@ that is! at its nearest "istance to N! so as to to7ch thecircle "escri5e" a5o7t N! with that "istance@ 5y the last of which +i5rations! expan"ing or contracting the $e"i7$ there! the ray is license" to rece"e again fro$ N! an" go on to $a9e thecolo7rs@ or f7rther 5ent a5o7t N! till the inter+al of the next wa+e o+erta9e it! an" gi+e it li5erty to gofro$ N! +ery nearly in the line it is then $o+ing! s7ppose towar" Q! to ca7se the faint light spo9en of a5o+e! yo7 will 7n"erstan" $e a little 5etter! 5y co$paring this with what was sai" of the colo7rs of thin transparent plates! co$paring the greatest "istance that the ray goes fro$ % H towar"s N! tothe thic9ness of one of those plates, So$ething too there is in 2ES CAR#ESS explication of therain5ows colo7rs! which wo7l" gi+e f7rther light in this, %7t I ha+e no ti$e left to insist f7rther 7ponpartic7lars@ nor "o I propo7n" this witho7t "iffi"ence! ha+ing not $a"e s7fficient o5ser+ation a5o7tit,

After rea"ing this "isco7rse! 8r, HOOE sai"! that the $ain of it was containe" in his 8icrographia!which 8r, NE>#ON ha" only carrie" farther in so$e partic7lars,

#he Society a"Fo7rne" till 2ece$5er -1,

;.(1<



2ece$5er -1, #here was rea" a letter to 8r, OL2EN%UR fro$ 8r, NE>#ON! "ate" atCa$5ri"ge .'st 2ece$5er! '()t ! in answer to what ha" 5een written to hi$ 5y 8r, OL2EN%URconcerning the want of s7ccess of his experi$ent $a"e with a glass r755e" Dc, #his letter was asfollows:

Upon yo7r letter I too9 another glass fo7r inches 5roa"! an" one fo7rth of an inch thic9! of s7chglass as telescopes are $a"e of! an" place" it a one sixth part of an inch fro$ the ta5le, It was set

in s7ch a piece of woo"! as the o5Fect0glasses of telescopes 7se to 5e set in: an" the experi$ents7ccee"e" well, After the r755ing was still! an" all was still! the $otion of the papers wo7l" contin7eso$eti$es while I co7nte" a h7n"re"! e+ery paper leaping 7p a5o7t twenty ti$es $ore or less! an""own as often, I trie" also with two other glasses that 5elong to a telescope! an" it s7ccee"e" with5oth@ an" I $a9e no B7estion 57t any glass will "o that! if it 5e excite" to electric +irt7e! as I thin9any $ay, If yo7 ha+e a $in" to any of these glasses! yo7 $ay ha+e the$@ 57t I s7ppose! if yo7cannot $a9e it "o in other glass! yo7 will fail in any I can sen" yo7, I a$ apt to s7spect the fail7rewas in the $anner of r755ing@ for I ha+e o5ser+e"! that the r755ing +ario7sly! or with +ario7s things!alters the case, At one ti$e I r755e" the aforesai" great glass with a nap9in! twice as $7ch as I7se" to "o with $y gown! an" nothing wo7l" stir@ an" yet presently r755ing it with so$ething else!the $otions soon 5egan, After the glass has 5een $7ch r755e" too! the $otions are not so lasting@an" the next "ay I fo7n" the $otions fainter an" "iffic7lter to excite than the first, If the Society ha+ea $in" to atte$pt it any $ore! I can gi+e no 5etter a"+ice than this: to ta9e a new glass not yet

r755e" &perhaps one of the ol" ones $ay "o well eno7gh after it has lain still a while* an" let this 5er755e"! not with linen! nor soft nappy woolen! 57t with st7ff! whose threa"s $ay ra9e the s7rface of the glass! s7ppose ta$erine! or the li9e! "o75le" 7p in the han"! an" this with a 5ris9 $otion as$ay 5e! till an h7n"re" or an h7n"re" an" fifty $ay 5e co7nte"! the glass lying all the while o+er thepapers, #hen! if nothing stir! r75 the glass with the finger en"s half a score of ti$es to an" fro! or 9noc9 yo7r fingeren"s as often 7pon the glass@ for this r755ing or 9noc9ing with yo7r fingers! after the for$er r755ing! con"7ces $ost to excite the papers, If nothing stir yet! r75 again with the clothtill sixty or eighty $ay 5e co7nte"! an" then r75 or 9noc9 again with yo7r fingers! an" repeat this tillthe electric +irt7e of the glass 5e so far excite" as to ta9e 7p the papers! an" then a +ery littler755ing or 9noc9ing now an" then will re+i+e the $otions, In "oing all this! let the r755ing 5e always"one as ni$5ly as $ay 5e@ an" if the $otion 5e circ7lar! li9e that of glass0grin"ing! it $ay "o 5etter,%7t if yo7 cannot $a9e it yet s7ccee"! it $7st 5e let alone till I ha+e so$e opport7nity of trying it

5efore yo7, As for the s7spicion of the papers 5eing $o+e" 5y the air! I a$ sec7re fro$ that@ yet inthe other! of "rawing leaf0gol" to a5o+e a foot "istance! which I ne+er went a5o7t to try $yself tillthe last wee9! I s7spect the air $ight raise the gol"! an" then a s$all attraction $ight "eter$ine ittowar"s the glass@ for I co7l" not $a9e it s7ccee",

;.('<

It was or"ere"! that 8r, NE>#ONS "irections in this letter sho7l" 5e o5ser+e" in the experi$ent to5e $a"e at the next $eeting of the Society,

8r, OL2EN%UR rea" a letter to hi$self fro$ 8r, OHN ASCOINE! "ate" at Liege! ')th2ece$5er! '()7 ! acB7ainting hi$ with the "eath of 8r, LINUS of the epi"e$ical "isease! whichthen rage" thro7gh so $any co7ntries! an" with the resol7tion of 8r, LINUSS "isciples! to try 8r,NE>#ONS experi$ent concerning light an" colo7rs $ore clearly an" caref7lly! an" 5efore $orewitnesses! accor"ing to the "irections gi+en the$ 5y 8r, NE>#ONS last letter: inti$ating withal!that if the sai" experi$ent 5e $a"e 5efore the Royal Society! an" 5e atteste" 5y the$ to s7ccee"!as 8r, NE>#ON affir$e"! they wo7l" rest satisfie",

It was or"ere"! that when the s7n sho7l" ser+e! the experi$ent sho7l" 5e $a"e 5efore the Society,

8r, AU%RE4 presente" the Society with his o5ser+ations $a"e in >iltshire! which 5eing rea"! hewas "esire" to en"ea+o7r to proc7re so$e of the iron0ore of Sein in that co7nty! sai" to 5e so rich!that the s$ith co7l" $elt it in his forge: as also to proc7re fro$ Easton0Peires in 8al$es57ryh7n"re"! so$e of the 5l7e clay! free fro$ san"! an" al$ost of the colo7r of 7ltra$arine@ which clay

8r, 2OIH# s7ppose" to 5e +ery fit for porcelane,

#he Society a"Fo7rne" till the '-th of an7ary following,



an7ary '-, Captain HENR4 SHEERES! OHN 8APLE#O6#! 8, 2,x ! an" Signor 6RANCISCO#RA3AINI were propose" can"i"ates! the first in the na$e of Sir OSEPH >ILLIA8SON! thesecon" 5y 8r, HOOE! an" the thir" 5y 8r, OL2EN%UR,

8r, NE>#ONS experi$ent of glass r755e"! to ca7se +ario7s $otions in 5its of paper 7n"erneath!5eing $a"e accor"ing to his $ore partic7lar "irections! s7ccee"e" +ery well, #he r755ing was$a"e 5oth with a scr755ing 5r7sh! $a"e of short hogs 5ristles! with a 9nife! the haft of the 9nife

$a"e of whale5one! an" with the nail of ones finger, It appeare"! that to7ching $any parts at oncewith a har" an" ro7gh 5o"y! pro"7ce" the effect expecte",

It was or"ere"! that 8r, NE>#ON sho7l" ha+e the than9s of the Society! for gi+ing hi$self thetro75le of i$parting to the$ s7ch f7ll instr7ctions for $a9ing the experi$ent,

8r, OL2EN%UR pro"7ce" an" rea" a Latin letter of 8r, 6LA8S#EA2 to Sir ONAS 8OORE!"ate" at reenwich! ./th 2ece$5er! '()y ! containing an acco7nt of his o5ser+ations $a"e of thelate eclipse of the $oon on the .'st 2ece$5er! p! $,

;.(.<

It was or"ere"! that 8r, OL2EN%UR sho7l" 5e "esire"! accor"ing to the $otion $a"e 5y 8r,6LA8S#EA2! to i$part these o5esr+ations to Signor CASSINI at Paris! an" to "esire hi$ toco$$7nicate to the Society his o5ser+ations on the sa$e eclipse,

8r, OL2EN%UR pro"7ce" li9ewise so$e papers of 8r, AU%RE4! containing his o5ser+ations of the co7nty of S7rry, %7t the ti$e 5eing elapse"! these papers were referre" to the next $eeting

an7ary .1, 8r, AU%RE4S papers of o5ser+ation on S7rrey were rea",

#here was also rea" the 5eginning of 8r, NE>#ONS "isco7rse! containing s7ch o5ser+ations! ascon"7ce to f7rther "isco+eries for co$pleting his theory of light an" colo7rs! especially as to theconstit7tion of nat7ral 5o"ies! on which their colo7rs or transparency "epen": in which he "escri5es

first the principal of his o5ser+ations! an" then consi"ers an" $a9es 7se of the$,

At this ti$e there were rea" the first fifteen of those o5ser+ations as follow :

I s7ppose yo7 7n"erstan"! that all transparent s75stances &as glass! water! air Dc,* when $a"e+ery thin 5y 5eing 5lown into 5755les! or otherwise for$e" into plates! "o exhi5it +ario7s colo7rs!accor"ing to their +ario7s thinness! altho7gh at a greater thic9ness they appear +ery clear an"colo7rless, In $y for$er "isco7rse a5o7t the constit7tion of light! I o$itte" these colo7rs! 5eca7sethey see$e" of a "iffic7lt consi"eration! an" were not necessary for the esta5lishing of the "octrine!which I propo7n"e"@ 57t 5eca7se they $ay con"7ce to f7rther "isco+eries for co$pleating thattheory! especially as to the constit7tion of the parts of nat7ral 5o"ies! on which their colo7rs or transparency "epen"! I ha+e now sent yo7 an acco7nt of the$, #o ren"er this "isco7rse short an""istinct! I ha+e first "escri5e" the principal of $y o5ser+ations! an" then consi"ere" an" $a"e 7seof the$, #he o5ser+ations are these:

O5s, ', Co$pressing two pris$s har" together! that their si"es &which 5y chance were a +ery littlecon+ex* $ight so$ewhere to7ch one another! I fo7n" the place! in which they to7che"! to 5eco$eaa5sol7tely transparent! as if they ha" 5een there one contin7e" piece of glass@ for when the light fellso o5liB7ely on the air! which in other places was 5etween the$! as to 5e all reflecte"! in that placeof contact it see$e" wholly trans$itte"@ inso$7ch that when loo9e" 7pon! it appeare" li9e a 5lac9or "ar9 spot! 5y reason of no sensi5le light was reflecte" fro$ thence! as fro$ other places@ an"when loo9e" thro7gh! it see$e"! as it were! a hole in that air! that was for$e" into a thin ;.(-<plate 5y 5eing co$presse" 5etween the glasses@ an" thro7gh this hole o5Fects! that were 5eyon"!$ight 5e seen "istinctly! which co7l" not at all 5e seen thro7gh other parts of the glasses! wherethe air was interFacent, Altho7gh the glasses were a little con+ex! yet this transparent spot was of a

consi"era5le 5rea"th! which 5rea"th see$e" principally to procee" fro$ the yiel"ing inwar"s of theparts of the glasses 5y reason of their $7t7al press7re@ for 5y pressing the$ +ery har" together! itwo7l" 5eco$e $7ch 5roa"er than otherwise,



O5s, ., >hen the plate of air! 5y t7rning the pris$s a5o7t their co$$on axis! 5eca$e so littleincline" to the inci"ent rays! that so$e of the$ 5egan to 5e trans$itte"! there arose in it $anyslen"er arcs of colo7rs! which at first were shape" al$ost li9e the conchoi"! as yo7 see the$ here"elineate", An" 5y contin7ing the $otion of the pris$s! these arcs increase" an" 5en"e" $ore an"$ore a5o7t the sai" transparent spot! till they were co$pleate" into circles or rings inco$passing it!an" afterwar"s contin7ally grew $ore an" $ore contracte",

#hese arcs! at their first appearance! were of a +iolet an" 5l7e colo7r! an" 5etween the$ werewhite arcs of circles! which presently 5eca$e a little tinge" in their inwar" li$5s with re" an" yellow!an" to their o7twar" li$5s the 5l7e was a"Facent@ so that the or"er of these colo7rs fro$ the central"ar9 spot! was at that ti$e white! 5l7e! +iolet! 5lac9! re"! orange! yellow! white! 5l7e! +iolet! Dc, 57tthe yellow an" re" were $7ch fainter than the 5l7e an" +iolet,

#he $otion of the pris$s a5o7t their axis 5eing contin7e"! these colo7rs contracte" $ore an" $ore!shrin9ing towar"s the whiteness on either si"e of it! 7ntil they totally +anishe" into it@ an" then thecircles in those parts appeare" 5lac9! an" white! witho7t any other colo7rs inter$ixe"@ 57t 5y f7rther $o+ing the pris$s a5o7t! the colo7rs again e$erge" o7t of the whiteness! the +iolet an" 5l7e at itsinwar" li$5! an" at its o7twar" li$5 the re" an" yellow@ so that now their or"er fro$ the central spotwas white! yellow! re"! 5lac9! +iolet! 5l7e! white! yellow! re" Dc, contrary to what it was 5efore,

O5s, -, >hen the rings or so$e parts appeare" only 5lac9 an" white! they were +ery "istinct an"well "efine"! an" the 5lac9ness see$e" as intense as that of the central spot@ also! in the 5or"ersof these rings! where the colo7rs 5egan to e$erge o7t of the whiteness! they were pretty "istinct!which $a"e the$ +isi5le to a +ery great $7ltit7"e, I ha+e so$eti$es n7$5ere" a5o+e thirtys7ccessions &rec9oning e+ery 5lac9 an" white ring for one s7ccession* an" seen $ore of the$!which 5y reason of their s$allness I co7l" not n7$5er, %7t in other positions of the pris$s! at whichthe rings appeare" of $any colo7rs! I co7l" not "isting7ish a5o+e eight or nine of the$! an" theexterior of those too were conf7se" an" "il7te,

;.(/<

In these two o5ser+ations! to see the rings "istinct! an" witho7t any other colo7r 57t 5lac9 an"white! I fo7n" it necesary that I hel" $y eye at a goo" "istance fro$ the$, 6or 5y approachingnearer! altho7gh in the sa$e inclination of $y eye! yet there e$erge" a 5l7ish colo7r o7t of thewhite! which 5y "ilating itself $ore an" $ore into a 5lac9! ren"ere" the circles less "istinct! an" leftthe white a little tinge" with re" an" yellow, I fo7n" also! that 5y loo9ing thro7gh a slit or o5longhole! which was narrower than the p7pil of $y eye! an" hel" close to it parallel to the pris$s! I co7l"see the circles $7ch "istincter an" +isi5le to a far greater n7$5er than otherwise,

O5s, /, #o o5ser+e $ore nicely the or"er of the colo7rs! which arose o7t of the white circles! as therays 5eca$e less an" less incline" to the plate of air@ I too9 two o5Fect0glasses! the one a plane0con+ex for a fo7rteen foot telescope! an" the other a large "o75le con+ex for one of fifty foot@ an"7pon this laying the other with its plane si"e "ownwar"s! I presse" the$ slowly together! to $a9ethe colo7rs s7ccessi+ely e$erge in the $i""le of the circles! an" then slowly lifte" the 7pper glassfro$ the lower! to $a9e the$ s7ccessi+ely +anish again in the sa$e place! where 5eing of aconsi"era5le 5rea"th! I co7l" $ore easily "iscern the$, An" 5y this $eans I o5ser+e" their s7ccession an" B7antity to 5e as followeth,

Next to the pell7ci" central spot $a"e 5y the contact of the glasses! s7ccee"e" +iolet! 5l7e! white!yellow! an" re", #he +iolet an" 5l7e were so +ery little in B7antity! that I co7l" not "iscern the$ inthe circles $a"e 5y the pris$s@ 57t the yellow an" re" were pretty copio7s! an" see$e" a5o7t as$7ch in extent as the white! an" fo7r or fi+e ti$es $ore than the 5l7e an" +iolet, #he next circ7it or or"er of colo7rs i$$e"iately enco$passing these was +iolet! 5l7e! green! yellow! an" re", An"these were all of the$ copio7s an" +i+i"! excepting the green! which was +ery little in B7antity! an"see$e" $7ch $ore faint an" "il7te than the other colo7rs, Of the other fo7r the +iolet was least!an" the 5l7e less than the yellow or re", #he thir" circ7it or or"er was also p7rple! 5l7e! green!

yellow! an" re"! in which the p7rple see$e" $ore re""ish than the +iolet in the for$er circ7it! an"the green was $7ch $ore conspic7o7s! 5eing as 5ris9 an" copio7s as any of the other colo7rsexcept the yellow@ 57t the re" 5egan to 5e a little fa"e"! inclining +ery $7ch to p7rple, After these



an" the other to yellow, %7t in this fo7rth circ7it there was neither +iolet! 5l7e! nor yellow! an" there" was +ery i$perfect an" "irty, Also the s7ccee"ing colo7rs 5eca$e $ore an" $ore i$perfectan" "il7te! till after three or fo7r $ore re+ol7tions they en"e" in perfect whiteness,

O5s, ), #o "eter$ine the inter+al of the glasses! or thic9ness of the interFacent air! 5y which eachcolo7r was pro"7ce"@ I $eas7re" the "ia$eter of the first six rings at the $ost l7ci" part of their or5its! an" sB7aring the$ I fo7n" their sB7ares to 5e in arith$etical progression of the o""

n7$5ers! ', -, ), (, =, '', An" since one of the glasses was plane an" the other spherical! their ;.()< inter+als at those rings $7st 5e in the sa$e progression, I $eas7re" also the "ia$eters of the "ar9 or faint rings 5etween the $ore l7ci" colo7rs! an" fo7n" their sB7ares to 5e in arith$eticalprogression! of the e+en n7$5ers .! /! ! ! '1! '.@ an" it 5eing +ery nice an" "iffic7lt to ta9e these$eas7res exactly! I repeate" the$ "i+ers ti$es! at "i+ers parts of the glasses! that 5y their agree$ent I $ight 5e confir$e" in the$@ an" the sa$e $etho" I 7se" in "eter$ining so$e othersof the following o5sera+tions,

O5s, , #he "ia$eter of the first ring! at the $ost l7ci" part of its or5it! was)'11 parts of an inch!an" the "ia$eter of the sphere! on which the "o75le con+ex o5Fect0glass was gro7n"! was anh7n"re" an" two foot! as I fo7n" 5y $eas7ring it@ an" conseB7ently the thic9ness of the air! or aereal inter+al of the glasses at that ring! was ''/))/ of an inch, 6or as the "ia$eter of the sai"sphere &an h7n"re" an" two foot! or twel+e h7n"re" an" twenty0fo7r inches* is to the se$i"ia$eter

of the ring .='11! so +ery nearly is that se$i"ia$eter to ''/))/! the sai" "istance of the glasses,Now! 5y the prece"ent o5ser+ations! the ele+enth part of this "istance &''11=/* is the thic9nessof the air at that part of the first ring! where the yellow wo7l" 5e $ost +i+i"! were it not $ixe" withother colo7rs in the white@ an" this "o75le" gi+es the "ifference of its thic9ness at the yellow in allthe other rings! +i, '11/(! or! to 7se a ro7n" n7$5er! the eighty tho7san" part of an inch,

O5s, (, #hese "i$ensions were ta9en! when $y eye was place" perpen"ic7larly o+er the glasses!in or near the axis of the rings@ 57t when I +iewe" the$ o5liB7ely! they 5eca$e 5igger! contin7allyswelling as I re$o+e" $y eye farther fro$ their axis@ an" partly 5y $eas7ring the "ia$eter of thesa$e circle at se+eral o5liB7ities of $y eye! partly 5y other $eans@ as also 5y $a9ing 7se of thetwo pris$s for +ery great o5liB7ities! I fo7n" its "ia$eter! an" conseB7ently the thic9ness of the air at its peri$eter in all those o5liB7ities! to 5e +ery nearly in the proportions expresse" in this ta5le,

Inci"ence Refraction 2ia$eter #hic9nesson the air into the air of the ring of the air gr, $in, gr, $in,

11 11 11 11 ,'1 ,'11 . '1 11 ,'1 1/13 ,'1 2/15

'. /) .1 11 ,'1 1/3 ,'1 2/3

' /= -1 11 ,'1 3/4 ,''1/2 ./ -1 /1 11 ,'' 2/5 ,'-.= -( )1 11 ,'. 1/2 ,') 1/2

-- ) 1 11 ,'/ ,.1-) /( ) 11 ,') 1/4 ,.- 1/3

-( '= (1 11 ,' 4/5 ,. 1/2

- -- () 11 ,'= 1/4 ,-(-= .( 1 11 ,.. 6/7 ,). 1/4

/1 11 ) 11 ,.= ,//1 '' =1 11 ,-) ,'.. 1/2

;.(<

In the first two col7$ns are expresse" the o5liB7ities of the rays to the plate of air@ that is! their angles of inci"ence an" refraction, In the thir" col7$n! the "ia$eter of any colo7re" ring of those

o5liB7ities is expresse" in parts! of which ten constit7te that "ia$eter! when the rays areperpen"ic7lar, An" in the fo7rth col7$n the thic9ness of the air at the circ7$ference of that ring isexpresse" in parts! of which also ten constit7te that thic9ness! when the rays are perpen"ic7lar,



O5s, , #he "ar9 spot in the $i""le of the rings increase" also 5y that o5liB7ation of the eye!altho7gh al$ost insensi5ly, %7t! if instea" of the o5Fect0glasses! the pris$s were $a"e 7se of! itsincrease was $ore $anifest! when +iewe" so o5liB7ely! that no colo7rs appeare" a5o7t it, It wasleast! when the rays were inci"ent $ost o5liB7ely on the interFacent air! an" increase" $ore an"$ore! 7ntil the colo7re" rings appeare"! an" then "ecrease" again! 57t not so $7ch as it increase"5efore, An" hence it is e+i"ent! that the transparency was not only at the a5sol7te contact of theglasses! 57t also where they ha" so$e little inter+al, I ha+e so$eti$es o5ser+e" the "ia$eter of

that spot to 5e 5etween half an" two fifth parts of the "ia$eter of the exterior circ7$ference of there" in the first circ7it or re+ol7tion of colo7rs! when +iewe" al$ost perpen"ic7larly@ whereas! when+iewe" o5liB7ely! it hath wholly +anishe"! an" 5eco$e opa9e an" white! li9e the other parts of theglass, >hence it $ay 5e collecte"! that the glasses "i" then scarcely! or not at all! to7ch oneanother@ an" that their inter+al of the peri$eter of that spot! when +iewe" perpen"ic7larly! wasa5o7t a fifth or sixth part of their inter+al at the circ7$ference of the sai" re",

O5s, =, %y loo9ing thro7gh the two contig7o7s o5Fect0glasses! I fo7n"! that the interFacent air exhi5ite" rings of colo7rs! as well 5y trans$itting light as 5y reflecting it, #he central spot was nowwhite! an" fro$ it the or"er of the colo7rs were yellowish! re"! 5lac9! +iolet! 5l7e! white! yellow! re"@+iolet! 5l7e! green! yellow! re"! Dc, 57t these colo7rs were +ery faint an" "il7te! 7nless when thelight was traFecte" +ery o5liB7ely thro7gh the glasses@ for 5y that $eans they 5eca$e pretty +i+i"!only the first yellowish re"! li9e the 5l7e in the fo7rth o5ser+ation! was so little an" faint as scarcely

to 5e "iscerne", Co$paring the colo7re" rings $a"e 5y reflection with these $a"e 5y trans$issionof the light! I fo7n"! that white was opposite to 5lac9! re" to 5l7e! yellow to +iolet! an" green to aco$po7n" of re" an" +iolet@ that is! those parts of the glass were 5lac9 when! loo9e" thro7gh! whichwhen loo9e" 7pon appeare" white! an" on the contrary@ an" so those! which in one case exhi5ite"5l7e! "i" in the other case exhi5it re"@ an" the li9e of the other colo7rs,

O5s, '1, >etting the o5Fect0glass a little at their e"ges! the water crept in slowly 5etween the$! an"the circles there5y 5eca$e less! an" the colo7rs $ore faint@ inso$7ch that! as the water creptalong! one half of the$! at which it first arri+e"! wo7l" appear 5ro9en off fro$ the other half! an"contracte" into a less roo$, %y $eas7ring the$ I fo7n" the proportion of their "ia$eters to the"ia$eters of the li9e circles $a"e 5y air! to 5e a5o7t se+en to eight@ ;.((< an" conseB7ently theinter+als of the glasses at li9e circles! ca7se" 5y these two $e"i7$s! water an" air! are as a5o7t

three to fo7r, Perhaps it $ay 5e a general r7le! that if any other $e"i7$! $ore or less "ense thanwater! 5e co$presse" 5etween the glasses! their inter+al at the rings! ca7se" there5y! will 5e totheir inter+al! ca7se" 5y interFacent air! as the sines are! which $eas7re the refraction $a"e o7t of that $e"i7$ into air,

O5s, '', >hen the water was 5etween the glasses! if I presse" the 7pper glass +ario7sly at itse"ges to $a9e the rings $o+e ni$5ly fro$ one place to another! a little 5right spot wo7l"i$$e"iately follow the center of the$! which! 7pon creeping in of the a$5ient water into that place!wo7l" presently +anish, Its appearance was s7ch! as interFacent air wo7l" ha+e ca7se"! an" itexhi5ite" the sa$e colo7rs@ 57t it was not air! for where any aereal 5755les were in the water theywo7l" not +anish, #he reflection $7st rather ha+e 5een ca7se" 5y a s75tiler $e"i7$! which co7l"rece"e thro7gh the glass at the creeping in of the water,

O5s, '., #hese o5ser+ations were $a"e in the open air, %7t f7rther! to exa$ine the effects of colo7re" light falling on the glasses! I "ar9ene" the roo$! an" +iewe" the$ 5y reflection of thecolo7rs of a pris$ cast on a sheet of white paper@ an" 5y this $eans the rings 5eca$e "istincter!an" +isi5le to a far greater n7$5er than in the open air,

I ha+e seen $ore than twenty of the$! whereas in the open air I co7l" not "iscern a5o+e eight or nine,

O5s, '-, Appointing an assistant to $o+e the pris$ to an" fro a5o7t its axis! that all its colo7rs$ight s7ccessi+ely fall on the sa$e place of the paper! an" 5e reflecte" fro$ the circles to $y eyewhilst I hel" it i$$o+ea5le@ I fo7n" the circles! which the re" light $a"e! to 5e $anifestly 5igger than those! which were $a"e 5y the 5l7e an" +iolet@ an" it was +ery pleasant to see the$ gra"7ally

swell or contract! accor"ingly as the colo7r of the light was change", #he inter+al of the glass at anyof the rings! when they were $a"e 5y the 7t$ost re" light! was to their inter+al at the sa$e ring!when $a"e 5y the 7t$ost +iolet! greater than three to two! an" less than thirteen to eight, %y the



sa$e in all o5liB7ities of $y eye! 7nless when two pris$s were $a"e 7se of instea" of the o5Fect0glasses: for then! at a certain great o5liB7ity! the rings $a"e 5y the se+eral colo7rs see$e" eB7al@an"! at a greater o5liB7ity! those $a"e 5y the +iolet wo7l" 5e greater than the sa$e rings $a"e 5ythe re",

O5s, '/, >hile the pris$ was t7rne" a5o7t 7nifor$ly! the contraction or "ilation of a ring $a"e 5yalll the se+eral colo7rs of the pris$ s7ccessi+ely reflecte" fro$ the o5Fect0glasses! was swiftest in

the re"! slowest in the +iolet! an" in inter$e"iate colo7rs it ha" inter$e"iate "egrees of celerity,Co$paring the extent! which each colo7r o5taine" 5y this contraction or Z"ilation![ I fo7n"! ;.(<that the 5l7e was sensi5ly $ore exten"e" than the +iolet! the yellow than the 5l7e! an" the re" thanthe yellow, An"! to $a9e a F7ster esti$ation of their proportions! I o5ser+e"! that the extent of there" was al$ost "o75le to that of the +iolet! an" that the light was of a $i""le colo7r 5etween yellowan" green at that inter+al of the glasses! which was an arith$etical $ean 5etween the twoextre$es@ contrary to what happens in the colo7rs $a"e 5y the refraction of a pris$! where the re"is $ost contracte"! the +iolet $ost expan"e"! an" in the $i"st of the$ is the confine of green an"5l7e,

O5s, '), #hese rings were not of +ario7s colo7rs! li9e those in the open air! 57t appeare" all o+er of that pris$atic colo7r only! with which they were ill7$inate": an"! 5y proFecting the pris$atic colo7rsi$$e"iately 7pon the glasses! I fo7n"! that the light! which fell on the "ar9 spaces! which were

5etween the colo7re" rings! was trans$itte" thro7gh the glasses witho7t any +ariation of colo7r,6or! on a white paper place" 5ehin"! it wo7l" paint rings of the sa$e colo7r with those! which werereflecte"! an" of the 5igness of their inter$e"iate spaces, An" fro$ hence the origin of these ringsis $anifest! na$ely! that the aereal inter+al of the glasses! accor"ing to its +ario7s thic9ness! is"ispose" in so$e places to reflect! an" in others to trans$it! the light of any colo7r@ an"! in thesa$e place to reflect one colo7r! where it trans$its another,

#hese o5ser+ations so well please" the Society! that they or"ere" 8r, OL2EN%UR to "esire 8r,NE>#ON to per$it the$ to 5e p75lishe"! together with the rest@ which! they pres7$e"! "i"correspon" with those! that ha" 5een now rea" to the$,

%esi"es! there was rea" a passage of 8r, NE>#ONS letter to 8r, OL2EN%UR! of .' 2ece$5er!

'()! stating the "ifference 5etween his hypothesis an" that of 8r, HOOE, >hich passage was asfollows:

As for 8r, HOOES insin7ation! that the s7$ of the hypothesis I sent yo7 ha" 5een "eli+ere" 5yhi$ in his 8icrography! I nee" not $7ch 5e concerne" at the li5erty he ta9es in that 9in": yet!5eca7se yo7 thin9 it $ay "o well! if I state the "ifference I ta9e to 5e 5etween the$! I shall "o it as5riefly as I can! an" that the rather! that I $ay a+oi" the fa+o7r of ha+ing "one any thing7nF7stifia5le or 7nhan"so$e towar"s 8r, HOOE, %7t! for this en"! I $7st first &to see what is his*cast o7t what he has 5orrowe" fro$ 2ES CAR#ES! or others! +i, that there is an ?thereal$e"i7$@ that light is the action of this $e"i7$@ that this $e"i7$ is less i$plicate" in the parts of soli" 5o"ies! an" so $o+es $ore freely in the$! an" trans$its light $ore rea"ily thro7gh the$! an"that after s7ch a $anner! as to accelerate the rays in a certain proportion@ that refraction arises fro$this acceleration! an" has sines proportional@ that light is at first 7nifor$@ that its colo7rs are so$e"ist7r5ance or new $o"ification of its rays 5y refraction or reflection@ that the colo7rs of a pris$ are$a"e 5y $eans of the B7iescent $e"i7$! accelerating so$e $otion of the rays on one si"e! wherere" appears! an" retar"ing it on ;.(=< the other si"e! where 5l7e appears@ an"! that there are 57tthese two original colo7rs! or colo7r0$a9ing $o"ifications of light! which 5y their +ario7s "egrees!or! as 8r, HOOE calls it! "il7tings! pro"7ce all inter$e"iate ones, #his reFecte"! the re$ain"er of his hypothesis is! that he has change" 2ES CAR#ESS pressing or progressi+e $otion of the$e"i7$ to a +i5rating one! the rotation of the glo57li to the o5ligation of p7lses! an" theaccelerating their rotation on the one han"! an" retar"ing it on the other! 5y the B7iescent $e"i7$!to pro"7ce colo7rs! to the li9e action of the $e"i7$ on the two en"s of his p7lses for the sa$e en", An" ha+ing th7s far $o"ifie" his 5y the Cartesian hypothesis! he has exten"e" it f7rther! toexplicate the ph?no$ena of thin plates! an" a""e" another explication of the colo7rs of nat7ral5o"ies! fl7i" an" soli",

#his! I thin9! is in short the s7$ of his hypothesis@ an" in all this I ha+e nothing in co$$on with hi$!57t the s7pposition! that ?ther is a s7scepti5le $e"i7$ of +i5rations! of which s7pposition I $a9e a



"ifference as is 5etween hi$ an" 2ES CAR#ES, %7t 5esi"es this! the $anner of refraction an"reflection! an" the nat7re an" pro"7ction of colo7rs in all cases &which ta9es 7p the 5o"y of $y"isco7rse* I explain +ery "ifferently fro$ hi$@ an" e+en in the colo7rs of thin transparents75stances! I explain e+ery thing after a way so "iffering fro$ hi$! that the experi$ents I gro7n" $y"isco7rse on! "estroy all he has sai" a5o7t the$@ an" the two $ain experi$ents! witho7t which the$anner of the pro"7ction of those colo7rs is not to 5e fo7n" o7t! were not only 7n9nown to hi$!when he wrote his 8icrography! 57t e+en last spring! as I 7n"erstoo"! in $entioning the$ to hi$,

#his therefore is the s7$ of what is co$$on to 7s! that ?ther $ay +i5rate@ an" so! if he thin9s fit to7se that notion of colo7rs! arising fro$ the +ario7s 5igness of p7lses &witho7t which his hypothesiswill "o nothing* his will 5orrow as $7ch fro$ $y answer to his o5Fections! as that I sen" yo7 "oesfro$ his 8icrography,

%7t! it $ay 5e! he $eans! that I ha+e $a"e 7se of his o5ser+ations! an" of so$e I "i"@ as! that of the inflection of rays! for which I B7ote" hi$@ that of opacity! arising fro$ the interstices of the partsof 5o"ies! which I insist not on@ an" that of plate" 5o"ies exhi5iting colo7rs! a ph?no$enon! for thenotice of which I than9 hi$, %7t he left $e to fin" o7t an" $a9e s7ch experi$ents a5o7t it! as $ightinfor$ $e of the $anner of the pro"7ction of those colo7rs! to gro7n" an hypothesis on@ he ha+inggi+en no f7rther insight to it than this! that the colo7r "epen"e" on so$e certain thic9ness of theplate@ tho7gh what that thic9ness was at e+ery colo7r! he confesses in his 8icrography! he ha"atte$pte" in +ain to learn@ an" therefore! seeing I was left to $eas7re it $yself! I s7ppose he will

allow $e to $a9e 7se of what I too9 the pains to fin" o7t, An" this I hope $ay +in"icate $e fro$what 8r, HOOE has 5een please" to charge $e with,

#he rea"ing of the rest of 8r, NE>#ONS "isco7rse was referre" to the next $eeting,

;.1<

an7ary .(, 8r, OL2EN%UR pro"7ce" fro$ his highness prince RUPER# a piece of $ar5le!ha+ing se+eral pict7res of 5oys an" trees painte" 7pon it in s7ch a $anner! that all the o7t0lines of the pict7res were exactly "efine" witho7t any flowing of the colo7rs a5roa"! an" the colo7rs fixe" 5ythe fire! an" afterwar"s so polishe"! that they wo7l" 5e per$anent! an" last as long as the $ar5le,

#his was ac9nowle"ge" 5y the $e$5ers to 5e a +ery great i$pro+e$ent of what ha" 5een "one atOxfor" 5y a certain stone0c7tter there@ an" that all! that ha" 5een perfor$e" 5efore in this art! wasnot co$para5le to this "egree of i$pro+e$ent,

8r, HOOE re$ar9e"! that he concei+e"! that there were 57t two colo7rs in this piece@ an" that heha" a $etho" of "oing it with $ost colo7rs! an" to paint with the$ 7pon $ar5le al$ost as c7rio7slyas with a pencil,

8r, NE>#ONS letter of an7ary .)! '()c ! in which he ac9nowle"ge" the fa+o7r of the Societyin their 9in" acceptance of his late papers@ an" "eclare"! that he 9new not how to "eny any thing!which they "esire" sho7l" 5e "one: 57t he reB7este"! that the printing of his o5ser+ations a5o7tcolo7rs $ight 5e s7spen"e" for a ti$e! 5eca7se he ha" so$e tho7ghts of writing s7ch another setof o5ser+ations for "eter$ining the $anner of the pro"7ction of colo7rs 5y the pris$: whicho5ser+ations! he sai"! o7ght to prece"e those now in the Societys possession! an" wo7l" 5e $ostproper to 5e Foine" with the$,

#here was also rea" a letter of 8r, PASCALL of So$ersetshire to 8r, AU%RE4! "ate" ' an7ary'()! containing so$e nat7ral o5ser+ations of that co7nty! +i, concerning the nat7re of the lea"0$ines in 8en"ip0Hills@ a well rese$5ling the s7lph7r0well near the Spaw in 4or9shire@ a springpetrifying far $ore than the "ropping0well at nares5oro7gh in the north@ the $otion of so$e7n"ergro7n" waters in the parishes of QOLAN2E! for$erly reco+ere" fro$ the sea Dc,

It was or"ere"! that the rea"ing of 8r, NE>#ONS o5ser+ations a5o7t colo7rs 5e contin7e" at thenext $eeting,

6e5r7ary -, #here was presente" fro$ 2r, >ALLIS his e"ition of ARCHI8E2ESS Arenari7s! with anew translation of his an" notes! printe" at Oxfor"! in '(,



#he rea"ing of 8r, NE>#ONS o5ser+ations on colo7rs was contin7e"! +i, that part! wherein heexplains 5y the si$plest of colo7rs the $ost reco$po7n"e"@ as follows:

O5s, ', #he sB7ares of the "ia$eters of these rings! $a"e 5y pris$atic colo7r! were in arith$eticalprogression! as in the fifth o5ser+ation, An" the "ia$eter of the sixth circle! when $a"e 5y theyellow! an" +iewe" al$ost perpen"ic7larly! was a5o7t )'11 parts of an inch! agreea5le to thesixth o5ser+ation,

;.'<

#he prece"ent o5ser+ations were $a"e with a rarer thin $e"i7$ ter$inate" 5y a "enser! s7ch aswas air or water co$presse" 5etwixt two glasses, In those! that follow! are set "own theappearances of a "enser $e"i7$ thinne" within a rarer@ s7ch as are the plates of 87sco+y0glass!5755les of water! an" so$e others thin s75stances ter$inate" on all si"es with air,

O5s, '(, If a 5755le 5e 5lown with water! first $a"e tenacio7s 5y "issol+ing a little soap in it! it is aco$$on o5ser+ation! that after a while it will appear tinge" with a great +ariety of colo7rs, #o"efen" these 5755les fro$ 5eing agitate" 5y the external air &where5y their colo7rs are irreg7larly$o+e" one a$ong another! so that no acc7rate o5ser+ation can 5e $a"e of the$* as soon as I ha"

5lown any of the$! I co+ere" it with a clear glass! an" 5y that $eans its colo7rs e$erge" in a +eryreg7lar or"er! li9e so $any concentric rings inco$passing the top of the 5755le, An" as the 5755legrew thinner 5y the contin7al s75si"ing of the water! these rings "ilate" slowly! an" o+ersprea" thewhole 5755le! "escen"ing in or"er to the 5otto$ of it! where they +anishe" s7ccessi+ely, In the$ean while! after all the colo7rs were e$erge" at the top! there grew in the center of the rings as$all! ro7n"! 5lac9 spot! li9e that in the first o5ser+ation! which contin7ally "ilate" itself! till it5eca$e so$eti$es $ore than one half or three fo7rths of an inch in 5rea"th! 5efore the 5755le5ro9e, At first I tho7ght there ha" 5een no light reflecte" fro$ the water in that place@ 57t o5ser+ingit $ore c7rio7sly! I saw within it se+eral s$aller! ro7n" spots! which appeare" $7ch 5lac9er an""ar9er than the rest! where5y I 9new! that there was so$e reflection at the other places! which werenot so "ar9 as those spots, An" 5y f7rther trial I fo7n"! that I co7l" see the i$ages &as of a can"le or the s7n* +ery faintly reflecte"! not only fro$ the great 5lac9 spot! 57t also fro$ the little "ar9er

spots! which were within it,

%esi"es the aforesai" colo7re" rings! there wo7l" often appear s$all spots of colo7rs ascen"ingan" "escen"ing 7p an" "own the si"e of the 5755le! 5y reason of so$e ineB7alities in thes75si"ing of the water@ an" so$eti$es s$all 5lac9 spots generate" at the si"es! wo7l" ascen" 7pto the larger 5lac9 spot at the top of the 5755le! an" 7nite with it,

O5s, ', %eca7se the colo7rs of these 5755les were $ore exten"e" an" li+ely than those of air thinne" 5etween two glasses! an" so $ore easy to 5e "isting7ishe"! I shall here gi+e yo7 a f7rther "escription of their or"er! as they were o5ser+e" in +iewing the$ 5y reflection of the s9ies! when of a white colo7r! whilst a 5lac9 s75stance was place" 5ehin" the 5755le: an" they were these@ re"!5l7e! re"! 5l7e@ re"! 5l7e@ re"! green@ re"! yellow@ green! 5l7e! p7rple@ re"! yellow! green! 5l7e!+iolet@ re"! yellow! white! 5l7e! 5lac9,

#he three first s7ccessions of re" an" 5l7e were +ery "il7te an" "irty! especially the first! where there" see$e" in a $anner to 5e white, A$ongst these there was scarcely any other colo7r sensi5le!only the 5l7es &an" principally the secon" 5l7e* incline" a little to green,

;..<

#he fo7rth re" was also "il7te an" "irty! 57t not so $7ch as the for$er three: after that s7ccee"e"little or no yellow! 57t a copio7s green! which at first was incline" a little to yellow! an" then 5eca$ea pretty 5ris9 an" goo" willow green! an" afterwar"s change" to a 5l7eish colo7r@ 57t theres7cce"e" neither 5l7e nor +iolet,

#he fifth re" at first was +ery $7ch incline" to p7rple! an" afterwar"s 5eca$e $ore 5right an" 5ris9!57t yet not +ery p7re, #his was s7ccee"e" with a +ery 5right an" intense yellow! which was 57t little



"eep! an" li+ely! than the for$er green, After that followe" an excellent 5l7e of a 5right s9y colo7r@an" then a p7rple! which was less in B7antity than the 5l7e! an" $7ch incline" to re",

#he sixth re" was at first of a +ery fair an" li+ely scarlet! an" soon after of a 5righter colo7r! 5eing+ery p7re an" 5ris9! an" the 5est of all the re"s, #hen! after a li+ely orange! followe" an intense!5right! an" copio7s yellow! which was also the 5est of all the yellows@ an" this change"! first to agreenish yellow! an" then to a greenish 5l7e@ 57t the green 5etween the yellow an" 5l7e was +ery

little an" "il7te! see$ing rather a greenish white than a green, #he 5l7e! which s7ccee"e"! 5eca$e+ery goo"! an" of a fair! 5right! s9y colo7r@ 57t yet so$ething inferior to the for$er 5l7e: an" the+iolet was intense an" "eep! with little or no re"ness in it! an" less in B7antity than the 5l7e,

In the last re" appeare" a tinct7re of scarlet next the +iolet! which soon change" to a 5righter colo7r! inclining to an orange: an" the yellow! which followe"! was at first pretty goo" an" li+ely! 57tafterwar"s it grew $ore an" $ore "il7te! 7ntil 5y "egrees it en"e" in perfect whiteness: an" thiswhiteness! if the water was +ery tenacio7s an" well te$pere"! wo7l" slowly sprea" an" "ilate itself o+er the greatest part of the 5755le! contin7ally growing paler at the top! where at length it wo7l"crac9! an" those crac9s! as they "ilate"! wo7l" appear of a pretty goo"! 57t yet o5sc7re an" "ar9!s9y0colo7r@ the white 5etween the 5l7e spots "i$inishing! 7ntil it rese$5le" the threa"s of anirreg7lar net0wor9! an" soon after +anishe" an" left all the 7pper part of the 5755le of the sai" "ar95l7e colo7r@ an" this colo7r! after the aforesai" $anner! "ilate" itself "ownwar"s! 7ntil so$eti$es it

hath o+ersprea" the whole 5755le, In the $ean while! at the top! which was of a "ar9er 5l7e thanthe 5otto$! an" appeare" also of $any ro7n" 5l7e spots! so$ething "ar9er than the rest! therewo7l" e$erge one or $ore +ery 5lac9 spots! an" within those! other spots of an intenser 5lac9ness!which I $entione" in the for$er o5ser+ation@ an" those contin7ally "ilate" the$sel+es 7ntil the5755le 5ro9e,

If the water was not +ery tenacio7s! the 5lac9 spots wo7l" 5rea9 forth in the white! witho7t anysensi5le inter+ention of the 5l7e: an" so$eti$es they wo7l" 5rea9 forth within the prece"ent yellow!or re"! or perhaps within the 5l7e of the secon" or"er! 5efore the inter$e"iate colo7rs ha" ti$e to"isplay the$sel+es,

;.-<

%y this "escription yo7 $ay percei+e! how great an affinity these colo7rs ha+e with those of air!"escri5e" in the fo7rth o5ser+ation! altho7gh set "own in a contrary or"er! 5y reason that they5egin to appear! when the 5755le is thic9est! an" are $ost con+eniently rec9one" fro$ the lowestan" thic9est part of the 5755le 7pwar"s,

O5s, '=, 3iewing! at se+eral o5liB7e positions of $y eye! the rings of colo7rs e$erging on the top of the 5755le! I fo7n"! that they were sensi5ly "ilate" 5y increasing the o5liB7ity! 57t yet not so $7ch5y far! as those $a"e 5y thinne" air in the se+enth o5ser+ation, 6or there they "isten"e" so $7ch!as! when +iewe" $ost o5liB7ely! to arri+e at a part of the plate $ore than twel+e lines thic9er thanthat where they appeare"! when +iewe" perpen"ic7larly@ whereas in this case the thic9ness of thewater! at which they arri+e" when +iewe" $ost o5liB7ely! was! to that thic9ness! which exhi5ite"the$ 5y perpen"ic7lar rays! so$ething less than eight to fi+e, %y the 5est of $y o5ser+ations! itwas 5etween fifteen an" fifteen an" a half to ten! an increase a5o7t twenty0fo7r ti$es less than inthe other case,

So$eti$es the 5755le wo7l" 5eco$e of an 7nifor$ thic9ness all o+er! except at the top of it near the 5lac9 spot! as I 9new! 5eca7se it wo7l" exhi5it the sa$e appearance of colo7rs in all positionsof the eye@ an" then the colo7rs! which were seen at its apparent circ7$ference 5y the o5liB7estrays! wo7l" 5e "ifferent fro$ those! that were seen in other places 5y rays less o5liB7e to it, An""i+ers spectators $ight see the sa$e part of it 5y "iffering colo7rs! 5y +iewing it at +ery "ifferingo5liB7ities, Now! o5ser+ing how $7ch the colo7rs at the sa$e place of the 5755le! or at "i+ersplaces of eB7al thic9ness! were +arie" 5y the se+eral o5liB7ities of the rays! 5y assistance of thefo7rth! fo7rteenth! sixteenth! an" eighteenth onser+ations! as they are hereafter explaine"! I

collecte" the thic9ness of the water! reB7isite to exhi5it any one the sa$e colo7r at se+eralo5liB7ities! to 5e +ery nearly in the proportion expresse" in this ta5le,



Inci"ence Refraction #hic9nesson the water into the water of the water "egr, $in, "egr, $in,

11 11 11 11 '1') 11 '' '' '1 1/4

-1 11 .. '1 '1 4/5 /) 11 -. .1 '' 4/5

1 11 /1 -1 '-() 11 / .) '/ 1/2

=1 11 / -) ') ')

In the two first col7$ns are expresse" the o5liB7ities of the rays to the s7perficies of the water@ thatis! their angles of inci"ence an" refraction@ where! I s7ppose! that the lines! which $eas7re the$!are in ro7n" n7$5ers! as three to fo7r! tho7gh pro5a5ly the "issol7tion of soap in the water $ay alittle alter its refracti+e +irt7e, In the thir" col7$n the thic9ness of the 5755le! ;./< at which anyone colo7r is exhi5ite" in those se+eral o5liB7ities! is exprest in parts! of which ten constit7te thatthic9ness! when the rays are perpen"ic7lar,

I ha+e so$eti$es o5ser+e" of the colo7rs! which arise on polishe" steel 5y heating it! or on 5ell$etal an" so$e other $etalline s75stances! when $elte" an" po7re" on the gro7n"! where it $aycool in the open air! that they ha+e! li9e those of water05755les! 5een a little change" 5y +iewingthe$ at "i+ers o5liB7ities@ an" partic7larly! that a "eep 5l7e or +iolet! when +iewe" +ery o5liB7ely!hath 5een change" to a "eep re", %7t the changes of these colo7rs are not so sensi5le as of those$a"e 5y water@ for the scoria! or +itrifie" part of the $etal! which $ost $etals! when heate" or $elte"! contin7ally protr7"e to their s7rface! where! 5y co+ering the$ in for$ of a thin glassy s9in! itca7ses these colo7rs! is $7ch "enser than water! an" I fin"! that the change $a"e 5y theo5liB7ation of the eye! is least in colo7rs of the "ensest thin s75stances,

O5s, .1, As in the ninth o5ser+ation! so here! the 5755le! 5y trans$itte" light appeare" of a

contrary colo7r to that! which it exhi5ite" 5y reflection,#h7s! when the 5755les! 5eing loo9e" on 5ythe light of the clo7"s reflecte" fro$ it! see$e" re" at its apparent circ7$ference! if the clo7"s atthe sa$e ti$e! or +ery s7""enly! were +iewe" thro7gh it! the colo7r at its circ7$ference wo7l" 5e5l7e, An"! on the contrary! when 5y reflecte" light it appeare" 5l7e! it wo7l" appear re" 5ytrans$itte" light,

O5s, .', %y wetting plates of 87sco+y0glass! whose thinness $a"e the li9e colo7rs appear! thecolo7rs 5eca$e $ore faint! especially 5y wetting the plates on that si"e opposite the eye@ 57t Ico7l" not percei+e any +ariation of their species, So that the thic9ness of a plate reB7isite topro"7ce any colo7r! "epen"s only on the "ensity of the plate! an" not of the a$5ient $e"i7$, An"hence! 5y the tenth an" sixteenth o5ser+ations! $ay 5e 9nown the thic9ness of 5755les of water or plates of 87sco+y0glass! or of any other s75stances! which they ha+e at any colo7r pro"7ce" 5ythe$,

O5s, .., A thin transparent 5o"y! which is "enser than its a$5ient $e"i7$! exhi5its $ore 5ris9 an"+i+i" colo7rs than that! which is so $7ch rarer@ as I ha+e partic7larly o5ser+e" in air an" glass: for!5lowing glass +ery thin at a la$p f7rnace! those plates enco$passe" with air "i" exhi5it colo7rs$7ch $ore +i+i" than those of air $a"e thin 5etween two glasses,

O5s, .-, Co$paring the B7antity of light reflecte" fro$ the se+eral rings! I fo7n" it was $ostcopio7s fro$ the first or in$ost! an" in the exterior rings 5eca$e gra"7ally less an" less, Also thewhiteness of the first ring was stronger than that reflecte" fro$ those parts of the thinne" $e"i7$!which were witho7t the rings! as I co7l" $anifestly percei+e 5y +iewing at "istance the rings $a"e5y the two o5Fect glasses@ or 5y co$paring two 5755les of water 5lown at "istant ti$es! in the firstof which the whiteness appeare"! which s7ccee"e" the colo7rs! an" the whiteness! which prece"e"

the$! in the other,



O5s, ./, >hen the two o5Fect0glasses were lai" 7pon one another! so as to $a9e the rings of colo7rs appear! tho7gh with $y na9e" eye I co7l" not "iscern a5o+e eight or nine of those rings!yet! 5y +iewing the$ thro7gh a pris$! I ha+e seen a far greater $7ltit7"e! inso$7ch! that I co7l"n7$5er $ore than forty! 5esi"es $any others! that were so +ery s$all an" close together! that Ico7l" not 9eep $y eye so stea"y on the$ se+erally as to n7$5er the$: 57t 5y their extent I ha+eso$eti$es esti$ate" the$ to 5e $ore than a h7n"re", An"! I 5elie+e! the experi$ent $ay 5e

i$pro+e" to the "isco+ery of far greater n7$5ers@ for they see$ to 5e really 7nli$ite"! tho7gh+isi5le only so far as they can 5e separate" 5y the refraction! as I shall hereafter explain,

%7t it was 57t one si"e of these rings! na$ely! that! towar"s which the refraction was $a"e! which5y that refraction was ren"ere" "istinct@ an" the other si"e 5eca$e $ore conf7se" than to thena9e" eye! inso$7ch that there I co7l" not "iscern a5o+e one or two! an" so$eti$es none of thoserings! of which I co7l" "iscern eight or nine with $y na9e" eye, An" their seg$ents! or arcs! whichon the other si"e appeare" so n7$ero7s! for the $ost part excee"e" not the thir" part of a circle, If the refraction was +ery great! or the pris$s +ery "istant fro$ the o5Fect0glasses! the $i""le part of those arcs 5eca$e also conf7se"! so as to "isappear an" constit7e an e+en whiteness! whilst oneither si"e their en"s! as also the whole arcs farthest fro$ the center! 5eca$e "istincter than5efore! appearing in the for$ yo7 see the$ here "esigne",

#he arcs! where they see$e" "istinctest! were only white an" 5lac9 s7ccessi+ely! witho7t any other colo7rs inter$ixe", %7t in other places there appeare" colo7rs whose or"er was in+erte" 5y therefraction! in s7ch $anner! that! if I first hel" the pris$ +ery near the o5Fect0glasses! an" thengra"7ally re$o+e" it farther off towar"s $y eye! the colo7rs of the secon"! thir"! fo7rth! an"following rings shr7n9 towar"s the white! that e$erge" 5etween the$! 7ntil they wholly +anishe"into it at the $i""le of the arcs! an" afterwar"s e$erge" again in a contrary or"er: 57t at the en" of the arcs they retaine" their or"er 7nchange",

I ha+e so$eti$es so lai" one o5Fect0glass 7pon the other! that! to the na9e" eye! they ha+e all o+er see$e" 7nifor$ly white! witho7t the least appearance of any of the colo7re" rings@ an" yet! 5y+iewing the$ thro7gh a pris$! great $7ltit7"es of those rings ha+e "isco+ere" the$sel+es, An"! inli9e $anner! plates of 87sco+y glass! an" 5755les of glass 5lown at a la$p f7rnace! which were not

so thin! as to exhi5it any colo7rs to the na9e" eye! ha+e thro7gh the pris$ exhi5ite" a great +arietyof the$! range" irreg7larly 7p an" "own! in the for$ of wa+es, An" so 5755les of water! 5efore they5egan to exhi5it their colo7rs to the na9e" eye of a 5y0stan"er! ha+e appeare"! thro7gh a pris$!gir"e" a5o7t with $any parallel an" horiontal rings@ to pro"7ce which effect! it was necessary tohol" the pris$ parallel! or +ery nearly parallel! to the horion! an" to "ispose it so! that the rays$ight 5e refracte" 7pwar"s,

;.<

Ha+ing gi+en $y o5ser+ations of these colo7rs! 5efore I $a9e 7se of the$ to 7nfol" the ca7ses of the colo7rs of nat7ral 5o"ies! it is con+enient! that! 5y the si$plest of the$! I first explain the $oreco$po7n"e"@ s7ch as are the secon"! thir"! fo7rth! ninth! twelfth! eighteenth! twentieth! an" twenty0fo7rth,

An" first! to show how the colo7rs in the fo7rth an" eighteenth o5ser+ations are pro"7ce"! let there5e ta9en in any right line the lengths 4 Q! 4 A! an" 4 H! in proportion as fo7r! nine! an" fo7rteen@an" 5etween Q A an" Q H ele+en $ean proportionals! of which let Q % 5e the secon"! Q C the thir"!Q 2 the fifth! Q E the se+enth! Q 6 the ninth! an" Q the tenth, An" at the points A! %! C! 2! E! 6! !H! let perpen"ic7lars A \! % ! Dc, 5e erecte"! 5y whose inter+als! the extent of the se+eral colo7rsset 7n"erneath against the$! is to 5e represente", #hen "i+i"e the line A \ in s7ch proportion asthe n7$5ers '! .! -@ )! ! (@ =! '1! ''! Dc, set at the point of "i+ision "enote, An" thro7gh those"i+isions fro$ 4 "raw lines ' I! . ! - L@ ) $! n! ( o! Dc,

Now! if A . 5e s7ppose" to represent the thic9ness of any thin transparent 5o"y! at which the

7t$ost +iolet is $ost copio7sly reflecte" in the first ring or series of colo7rs! then! 5y the thirteentho5ser+ation! H will represent its thic9ness! at which the 7t$ost re" is $ost copio7sly reflecte" inthe sa$e series, Also! 5y the fifth an" sixteenth o5ser+ations! A ! an" H n! will "enote the



on, An" the thic9ness! at which any of the inter$e"iate colo7rs are reflecte" $ost copio7sly! will!accor"ing to the fo7rteenth o5ser+ation! 5e "efine" 5y the in0 ;.(< ter$e"iate parts of the lines .! n! Dc, against which the na$es of those colo7rs are written 5elow,

%7t farther! to "efine the latit7"e of these colo7rs in each ring or series! let A ' "esign the leastthic9ness! an" A - the greatest thic9ness! at which the extre$e +iolet in the first series is reflecte"@an" let H I an" H L "esign the li9e li$it for the extre$e re"! an" the inter$e"iate colo7rs 5e li$ite"

5y the inter$e"iate parts of the lines! ' I an" - L@ against which the na$es of those colo7rs arewritten, An" in the secon" series! let those li$its 5e the lines ) 8 an" ( O@ an" so on: 57t yet withthis ca7tion! that the reflections 5e s7ppose" strongest at the inter$e"iate spaces! . ! N! '1 R!Dc, an" to "ecrease gra"7ally towar"s these li$its! ' I! - L@ ) 8! ( O! Dc, on either si"e! where yo7$7st not concei+e the$ to 5e precisely li$ite"! 57t to "ecay in"efinitely, An" whereas I ha+e"esigne" the sa$e latit7"e to e+ery series! I "i" it! 5eca7se! altho7gh the colo7rs in the first seriessee$ to 5e a little 5roa"er than the rest! 5y reason of a stronger reflection there@ yet that ineB7alityis so insensi5le as scarcely to 5e "eter$ine" 5y o5ser+ation,

Now! accor"ing to this "escription! concei+ing! that the rays! in which se+eral colo7rs in here! are 5yt7rns reflecte" at the space ' ! - L! ) 8! O (! = P! R ''! Dc, an" trans$itte" at the spaces A H I '!- L! 8 )! ( O! P =! Dc, it is easy to 9now what colo7r in the open air $7st 5e exhi5ite" at anythic9ness of a transparent thin 5o"y, 6or! if a r7ler 5e applie" parallel to A H! at that "istance fro$ it

5y which the thic9ness of the 5o"y is represente"! the alternate spaces ' I! L -! ) 8! O (! Dc, whichit crosseth! will "enote the reflecte" original colo7rs! of which the colo7r exhi5ite" in the open air isco$po7n"e", #h7s! if the constit7tion of the green in the thir" series of colo7rs 5e "esire"@ applythe r7ler! as yo7 see! at M V X ! an" 5y its passing thro7gh so$e of the 5l7e at M! an" yellow at X!as well as thro7gh the green V! yo7 $ay concl7"e! that green! exhi5ite" at that thic9ness of the5o"y! is principally constit7te" of original green! 57t not witho7t a $ixt7re of so$e 5l7e an" yellow,%y this $eans yo7 $ay 9now! how the colo7rs fro$ the center of the rings o7twar"s o7ght tos7ccee" in or"er! as they were "escri5e" in the fo7rth an" eighteenth o5ser+ations: for! if yo7 $o+ethe r7ler gra"7ally fro$ A H thro7gh all "istances! ha+ing past o+er the first space! which "enoteslittle or no reflection to 5e $a"e 5y thinnest s75stances! it will first arri+e at '! the +iolet! an" then+ery B7ic9ly at the 5l7e an" green! which! together with that +iolet co$po7n"e" 5l7e! an" then atthe yellow an" re"! 5y whose f7rther a""ition! that 5l7e is con+erte" into whiteness! which

whiteness contin7es "7ring the transit fro$ I to -@ an" after that! 5y the s7ccessi+e "eficience of itsco$ponent colo7rs! t7rns first to co$po7n" yellow! an" then to re"! an" last of all the re" ceasethat L #hen 5egin the colo7rs of the secon" series! which s7ccee" in or"er 5etween ) an" O! an" are$ore li+ely than 5efore! 5eca7se $ore expan"e" an" se+ere", An"! for the sa$e reason! instea" of the for$er white! there interce"es 5etween the 5l7e an" yellow a $ixt7re of orange! yellow! green!5l7e an" in"ico! all which together o7ght to exhi5it a "il7te an" i$perfect green, So the colo7rs of the thir" series all s7ccee" in ;.< or"er@ first the +iolet! which a little interferes with the re" of thesecon" or"er! an" is there5y incline" to a re"ish p7rple@ then the 5l7e an" green! which are less$ixe" with other colo7rs! an" conseB7ently $ore li+ely than 5efore! especially the green, #henfollows the yellow! so$e of which towar"s the green is "istinct an" goo"@ 57t that part of it towar"sthe s7ccee"ing re"! as also that re"! is $ixe" with the +iolet an" 5l7e of the fo7rth series! where5y+ario7s "egrees of re"! +ery $7ch inclining to p7rple! are co$po7n"e", #he +iolet an" 5l7e! whichsho7l" s7ccee" this re"! 5eing $ixe" with! an" hi""en in it! there s7ccee"s a green@ an" this at firstis $7ch incline" to 5l7e! 57t soon "eco$es a goo" green@ the only 7n$ixe" an" li+ely colo7r in thisfo7rth series: for as it +erges towar"s the yellow! it 5egins to interfere with the colo7rs of the fifthseries! 5y whose $ixt7re the s7ccee"ing yellow an" re" are +ery $7ch "il7te"! an" $a"e "irty!especially the yellow! which 5eing the wea9er colo7r! is scarce a5le to shew itself, After this these+eral series interfere $ore an" $ore! an" their colo7rs 5eco$e $ore an" $ore inter$ixe"! tillafter three or fo7r re+ol7tions &in which the re" an" 5l7e pre"o$inate 5y t7rns* all sorts of colo7rsare in all places pretty eB7ally 5len"e"! an" co$po7n" one e+en whiteness,

An" since! 5y the fifteenth o5ser+ation! the rays in"7e" with one colo7r are trans$itte"! wherethose of another colo7r are reflecte"! the reason of the colo7rs $a"e 5y the trans$itte" light! in theninth an" twentieth o5ser+ations! is also fro$ hence e+i"ent,

If not only the or"er an" species of these colo7rs! 57t also the precise thic9ness of the plate! or thin5o"y! at which they are exhi5ite"! 5e "esire" in parts of an inch! that $ay 5e also perfor$e" 5yassistance of the sixth or sixteenth o5ser+ation, 6or! accor"ing to those o5ser+ations! the thic9ness



was ''/))/ parts of an inch, Now! s7ppose this thic9ness 5e represente" 5y ]! an" the ele+enthpart of it! J! will 5e a5o7t ''1111 of an inch, An" so K! 3! Y! o! will 5e -'1111!)'1111! ('1111! an" ='1111, An" this 5eing 9nown! it is easy to "eter$ine what thic9ness of air is represente" 5y ! or any other "istance of the r7ler fro$ A H,

%7t f7rther! since! 5y the tenth o5ser+ation! the thic9ness of air was to the thic9ness of water! which5etween the sa$e glasses exhi5ite" the sa$e colo7r! as fo7r to three@ an"! 5y the twenty0first

o5ser+ation! the colo7rs of thin 5o"ies are not +arie" 5y +arying the a$5ient $e"i7$@ the thic9nessof a 5755le of water exhi5iting any colo7r will 5e three fo7rths of the thic9ness of air pro"7cing thesa$e colo7r, An" so! accor"ing to the sa$e tenth an" twenty0first o5ser+ations! the thic9ness of aplate of glass! whose refraction is $eas7re" 5y the proportion of the sines thirty0one to twenty! $ay5e .1-' of the thic9ness of air pro"7cing the sa$e colo7rs: an" the li9e of other $e"i7$s, Onthese gro7n"s I ha+e co$pose" the following ta5le@ wherein the thic9ness of air! water! an" glass!at which each colo7r is $ost intense an" specific! is expresse" in parts of an inch "i+i"e" into tenh7n"re" tho7san" eB7al parts,

;.=<

Now! if this ta5le 5e co$pare" with the thir" sche$e! yo7 will there see the constit7tion of each

colo7r! as to its ingre"ients! or the original colo7rs! of which it is co$po7n"e"! an" thence 5eena5le" to F7"ge of its intenseness or i$perfection! which $ay s7ffice in explication of the fo7rthan" eighteenth ;.=1< o5ser+ations! 7nless it 5e f7rther "esire" to "elineate the $anner! how thecolo7rs appear! when the two o5Fect0glasses are lai" 7pon one another: to "o which let there 5e"escri5e" a large arc of a circle an" a strait line! which $ay to7ch that arc@ are parallel to thattangent se+eral occ7lt lines at s7ch "istances fro$ it! as the n7$5ers set against the se+eralcolo7rs in the ta5le "enote, 6or the arc an" its tangent will represent the s7perficies of the glasses!ter$inating the interFacent air! an" the places! where the occ7lt lines c7t the arc! will show at what"istances fro$ the center! or point of the contact! each colo7r is reflecte",

#here are also other 7ses for this ta5le@ for 5y its assistance the thic9ness of the 5755le! in thenineteenth o5ser+ation! was "eter$ine" 5y the colo7rs! which it exhi5ite", An" so the 5igness of

the parts of nat7ral 5o"ies $ay 5e conFect7re" at 5y their colo7rs! as shall 5e hereafter shown, Also! if two or $ore +ery thin plates 5e lai" one 7pon another! so as to co$pose one plate!eB7alling the$ all in thic9ness! the res7lting colo7r $ay 5e here5y "eter$ine", 6or instance! 8r,HOOE! in his 8icrographia! o5ser+es! that a faint yellow plate of 87sco+y glass! lai" 7pon a 5l7eone! constit7te" a +ery "eep p7rple, #he yellow of the first or"er is a faint one! an" the thic9ness of the plate exhi5iting it! accor"ing to the ta5le! is )'/! to which a"" ='.! the thic9ness exhi5iting5l7e of the secon" or"er! an" the s7$ will 5e '/ -/! which $ost nearly approaches '//)! thethic9ness exhi5iting the p7rple of the thir" or"er,

#o explain! in the next place! the circ7$stances of the secon" an" thir" o5ser+ations! that is! howthe colo7rs &5y t7rning the pris$s a5o7t their co$$on axis the contrary way to that expresse" inthose o5ser+ations* $ay 5e con+erte" into white an" 5lac9 rings! an" afterwar"s into colo7rs againin an in+erte" or"er@ it $7st 5e re$e$5ere"! that those colo7rs are "ilate" 5y o5liB7ation of therays to the air! which interce"es the glasses@ an" that! accor"ing to the ta5le in the se+entho5ser+ation! their "ilation or reflection fro$ the co$$on center is $ost $anifest an" spee"y whenthey are o5liB7est, Now! the rays of yellow 5eing $ore refracte" 5y the first s7perficies of the sai"air than those of re"! are there5y $a"e $ore o5liB7e to the secon" s7perficies! at which they arereflecte"! to pro"7ce the colo7re" rings@ an" conseB7ently! the yellow in each ring will 5e $ore"ilate" than the re"@ an" the excess of its "ilation will 5e so $7ch the greater! 5y how $7ch thegreater is the o5liB7ity of the rays! 7ntil at last it 5eco$e of eB7al extent with the re" of the sa$ering, An"! for the sa$e reason! the green! 5l7e! an" +iolet! will 5e also so $7ch "ilate" 5y the stillgreater o5liB7ity of their rays! as to 5eco$e all +ery nearly of eB7al extent with the re"@ that is!eB7ally "istant fro$ the center of the rings, An" then all the colo7rs of the sa$e series $7st 5ecoinci"ent! an" 5y their $ixt7re exhi5it a white ring@ an" these white rings $7st ha+e 5lac9 or "ar9rings 5etween the$! 5eca7se they "o not sprea" an" interfere with one another as 5efore@ an"! for

that reason also! they $7st 5eco$e "istincter! an" +isi5le to far greater n7$5ers, %7t yet the +iolet!5eing ;.='< o5liB7est! will 5e so$ething $ore "ilate" in proportion than the other colo7rs@ an" so+ery apt to appear at the exterior +erges of the white,



Afterwar"s! 5y a greater o5liB7ity of the rays! the +iolet an" the 5l7e 5eco$e sensi5ly $ore "ilate"than the re" an" yellow@ an" so 5eing f7rther re$o+e" fro$ the center of the rings! the colo7rs $7ste$erge o7t of the white in an or"er contrary to that which they ha" 5efore! the +iolet an" 5l7e at theexterior li$5s! an" the re" an" yellow at the interior, An" the +iolet! 5y reason of the greatesto5liB7ity of its rays! 5eing! in proportion! $ost of all expan"e"! will soonest appear at the exterior li$5 of each white ring! an" 5eco$e $ore conspic7o7s than the rest, An" the se+eral series of colo7rs! 5y their 7nfol"ing an" sprea"ing! will 5egin again to interfere! an" there5y ren"er the rings

less "istinct! an" not +isi5le to so great n7$5ers,

If! instea" of the pris$s! the o5Fect0glasses 5e $a"e 7se of! the rings! which they exhi5it! 5eco$enot white an" "istinct 5y the o5liB7ity of the eye! 5y reason! that the rays! in their passage thro7ghthat air! which interce"e" the glasses! are +ery nearly parallel to the$sel+es! when first inci"ent onthe glasses@ an" conseB7ently! those in"7e" with se+eral colo7rs are not incline" one $ore thananother to that air! as it happens in the pris$s,

#here is yet another circ7$stance of these experi$ents to 5e consi"ere"@ an" that is! why the 5lac9an" white rings! which! when +iewe" at a "istance! appear "istinct! sho7l" not only 5eco$econf7se" 5y +iewing the$ near at han"! 57t also yiel" a +iolet colo7r at 5oth the e"ges of e+erywhite ring: an" the reason is! that the rays! which enter the eye at se+eral parts of the p7pil! ha+ese+eral o5liB7ities to the glasses! an" those! which are $ost o5liB7e! if consi"er" apart! wo7l"

represent the rings 5igger than those! which are the least o5liB7e, >hence the 5rea"th of theperi$eter of e+ery white ring is expan"e" o7twar"s 5y the o5liB7est rays! an" inwar"s 5y the leasto5liB7e, An" this expansion is so $7ch the greater! 5y how $7ch the greater is the "ifference of theo5liB7ity@ that is! 5y how $7ch the p7pil is wi"er! or the eye nearer to the glasses: an" the 5rea"thof the +iolet $7st 5e $ost expan"e"! 5eca7se the rays! apt to excite a sensation of that colo7r! are$ost o5liB7e to the secon" or f7rther s7perficies of the thinne" air! at which they are reflecte"@ an"ha+e also the greatest +ariation of o5liB7ity! which $a9es that colo7r soonest e$erge o7t of thee"ges of the white, An"! as the 5rea"th of e+ery ring is th7s a7g$ente"! the "ar9 inter+als $7st 5e"i$inishe"! 7ntil the neigh5o7ring rings 5eco$e contin7o7s! an" are 5len"e"! the exterior first! an"then those nearer the center@ so that they can no longer 5e "isting7ishe" a0part! 57t see$ toconstit7te an e+en an" 7nifor$ whiteness,

A$ongst all the o5ser+ations there is none acco$panie" with so o"" circ7$stances as the twenty0fo7rth, Of those the principal are! that in thin plates! which! to the na9e" eye! see$ of an e+en an"7nifor$ transparent whiteness! the refraction of a pris$ sho7l" $a9e the rings of colo7rs appear@whereas it 7s7ally $a9es o5Fects to appear colo7re" only! where they are ter$inate" with ;.=.<sha"ows! or ha+e parts 7neB7ally l7$ino7s@ an" that it sho7l" $a9e those rings excee"ingly "istinctan" white! altho7gh it 7s7ally ren"ers those o5Fects conf7se" an" colo7re", #he ca7se of thesethings yo7 will 7n"erstan" 5y consi"ering! that all the rings of colo7rs are really in the plate! when+iewe" 5y the na9e" eye! altho7gh! 5y reason of the great 5rea"th of their circ7$ferences! they so$7ch interfere! an" are 5len"e" together! that they see$ to constit7te an e+en whiteness, %7t!when the rays pass thro7gh the pris$ to the eye! the or5its of the se+eral colo7rs in e+ery ring arerefracte"! so$e $ore than others! accor"ing to their "egree of refrangi5ility@ 5y which $eans thecolo7rs on one si"e of the ring 5eco$e $ore 7nfol"e" an" "ilate"! an" on the other si"e $oreco$plicate" an" contracte", An" where! 5y a "7e refraction! they are so $7ch contracte"! that these+eral rings 5eco$e narrower than to interfere with one another! they $7st appear "istinct! an"also white! if the constit7ent colo7rs 5e so $7ch contracte" as to 5e wholly coinci"ent: 57t on theother si"e! where e+ery ring is $a"e 5roa"er 5y the f7rther 7nfol"ing its colo7rs! it $7st interfere$ore with other rings than 5efore! an" so 5eco$e less "istinct,

#o explain this a little f7rther@ s7ppose the concentric circles! A % an" C 2! represent the re" an"+iolet of any or"er! which! together with the inter$e"iate colo7rs! constit7te any one of these rings,Now! these 5eing +iewe" thro7gh a pris$! the +iolet circle! % C! will! 5y a greater refraction! 5ef7rther translate" fro$ its place than the re"! A 2! an" so approach nearer to it on that si"e towar"swhich the refractions are $a"e, 6or instance! if the re" 5e translate" to a "! the +iolet $ay 5etranslate" to 5 c! so as to approach nearer to it at c than 5efore@ an"! if the re" 5e f7rther translate"to a "! the +iolet $ay 5e so $7ch f7rther translate" to 5 c! as to con+ene with it at c! an"! if the re"

5e yet f7rther translate" to \ T! the +iolet $ay 5e still so $7ch f7rther translate" to ! as to pass5eyon" it at ! an" con+ene with it at e an" f, An" this 5eing 7n"erstoo"! not only of the re" an" the+iolet! 57t of all the inter$e"iate colo7rs@ an" also of e+ery re+ol7tion of those colo7rs! yo7 will



their coinci"ence at c "! e an" f! o7ght to constit7te pretty "istinct arcs of circles! especially at c "!or at e an" f! an" that they will appear se+eral at c "! at c " exhi5it whiteness 5y their coinci"ence!an" again appear se+eral at T ! 57t yet in a contrary or"er to that which they ha" 5efore! an" stillretain 5eyon" e an" f, %7t! on the other si"e! at a 5! a 5! or \ ! these colo7rs $7st 5eco$e $7ch$ore conf7se" 5y 5eing "ilate"! an" sprea" so as to interfere with those of other or"ers, An" thesa$e conf7sion will happen at T 5etween e an" f! if the refraction 5e ;.=-< +ery great! or thepris$ +ery "istant fro$ the o5Fect0glasses@ in which case no parts of the ring will 5e seen! sa+e only

two little arcs at e an" f! whose "istance fro$ one another will 5e a7g$ente" 5y re$o+ing the pris$still f7rther fro$ the o5Fect0glasses, An" these little arcs $7st 5e "istinctest an" whitest at their $i""le@ an" at their en"s! where they grow conf7se"! they $7st 5e colo7re"@ an" the colo7rs at oneen" of e+ery arc $7st 5e in a contrary or"er to those at the other en"! 5y reason that they cross inthe inter$e"iate white@ na$ely! their en"s! which +erge towar"s T ! will 5e re"! an" yellow on thatsi"e next the center! an" 5l7e an" +iolet on the other si"e, %7t their other en"s! which +erge fro$ T! will! on the contrary! 5e 5l7e an" +iolet on that si"e towar"s the center! an" on the other si"e re"an" yellow,

6or confir$ation of all this! I nee" to alle"ge no $ore! than that it is $athe$atically "e$onstra5lefro$ $y for$er principles, %7t I shall a""! that they! which please to ta9e the pris$s! $ay 5e thetesti$ony of their senses 5e ass7re"! that these explications are not hypothetical! 57t infalli5ly tr7ean" gen7ine: for in a "ar9 roo$! 5y +iewing these rings thro7gh a pris$! 5y reflection of the se+eral

pris$atic colo7rs! which an assistant ca7ses to $o+e to an" fro 7pon a wall or paper! fro$ whencethey are reflecte"! whilst the spectators eye! the pris$! an" o5Fect0glasses &as in the thirteentho5ser+ation* are place" ste""y! the position of the circles! $a"e s7ccessi+ely 5y the se+eralcolo7rs! will 5e fo7n" s7ch! in respect of one another! as I ha+e "escri5e" at a 5 c "! or a 5 c "! or \ T, An" 5y the sa$e $etho" the tr7th of the explications of the other o5ser+ations is to 5eexa$ine",

%y what hath 5een sai"! the li9e ph?no$ena of water05755les an" thin plates of glass $ay 5e7n"erstoo", %7t in s$all frag$ents of those plates! there is this f7rther o5ser+a5le! that! if they!lying flat 7pon a ta5le! 5e t7rne" a5o7t their center! whilst they are +iewe" thro7gh a pris$! so$e of the$ exhi5it wa+es in one or two positions only@ 57t the $ost of the$ "o in all positions exhi5itthose wa+es! an" that for the $ost part appearing al$ost all o+er the glass, #he reason is! that the

s7perficies of s7ch plates are not e+en! 57t ha+e $any ca+ities an" swellings! which! how shallowsoe+er! "o a little +ary the thic9ness of the plate@ an" 5y the se+eral si"es of those ca+ities there$7st 5e pro"7ce" wa+es in se+eral post7res of the pris$, Now! tho7gh it 5e 57t so$e +ery s$allan" narrow parts of the glass! 5y which these wa+es for the $ost part are ca7se"! yet they $aysee$ to exten" the$sel+es o+er the whole glass! 5eca7se fro$ the narrowest of those parts thereare colo7rs of se+eral or"ers conf7se"ly reflecte"! which 5y refraction of the pris$ are 7nfol"e"!an" "isperse" to se+eral places! so as to constit7te so $any se+eral wa+es as there were "i+ersor"ers of the colo7rs pro$isc7o7sly reflecte" fro$ that part of the glass,

#hese are the principal ph?no$ena of thin plates or 5755les! whose explications "epen" on theproperties of light! that I ha+e heretofore "eli+ere": an" these! yo7 see! "o necessarily follow fro$the$! an" agree with the$ e+en to their least circ7$stances@ an" not only so! 57t "o +ery $7chten" to ;.=/< their proof, #h7s! 5y the twenty0fo7rth o5ser+ation! it appears! that the rays of se+eral colo7rs! $a"e! as well 5y thin plates or 5755les! as 5y the refractions of a pris$! ha+ese+eral "egrees of refrangi5ility! where5y those of each or"er! which! at their reflection fro$ theplate or 5755le! are inter$ixe" with those of other or"ers! are separate" fro$ the$ 5y refraction!an" associate" together! so as to 5eco$e +isi5le 5y the$sel+es! li9e arcs of circles, 6or! if the rayswere all ali9e refrangi5le! it is i$possi5le! that the whiteness! which to the na9e" sense appears7nifor$! sho7l" 5y refraction ha+e its parts transpose"! an" range" into those 5lac9 an" white arcs,

It appears also! that the 7neB7al refractions of "iffor$ rays procee" not fro$ any contingentirreg7larities! s7ch as are +eins! an 7ne+en polish! or fort7ito7s position of the pores of glass!7neB7al $otions in the air or ?ther! sprea"ing! 5rea9ing! or "i+i"ing the sa$e ray into $any"i+erging parts! or the li9e, 6or! a"$itting any s7ch irreg7larities! it wo7l" 5e i$possi5le for refractions to ren"er those rings so +ery "istinct an" well "efine"! as they "o in the twenty0fo7rth

o5ser+ation, It is necessary therefore! that e+ery ray ha+e its proper an" constant "egree of refrangi5ility connate with it@ accor"ing to which its refraction is e+er F7stly an" reg7larly perfor$e"!an" that se+eral rays ha+e se+eral of those "egrees,



An" what is sai" of their refrangi5ility $ay 5e 7n"erstoo" of their reflexi5ility@ that is! of their "ispositions to 5e reflecte"! so$e at a greater! an" others at a less thic9ness of thin plates or 5755les! na$ely! that those "ispositions are also connate with the rays! an" i$$7ta5le! as $ayappear 5y the thirteenth! fo7rteenth! an" fifteenth o5ser+ations! co$pare" with the fo7rth an"eighteenth,

%y the prece"ent o5ser+ations it appears also! that the whiteness is a "issi$ilar $ixt7re of all

colo7rs! an" that light is a $ixt7re of rays en"owe" with all those colo7rs, 6or! consi"ering the$7ltit7"e of the rings of colo7rs in the thir"! twelfth! an" twenty0fo7rth o5ser+ations! it is $anifest!that! altho7gh in the fo7rth an" eighteenth o5ser+ations there appear $ore than eight or nine of those rings! yet there are really a far greater n7$5er! which so $7ch interfere an" $ingle with oneanother! as! after those eight or nine re+ol7tions! to "il7te one another wholly! an" constit7te ane+en an" sensi5le 7nifor$ whiteness, An" conseB7ently! that whiteness $7st 5e allowe" a $ixt7reof all colo7rs! an" the light! which con+eys it to the eye! $7st 5e a $ixt7re of rays en"7e" with allthose colo7rs,

%7t f7rther! 5y the twenty0fo7rth o5ser+ation it appears! that there is a constant relation 5etweencolo7rs an" refrangi5ility! the $ost refrangi5le rays 5eing +iolet! the least refrangi5le re"! an" thoseof inter$e"iate colo7rs ha+ing proportionally inter$e"iate "egrees of refrangi5ility, An"! 5y thethirteenth! fo7rteenth! an" fifteenth o5ser+ations! co$pare" with the fo7rth or eighteenth! there

appears to 5e the sa$e constant relation 5etween colo7r an" refrangi5ility@ the +iolet 5eing oneB7al ter$s reflecte" at least thic9ness of any thin ;.=)< plate or 5755le@ the re" at greatestthic9ness! an" the inter$e"iate colo7rs at inter$e"iate thic9nesses: whence it follows! that thecolorific "ispositions of rays are also connate with the$! an" i$$7ta5le@ an" 5y conseB7ence! thatall the pro"7ctions an" appearances of colo7rs in the worl" are "eri+e"! not fro$ any physicalchange ca7se" in light 5y refraction of reflection! 57t ony fro$ the +ario7s $ixt7res or separationsof rays! 5y +irt7e of their "ifferent refrangi5ility, An"! in this respect it is! that the science of colo7rs5eco$es a spec7lation $ore proper for $athe$aticians than nat7ralists,

#his 5eing rea"! occasion was ta9en to "isco7rse of 8r, NE>#ONs theory itself! an" to "e5ate!whether the rays of light! which! tho7gh ali9e inci"ent in the sa$e $e"i7$! yet exhi5it "ifferentcolo7rs! $ay not reasona5ly 5e sai" to owe that exhi5ition of "ifferent colo7rs to the se+eral

"egrees of the +elocity of p7lses! rather than! as 8r, NE>#ON tho7ght! to the se+eral connate"egrees of refrangi5ility in the rays the$sel+esG

8r, HOOE was of opinion! that the for$er of these ways was s7fficient to gi+e a goo" acco7nt of the "i+ersity of colo7rs

6e5r7ary '1, 2r, 8APLE#O6# was electe" an" a"$itte",

Capt, SHEERES! 8r, HALL! an" Signor #RA3AINO were electe",

8r, %ERCHENSHA> presente" hi$self to the Society! an" shewe" the$ his scale of $7sic!wherein were containe"!

', A ta5le of all consonant an" "issonant inter+als s7ita5le to $7sical har$ony! which arepractica5le! an" $ay 5e expresse" 5y the +oice an" other instr7$ents, #o these respecti+einter+als apt an" proper n7$5ers were assigne"! 5y which their ratios an" proportions were"e$onstrate",

., A syste$ of all the 9eys! 5y which the aforesai" inter+als were co$plete"@ of which 9eys so$ewere nat7ral@ so$e inten"e" to the first "egree of ac7teness@ so$e re$itte" to the first "egree of gra+ity@ so$e twice spissate"@ so$e twice asperate",

-, In this scale the $agnit7"e! "i$ension! an" proportion of the sai" 9eys were exactly"e$onstrate" accor"ing to the proportional parts of a chor"! the chor" 5eing s7ppose" thirty0sixinches long,

If it were "e$an"e"! whether there was any thing in this ta5le an" syste$! that was not to 5e fo7n"in the scales an" writings of other $7siciansG he answere"!



', #hat the inter+als in this ta5le were perfect an" co$plete, #here was not one too $any! nor onewanting! which $ight con"7ce to the $a9ing of har$ony,

;.=<

., #hat the so7n"s or $7sical n7$5ers containe" in this syste$ arose o7t of the 7nison! an" fro$

one another! accor"ing to the reason of fig7rate! not si$ple n7$5ers! &as! he sai"! he co7l""e$onstrate 5y n7$5ers assigne" to the respecti+e inter+als in the ta5le* for that so the reason of the state of $7sic reB7ire",

-, #hat there are neither $ore or less 9eys in this syste$! than wo7l" co$plete the aforesai"inter+als,

/, #hat in this scale all the tones are of the sa$e ratio! an" that so are all the se$itones!se$i"itones! "itones! an" other inter+als,

), #hat the tr7e $agnit7"e an" "i$ension of e+ery one of the sai" 9eys are "e$onstrate"accor"ing to the proportional parts of a chor",

, #hat the nat7ral! gen7ine! an" tr7e reason of the excellency an" f7llness of the har$ony of three!fo7r! fri+e! six! an" se+en parts! $ay clearly 5e "iscerne" 5y the syste$ of se+en parts,

He a""e"! that $any other things were to 5e fo7n" in this ta5le an" scale! of which little or no$ention is $a"e in the scales an" writings of either $o"ern or antient $7sical a7thors@ which! hesai"! he inten"e" to "isco+er! an" to write of the$ at large! as he sho7l" 5e ena5le" there7nto,

He was exhorte" to finish this wor9! or at least to p75lish this syste$ with an explanation thereof,

After this was rea" the last part of 8r, NE>#ONS o5ser+ations! wherein he consi"ere" in ninepropositions! how the ph?no$ena of thin transparent plates stan" relate" to those of all other nat7ral 5o"ies: of which 5o"ies ha+ing 5efore $entione"! that they appear of "i+ers colo7rs!

accor"ing as they are "ispose" to reflect $ost copio7sly the rays in"7e" with these colo7rs! he nowinB7ires after their constit7tions,

Here! a$ong $any other consi"era5le things! he shews! how the 5igness of the co$ponent parts of nat7ral 5o"ies $ay 5e conFect7re" 5y their colo7rs: as also! that the ca7se of reflexion is not thei$pinging of light on the soli" an" i$per+io7s parts of 5o"ies! as was co$$only s7ppose",

#his last part was as follows:

I a$ now co$e to the last part of this "esign@ which is! to consi"er! how the ph?no$ena of thintransparent plates stan" relate" to those of all other nat7ral 5o"ies, Of these 5o"ies I ha+e alrea"ytol" yo7! that they appear of "i+ers colo7rs! accor"ing as they are "ispose" to reflect $ostcopio7sly the rays en"7e" with those colo7rs, %7t their constit7tions! where5y they reflect so$erays ;.=(< $ore copio7sly than others! re$ains to 5e inB7ire" after, An" this I shall en"ea+o7r inthe following propositions,

Prop, ', #hose s7perficies reflect the greatest B7antity of light! which ha+e the greatest refractingpower@ that is! which intercee"s $e"i7$s! that "iffer $ost in their refracting "ensities@ an" in theconfines of eB7ally "ense $e"i7$s there is no reflection,

#he analogy 5etween reflection an" refraction will appear 5y consi"ering! that when light passetho5liB7ely o7t of one $e"i7$ into another! which refracts fro$ the perpen"ic7lar! the greater is the"ifference of their "ensity! the less o5liB7ity is reB7isite to ca7se a total reflection@ 5eca7se as thesines are! which $eas7re the refraction! so is the sine of inci"ence! at which the total reflection5egins! to the ra"i7s of the circle@ an" conseB7ently that inci"ence is least! where there is the great

"ifference of the sines, #h7s in the passing of light o7t of water into air! where the refraction is$eas7re" 5y the ratio of the sines! - to /! the total reflection 5egins! when the angle of theinci"ence is a5o7t forty0 eight "egrees an" thirty0fi+e $in7tes, In passing o7t of glass into air! where



angle of inci"ence is forty "egress an" ten $in7tes: an" so! in passing o7t of crystal! or $orestrongly refracting $e"i7$s! into air! there is still a less o5liB7ity reB7isite to ca7se a total reflection,S7perficies therefore! which refract $ost! "o soonest reflect all the light! which is inci"ent on the$!an" so $7st 5e allowe" $ost strongly reflecti+e,

%7t the tr7th of this proposition will f7rther appear! 5y o5ser+ing! that in the s7perficies! intercee"ingany two of those $e"i7$s! air or water! or other liB7ors! co$$on glass! crystal! an" $etalline

glasses! the reflection is stronger or wea9er accor"ingly as the s7perficies hath a greater or lessrefracting power, #h7s! when other $e"i7$s are contig7o7s to air! the reflection is stronger in thes7perficies of glass than of water! still stronger in the s7perficies of crystal! an" strongest in thes7perficies of $etalline glass, So! in the confine of water an" co$$on glass! the reflection is +erywea9! 57t yet stronger than in the confine of water an" oil! or al$ost any two liB7ors! an" stillstronger in the confine of water an" crystal! or $etalline glass: accor"ingly as those $e"i7$s "iffer $ore or less in "ensity! so in the confine of co$$on glass an" crystal there is a wea9 reflection!an" a stronger relection in the confine of co$$on an" $etalline glass: 57t in the confine of twoglasses of eB7al "ensity! there is not any sensi5le reflection! as was shewn in the first o5ser+ation, An" the sa$e $ay 5e 7n"erstoo" of the s7perficies of two crystals or liB7ors! or any other s75stances! in which no refraction is ca7se": whence it co$es to pass! that 7nifor$ $e"i7$s ha+eno sensi5le reflexion 57t in their external s7perficies! where they are a"Facent to their $e"i7$s of a"ifferent "ensity,

Prop, ., #he least parts of nat7ral 5o"ies are in so$e $eas7re transparent@ an" the opacities of those 5o"ies arise fro$ the $7ltit7"e of reflections ca7se" in their internal parts,

;.=<

#hat this is so! will easily 5e grante" 5y the$! that ha+e 5een con+ersant with $icroscopes: an" it$ay also 5e trie" 5y applying any s75stance to a hole! thro7gh which the light is e$itte" into a "ar9roo$@ for how opa9e soe+er that s75stance $ay see$ in the open air! it will! 5y that $eans! appear +ery $anifestly transparent! if it 5e of a s7fficient thic9ness: only $etalline 5o"ies $7st 5eexe$pte"! which! 5y reason of their excessi+e "ensity see$ to reflect al$ost all the light inci"ent on

their first s7perficies,

Prop, -, %etween the parts of opa9e or colo7re" 5o"ies are $any interstices! replenishe" with$e"i7$s of other "ensities! as water 5etween the tinging corp7scles! wherewith any liB7or isi$pregnate"@ air 5etween the aB7eo7s glo57les that constit7te clo7"s or $ists@ an" for the $ostpart spaces +oi" of 5oth air an" water@ 57t yet perhaps replenishe" with so$e s75tiler $e"i7$5etween the parts of har" 5o"ies,

#he tr7th of this is e+ince" 5y the two prece"ent propositions: for 5y the secon" proposition thereare $any reflections fro$ the internal part of 5o"ies! which 5y the first proposition wo7l" nothappen! if the parts of those 5o"ies were contin7e" witho7t any s7ch interstices 5etween the$!5eca7se reflections are ca7se" only in s7perficies! which intercee" $e"i7$s of a "ifferent "ensity,

%7t f7rther! that this "iscontin7ity of parts is the principal ca7se of the opacity of 5o"ies! will appear 5y consi"ering! that opa9e s75stances 5eco$e transparent 5y filling their pores with any s75stanceof eB7al! or al$ost eB7al "ensity with their parts, #h7s paper "ippe" in water or oil! the oc7l7s$7n"i stone steepe" in water! linen0cloth oile" or +arnishe"! an" $any other s75stances soa9e" ins7ch liB7ors! as will inti$ately per+a"e their little pores! 5eco$e 5y that $eans $ore transparentthan otherwise, So! on the contrary! the $ost transparent s75stances $ay! 5y separating their parts! 5e ren"ere" s7fficiently opa9e@ as glass! 5y 5eing re"7ce" to pow"er! or otherwise flawe"!water 5y 5eing for$e" into $any s$all 5755les! either alone in the for$ of froth! or 5y sha9ing ittogether with oil of t7rpentine! or so$e other con+enient liB7or! with which it will not incorporate!an" horn 5y 5eing scrape",

#o the increase of the opacity of these 5o"ies it con"7ces so$ething! that 5y the twenty thir"

o5ser+ation! the refractions of +ery thin transparent s75stances are consi"era5ly stronger thanthose $a"e 5y the sa$e s75stances of a greater thic9ness, An" to the reflection of soli" 5o"ies it$ay 5e f7rther a""e"! that the interstices of their parts are +oi" of air, 6or that for the $ost part they



ca+ities! as appears 5y the ascension of water in slen"er glass0pipes! paper! cloth! an" other s7chli9e s75stances! whose pores are fo7n" too s$all to 5e replenishe" with air! an" yet large eno7ghto a"$it water@ an" 5y the "iffic7lty! wherewith air per+a"es the pores of a 5la""er! thro7gh whichwater fin" rea"y passage, An" accor"ing to the ele+enth o5ser+ation! the ca+ities th7s +oi" of air will ca7se the sa$e 9in" of effects as to reflection! which those "o! that are replenishe" with it@ 57tyet so$ething $ore $anifestly! 5e0 ;.==< ca7se the $e"i7$ in relation to refractions is rarest!when $ost e$pty of air as 8r, HOOE hath pro+e" in his 8icrographia@ in which 5oo9 he hath also

largely "isco7rse" of this an" the prece"ent proposition! an" "eli+ere" $any other +ery excellentthings concerning the colo7rs of thin plates! an" other nat7ral 5o"ies! which I ha+e not scr7ple" to$a9e 7se of so far as they were for $y p7rpose,

Prop, /, #he parts of 5o"ies an" their interstices $7st not 5e less than of so$e "efiniti+e 5igness!to ren"er the$ opa9e an" colo7re"@ for the opa9est 5o"ies! if their parts 5e s75tilly "i+i"e" &as$etals 5y 5eing "issol+e" in aci" $enstr77$s! Dc,* 5eco$e prefectly transparent, An" yo7 $ayalso re$e$5er! that in the eighth o5ser+ation there was no reflection at the s7perficies of theo5Fect0glasses! where they were +ery near one another! tho7gh they "i" not a5sol7tely to7ch, An"in the se+enteenth o5ser+ation! the reflection of the water05755le! where it 5eca$e thinnest! wasal$ost insensi5le! so as to ca7se the apparitions of +ery 5lac9 spots,

On these gro7n"s I concei+e it is! that water! salt! glass! stones! an" s7ch li9e s75stances! are

transparent@ for! 7pon "i+ers consi"erations! they see$ to 5e as poro7s as other 5o"ies! 57t yettheir pores an" parts too s$all to ca7se any opacity,

Prop, ), #he transparent parts of 5o"ies! accor"ing to their se+eral sies! $7st reflect rays of onecolo7r! an" trans$it those of another! on the sa$e gro7n"s! that thin plates or 5755les "o reflect or trans$it those rays: an" this I ta9e to 5e the gro7n" of all their colo7rs,

6or! if a thinne" or plate" 5o"y! which 5eing of an e+en thic9ness appears all o+er of one 7nifor$colo7r! sho7l" 5e 5ro9en into frag$ents of the sa$e thic9ness with the plate! I see no reason! whya heap of those frag$ents sho7l" not constit7te a pow"er of the sa$e colo7r! which the plateexhi5ite" 5efore it was 5ro9en, An" the parts of all nat7ral 5o"ies! 5eing li9e so $any frag$ents of a plate! $7st on the sa$e gro7n"s exhi5it the sa$e colo7rs,

Now! that they "o so! will f7rther appear 5y the affinity of their properties: as that the inf7sion of nephritic0woo"! an" $any other s75stances reflect one colo7r! an" trans$it another! li9e thin5o"ies in the ninth an" twentieth o5ser+ations, #hat the colo7rs of sil9s! cloaths! an" other s75stances! which water or oil can inti$ately penetrate! 5eco$e $ore faint an" o5sc7re 5y 5einge$erge" in those liB7ors! an" reco+er their +igo7r again 5y 5eing "rie"! $7ch after the $anner "eclare" of thin 5o"ies! in the tenth an" twenty first o5ser+ations: an" that so$e of those colo7re"pow"ers! which painters 7se! $ay ha+e their colo7rs a little change"! 5y 5eing +ery ela5orately an"finely gro7n", >here I see not! what can 5e F7stly preten"e" for those changes! 5esi"es the5rea9ing of their parts into less parts 5y that contrition! after the sa$e $anner that the colo7r of aplate is change" 5y +arying its thic9ness, 6or which reason also it is! that $any flowers! 5y 5eing5r7ise"! 5eco$e $ore transparent ;-11< than 5efore! or! at least in so$e "egree or other! changetheir colo7rs, Nor is it $7ch less to $y p7rpose! that! 5y $ixing "i+ers liB7ors! +ery o"" an"re$ar9a5le pro"7ctions an" changes of colo7rs $ay 5e effecte"! of which no ca7se can 5e $oreo5+io7s an" nat7ral! than that the saline corp7scles of one liB7or "o +ario7sly act 7pon! or 7nitewith! the tinging corp7scles of another@ so as to $a9e the$ swell or shrin9 &where5y not only their 57l9! 57t their "ensity also $ay 5e change"* or to "i+i"e the$ into s$aller corp7scles! or $a9e$any of the$ associate into one cl7ster@ for we see how apt those saline $enstr77$s are topenetrate an" "issol+e s75stances! to which they are applie"@ an" so$e of the$ to precipitate whatothers "issol+e, In li9e $anner! if we consi"er the +ario7s ph?no$ena of the at$osphere! we $ayo5ser+e! that when +apo7rs are first raise"! they hin"er not the transparency of the air! 5eing"i+i"e" into parts too s$all to ca7se any reflection in their s7perficies: 57t when! in or"er toco$pose "rops of rain! they 5egan to coalesce an" constit7te glo57les of all inter$e"iate sies!those glo57les! when they 5eco$e of a con+enient sie to reflect so$e colo7rs! an" trans$itothers! $ay constit7te clo7"s of +ario7s colo7rs! accor"ing to their sies, An" I see not what can 5e

rationally concei+e"! in so transparent a s75stance as water for the pro"7ction of these colo7rs!5esi"es the +ario7s sies of its parcels! which see$ to affect a glo57lar fig7re $ost@ 57t yet perhapsnot witho7t so$e insta5ility in the s$allest of the$! 5y reason that those are $ost easily agitate" 5y



Prop, , #he parts of 5o"ies! on which their colo7rs "epen"! are "enser than the $e"i7$! whichper+a"es their interstices,

#his will appear 5y consi"ering! that the colo7r of a 5o"y "epen"s not only on the rays! which areinci"ent perpen"ic7larly or its parts! 57t on those also! which are inci"ent at all other angles, An"that! accor"ing to the se+enth o5ser+ation! a +ery little +ariation of o5liB7ity will change the reflecte"

colo7r! where the thin 5o"y or s$all particle is rarer than the a$5ient $e"i7$! in so$7ch that s7cha s$all particle will! at "i+ersly o5liB7e inci"ents! reflect all sorts of colo7rs! in so great a +ariety!that the colo7r! res7lting fro$ the$ all conf7se"ly reflecte" fro$ a heap of s7ch particles! $7strather 5e a white or grey! than any other colo7r! or at 5est it $7st 5e 57t a +ery i$perfect an" "irtycolo7r@ whereas! if the thin 5o"y or s$all particle 5e $7ch "enser than the a$5ient $e"i7$! thecolo7rs! accor"ing to the nineteenth o5ser+ation! are so little change" 5y the +ariation of o5liB7ity!that the rays! which are reflecte" least o5liB7ely! $ay pre"o$inate o+er the rest so $7ch! as toca7se a heap of s7ch particles to appear +ery intensly of their colo7r,

It concl7"es also so$ething to this proposition! that! accor"ing to the twenty0secon" o5ser+ation!the colo7rs exhi5ite" 5y the "enser thin 5o"y within the rarer are $ore 5ris9 than those exhi5ite" 5ythe rarer within the "enser,

Prop, (, #he 5igness of the co$ponent parts of nat7ral 5o"ies $ay 5e conFect7re" 5y their colo7rs,

;-1'<

6or since the parts of these 5o"ies! 5y proposition ), "o $ost pro5a5ly exhi5it the sa$e colo7rswith a plate of eB7al thic9ness! pro+i"e" they ha+e the sa$e refracti+e "ensity@ an" since their partssee$ for the $ost part to ha+e $7ch the sa$e "ensity with water or glass! as 5y $anycirc7$stances is o5+io7s to collect: to "eter$ine the sies of these parts! yo7 nee" only ha+ereco7rse to the prece"ent ta5les! in which the thic9ness of water or glass exhi5iting any colo7r isexpresse", #h7s! if it 5e "esire" to 9now the "ia$eter of a corp7scle! which 5eing of eB7al "ensitywith glass! shall reflect green of the thir" or"er@ the n7$5er '('. shows it to 5e a5o7t

'('.'111111 parts of an inch,

#he greatest "iffic7lty is here to 9now! of what or"er the colo7r of any 5o"y is@ an" for this en" we$7ct ha+e reco7rse to the fo7rth an" eighteenth o5ser+ations! fro$ whence $ay 5e collecte" thesepartic7lars,

Scarlets! an" other re"s! oranges an" yellows! if they 5e p7re an" intense! are $ost pro5a5ly of thesecon" or"er, #hose of the first an" thir" or"er also $ay 5e pretty goo"@ only the orange an" re" of the thir" or"er ha+e too great a $ixt7re of +iolet an" 5l7e,

#here $ay 5e goo" greens of the fo7rth or"er! 57t the p7rest are of the thir": an" of this or"er thegreen of all +egeta5les see$s to 5e! partly 5y reason of the intenseness of their colo7rs! an" partly5eca7se when they wither! so$e of the$ t7rn to a greenish yellow! an" others to a $ore perfectyellow or orange! or perhaps to re"@ passing first thro7gh all the aforesai" inter$e"iate colo7rs!which changes see$ to 5e effecte" 5y the exhaling of the $oist7re! which $ay lea+e the tingingcorp7scles $ore "ense! an" so$ething a7g$ente" 5y the accretion of the oily an" earthy part of that $oist7re, Now the green! witho7t "o75t! is of the sa$e or"er with those colo7rs! into which itchangeth! 5eca7se the changes are gra"7al! an" those colo7rs! tho7gh 7s7ally not +ery p7re! yetfor the $ost part are too p7re an" li+ely to 5e of the fo7rth or"er,

%l7es an" p7rples $ay 5e either of the secon" or thir" or"er@ 57t the 5est are of the thir", #h7s thecolo7r of +iolet see$s to 5e of that or"er@ 5eca7se their syr7p! 5y aci" liB7ors! t7rns re"! an" 5y7rino7s an" al9alaite t7rns green, 6or since it is of the nat7re of aci"s to "issol+e or atten7ate! an"of alcalis to precipitate or incrassate! if the p7rple colo7r of the syr7p was of the secon" or"er! anaci" liB7or 5y atten7ating its tinging corp7scles wo7l" tinge it to a re" of the first or"er! an" an

alcali! 5y incrassating the$! wo7l" change it to a green of the secon" or"er@ which re" an" green!especially the green! see$ too i$perfect to 5e the colo7rs pro"7ce" 5y these changes, %7t if thesai" p7rple 5e s7ppose" of the thir" or"er! its change to re" of the secon" an" green of the thir"



If there 5e fo7n" any 5o"y of a "eeper an" less re""ish p7rple than that of +iolets! its colo7r $ostpro5a5ly is of the secon" or"er, %7t yet there 5eing no 5o"y co$$only 9nown! whose colo7r isconstantly $ore "eep than theirs! ;-1.< I ha+e $a"e 7se of their na$e to "enote the "eepest an"least re""ish p7rples! s7ch as $anifestly transcen" their colo7r in p7rity,

#he 5l7e of the first or"er! tho7gh +ery faint an" little! $ay possi5ly 5e the colo7r of so$e

s75stances@ an" partic7larly the a7re colo7r of the s9ies see$s to 5e of this or"er, 6or all +apo7rs!when they 5egin to con"ense an" coalesce into s$all parcels! 5eco$e first of that 5igness!where5y s7ch an a7re $7st 5e reflecte"! 5efore they can constit7te clo7"s of other colo7rs, An"so this 5eing the first colo7r! which +apo7rs 5egin to reflect! it o7ght to 5e the colo7r of the finestan" $ost transparent s9ies! in which +apo7rs are not arri+e" to that grossness reB7isite to reflectother colo7rs! as we fin" it is 5y experience,

>hiteness! if it 5e intense! is either that in the first or"er of colo7rs! of which sort perhaps is thecolo7r of white lea"@ or else it is a $ixt7re of those s7ccee"ing the thir" or fo7rth or"er! s7ch as isthe colo7r of paper! linen! an" $ost white s75stances, If corp7scles of +ario7s sies! exhi5iting thecolo7rs of the secon" an" thir" or"er! 5e $ixe"! they sho7l" rather constit7te an i$perfectwhiteness or grey! of which I ha+e alrea"y spo9en: 57t yet it see$s not i$possi5le for the$ toexhi5it an intense whiteness! if they 5e "ispose" to trans$it all the light! which they reflect not! an"

"o not retain an" stifle $7ch of it, 6or th7s I tol" yo7! that froth at a "istance hath appeare" +erywhite! an" yet! near at han"! the se+eral 5755les! of which it was constit7te"! were seen tinge" allo+er with rings of colo7rs of the fo7r or fi+e first or"ers,

Lastly! for the pro"7ction of 5lac9! the corp7scles $7st 5e less than any of those! which exhi5itcolo7rs, 6or at all greater sies there is too $7ch light reflecte" to constit7te this colo7r, %7t if they5e s7ppose" a little less than is reB7isite to reflect the 5l7e of the first or"er! they will! accor"ing tothe fo7rth! eighth! se+enteenth! an" eighteenth o5ser+ations! reflect so +ery little light as to appear intensely 5lac9! an" yet $ay perhaps +ario7sly refract it to an" fro within the$sel+es so long! 7ntil ithappen to 5e stifle" an" lost@ 5y which $eans they will appear 5lac9 in all positions of the eyewitho7t any transparency, An" fro$ hence $ay 5e 7n"erstoo"! why fire! an" the $ore s75til"issol+er! p7trefaction! t7rn s75stances to 5lac9@ why s$all B7antities of 5lac9 s75stances i$part

their colo7r +ery freely an" intensely to other s75stances! to which they are applie"@ why glassgro7n" +ery ela5orately! on a copper0plate! till it 5e well polishe"! $a9es the san"! together withwhat is worn off fro$ the glass! an" copper! 5eco$e +ery 5lac9@ why 5lac9 s75stances "o soonestof all others 5eco$e hot an" 57rn! which effect $ay procee"! partly fro$ the $7ltit7"e of refractionsin a little roo$! an" partly fro$ the easy co$$otion of so +ery s$all corp7scles@ an" why 5lac9s are7s7ally a little incline" to a 5l7ish colo7r, 6or that they are so! $ay 5e seen 5y ill7$inating whitepaper 5y reflection fro$ 5lac9 s75stances! which will 7s7ally appear of a 5l7ish white, An" thereason is! that 5lac9 5or"ers on the o5sc7re 5l7e of the first or"er! "escri5e" in the eighteentho5ser+ation! whence the corp7scles of 5lac9 s75stances are $ost apt to reflect that colo7r,

;-1-<

In these "escriptions I ha+e 5een the $ore partic7lar! 5eca7se it is not i$possi5le! 57t that$icroscopes $ay at length 5e i$pro+e" to the "isco+ery of corp7scles of 5o"ies! on which their colo7rs "epen", 6or if those instr7$ents co7l" 5e so far i$pro+e"! as with s7fficient "istinctness torepresent o5Fects fi+e or six h7n"re" ti$es 5igger than at a foot "istance they appear to o7r na9e"eyes, I sho7l" hope! that we $ight 5e a5le to "isco+er so$e of the greatest of those corp7scles, An" 5y one! that wo7l" $agnify three or fo7r tho7san" ti$es! perhaps they $ight all 5e "isco+ere"57t those! which pro"7ce 5lac9ness, In the $ean while! I see nothing $aterial! that rationally can 5e"o75te" of! excepting this position! that transparent corp7scles of the sa$e thic9ness an" "ensitywith a plate "o exhi5it the sa$e colo7r, An" this I wo7l" ha+e 7n"erstoo" not witho7t so$e latit7"e!as well 5eca7se those corp7scles $ay 5e of irreg7lar fig7res! an" $any rays $7st 5e o5liB7elyinci"ent! an" so ha+e a shorter way thro7gh the$ than the length of their "ia$eter@ as 5eca7se thestraitness of the $e"i7$! pent in on all si"es! $ay a little alter its $otions! or other B7alities! on

which the reflexion "epen"s, %7t yet I cannot $7ch s7spect the last! 5eca7se I ha+e o5ser+e" of so$e s$all plates of 87sco+y0glass! which were of an e+en thic9ness! that thro7gh a $icroscopethey ha+e appeare" of the sa$e colo7r at their e"ges an" corners! where the incl7"e" $e"i7$ was



if those corp7scles co7l" 5e "isco+ere" with $icroscopes! which if we shall e+er attain to! I fear itwill 5e the 7t$ost i$pro+e$ent of this sense@ for it see$s i$possi5le to see the $ore secret an"no5le wor9s of nat7re within those corp7scles! 5y reason of their transparency,

#his $ay s7ffice concerning the constit7tion of nat7ral 5o"ies! on which their colo7rs "epen", %7tfor f7rther 7n"erstan"ing the nat7re of reflections! I shall a"" these two following propositions,

Prop, , #he ca7se of the reflection is not the i$pinging of light on the soli" an" i$per+io7s parts of 5o"ies! as is co$$only s7ppose",

#his will appear 5y the following consi"erations: first! that in the passage of light o7t of glass intoair! there is a reflection as strong or stronger than in its passage o7t of air into glass! an" 5y $any"egrees stronger than in its passage o7t of glass into water, An" it see$s not pro5a5le! that air sho7l" ha+e $ore reflecting parts than water or glass, %7t if that sho7l" possi5ly 5e s7ppose"! it willa+ail nothing@ for the reflection is as strong! if not stronger! when the air is "rawn away fro$ theglass &s7ppose in the air0p7$p in+ente" 5y 8r, %O4LE* as when it is a"Facent to it, Secon"ly! if light in its passage o7t of glass into air 5e inci"ent $ore o5liB7ely than at an angle of forty or forty0one "egrees! it is wholly reflecte"@ if less o5liB7ely! it is in great $eas7re trans$itte", Now it is notto 5e i$agine"! that light at one "egree of o5liB7ity sho7l" $eet with pores eno7gh in the air totrans$it the greater part of it! an" at another "egree of o5liB7ity $eet with nothing 57t parts to

reflect it wholly@ especially consi"ering! that in its passage o7t of air into glass! how o5liB7e soe+er 5e its inci"ence! it fin"s pores eno7gh in the glass to trans$it the greatest part of it, ;-1/< If any$an s7ppose! that it is not reflecte" 5y the air! 57t 5y the 7t$ost s7perficial parts of the glass! thereis still the sa$e "iffic7lty@ 5esi"es! that s7ch a s7pposition is 7nintelligi5le@ an" will also appear to5e false! 5y applying water 5ehin" so$e part of the glass instea" of air, 6or so in a con+eniento5liB7ity of the rays! s7ppose of forty0fi+e or forty0six "egrees! at which they are all reflecte"! wherethe air is a"Facent to the glass! they shall 5e in great $eas7re trans$itte"! where the water isa"Facent to it@ which arg7es! that their reflection or trans$ission "epen"s on the consit7tion of theair an" water 5ehin" the glass! an" not on the parts of the glass,

#hir"ly! if the colo7rs $a"e 5y a pris$! place" at the entrance of a 5ea$ of light into a "ar9ene"roo$! 5e s7ccessi+ely cast on a secon" pris$ place" at a great "istance fro$ the for$er! in s7ch

$anner that they are all ali9e inci"ent 7pon it@ the secon" pris$ $ay 5e so incline" to the inci"entrays! that those! which are of a 5l7e colo7r! shall 5e all reflecte" 5y it@ an" yet those of a re" colo7r pretty copio7sly trans$itte", Now if the reflection 5e ca7se" 5y the parts of air or glass! I wo7l" as9!why at the sa$e o5liB7ity of inci"ence the 5l7e sho7l" wholly i$pinge on those parts so as to 5e allreflecte"! an" yet the re" fin" pores eno7gh to 5e in great $eas7re trans$itte", 6o7rthly! where twoglasses to7ch one another! there is no sensi5le reflection! as was "eclare" in the first o5ser+ation@an" yet I see no reason! why the rays sho7l" not i$pinge on the parts of glass! when contig7o7s toanother glass! as $7ch as when contig7o7s to air, 6ifthly! when the top of a water05755le &in these+enteenth o5ser+ation* 5y the contin7al s75si"ing an" exhaling of the water grew +ery thin! therewas s7ch a little an" al$ost insensi5le B7antity of light reflecte" fro$ it! that it appeare" intensely5lac9@ whereas! ro7n" a5o7t that 5lac9 spot! where the water was thic9er! the reflection was sostrong as to $a9e the water see$ +ery white, Nor is it only at the least thic9ness of thin plates or 5755les that there is no $anifest reflection! 57t at $any other thic9nesses contin7ally greater an"greater, 6or in the fifteenth o5ser+ation! the rays of the sa$e colo7r were 5y t7rns trans$itte" atone thic9ness! an" reflecte" at another thic9ness! for an inter$e"iate n7$5er of s7ccessions, An"yet in the s7perficies of the thir" 5o"y! where it is of any one thic9ness! there are as $any parts for the rays to i$pinge on! as where it is of any other thic9ness,

Lastly! if reflection were ca7se" 5y the parts of reflecting 5o"ies! it wo7l" 5e i$possi5le for thinplates or 5755les! at the sa$e place to reflect the rays of one colo7r! an" trans$it those of another!as they "o accor"ing to the thirteenth an" fifteenth o5ser+ations, 6or it is not to 5e i$agine"! that atone place the rays! which! for instance! exhi5it a 5l7e colo7r! sho7l" ha+e the fort7ne to "ash 7ponthe parts! an" those! which exhi5it a re"! to hit 7pon the pores of the 5o"y@ an" then at another place! where the 5o"y is either a little thic9er! or a little thinner! that on the contrary the 5l7e sho7l"hit 7pon its pores! an" the re" 7pon its parts,

Prop, =, It is $ost pro5a5le! that the rays! which i$pinge on the soli" parts of any 5o"y! are notreflecte" 57t stifle" an" lost in that 5o"y,



;-1)<

#his is consentaneo7s to the prece"ent proposition! an" will f7rther appear 5y consi"ering! that if allthe rays sho7l" 5e reflecte"! which i$pinge on the internal parts of clear water or crystal! thoses75stances sho7l" rather ha+e a clo7"y than so +ery clear transparency,

An" f7rther! there wo7l" 5e no principle of the o5sc7rity or 5lac9ness! which so$e 5o"ies ha+e inall positions of the eye, 6or to pro"7ce this effect! it is necessary! that $any rays 5e retaine" an"lost in the 5o"y! an" it see$s not pro5a5le! that any rays can 5e stoppe" an" retaine" in it! which"o not i$pinge on its parts,



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Hypothesis explaining the Properties of Light

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