Opticks (1721, 3rd Edition) — part 1 of 18

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THE LIBRARY

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THE UNIVERSITY

OF CALIFORNIA

LOS ANGELES

GIFT

Dr. K. N. Beigelman

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OPTICKS:

OR, A i^'}pqi^

TREATISE

OF THE

Kefled'ions , Refracfions Inflexions and Colours

O F

LIGHT.

The Third Edition y_ Con eel ed.

By Sir Isaac Newton, Knt.

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L O N B O N :

Printed for William and John Inny5 at the] Weft End of St, Taul's, 'J'l.

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Ad-VERTISEMENT

ART of the enfiung D/f- cotirfe about Light was written at the dejire of fome Gentlemen of the Royal So- ciety, ht the tear 1675, and _ then fent to their Secretary, and read at their Meetings, and the reft was added about twelve Tears after to complete the Theory ; ex- cept the third Book, and the laft Fropofition of the Second, which were fince put together out of fcatterd Papers. To avoid be- ing engaged in Difputes aboiit

2 thefe

Advertifement

tljcfe Matters, I have hitherto de- layed the printing, and fiould fiill have delayed it, had not the Importunity of Friends prevailed upon me. If any other Papers writ on this Stiljeci are got out of my Hands they are imperfeB, and were perhaps written hefore I had tried all the Experiments here fet down , and fully fatisfi- ed 7ny felf ahoiit the Laws of Re- fractions and Compofition of Co- lours ^ I have here publijlod what I think proper to come abroad, wijlnng that it may not l?e tranjla- ted into another Language with- out my Confent.

The Crowns of Colours, which fometimes appear about the Sun and Moon, • / have endeavoured to give an jlccount of; but for

want

Advertifement.

want of fufficient Ohfervattons leave that Matter to he farther examined. The SuljeB of the Third Book I haw alfo left im- perfeB, not having tried all the Experi7nents which I intended when I was aloitt thefe Matters^ nor repeated fome of thofe which I did try, tintil I had fatisfied my felf alout all their Circitm- fiances. To communicate what I have tried, and leave the reji to others for farther Enquiry, is all my Defign in piihlijJnng thefe Papers.

In a Letter written to Mr. Leib- nitz/// the Tear 1619, and puh lijljedly jDr.Wallis, Imentiorid a Method hy which I had found fome general TIjeorems ahout fquaring Curvilinear Figures,

or

Advertifement.

or comparing them with the Co- 711C Semons, or other the fimplejl Figures with which they may he compared. Afid fome Tears ago I lent out a Mannfcript contain- ing fuch Theorems^ and having fince met with fome Things copi- ed out of it, I have on this Occa- fion made it piiMick, prefixing to it an Introdiidiion, and f unjoin- ing a Scholium concerning that Method, And I have joined with it another fmall Tract concerning the Qirvilinear Figures of the Second Kind, which was alfo written many Tears ago ^ and made known to fome Friends^ who have folicited the making it piMich *

ilpril T. T

1704. X<i

Advertisement

N this Second Edition of thefe Opticks I have o- mitted the Mathematical TraBs puUifid at the End of the former Edition^ as not l?e- longing to the SiibjeSi And at the End of the Third Book I have added fome ^leflions. And to JJjew that I do not take Gra- vity for an EJfential Property of Bodies, I have added one ^le- ftion concerning its Cat/fe, chii- fing to propofe it hy way of a ^tejiion, lecanfe I am not yet fatisfied aloiit it for want of Experiments.

July 1 5.

I. N.

errata;

Pap. 341. lin. 6. for [even, read [even and a half. lb. lin. 8. for 14, read 15. lb. lin. 10. for 21, zS or 35, read iz4-, 30 or 38. lb. lin. 12. for 70, 140, no. read 76, 152, 228. Pag. 366.

1. 22. for height read dijlance..

r I ]

THE

FIRST BOOK

O F

OPTICKS.

PART I.

Y Defign in this Book is not to ex- plain the Properties of Light by Hy- pothefes, but to propofe and prove

them by Reafon and Experiments: In

order to which I fliall premile the following Definitions and Axioms.

B

DEFL

[^

DEFINITIONS. D E F I N. I.

T tJoe Rays of Light I underflmtd its lea ft

\ ^artSy and thofe as well SucceJJive in the

fame lines as Contemporary in fever al lines. For it is manifeft that Light confiits of parts both Succeffive and Contemporary ; becaufe in the fame place you may ftop that which comes one moment, and let pafs that which comes pre- fently after; and in the fame time you may flop it in any one place, and let it pafs in any other. For that part of Light which is flopt cannot be the fame with that which is let pafs. The ieaft Light or part of Light, which may be llopt alone without the reit of the Light, or propagated alone, or do or fuifer any thing alone which the refl of the Light doth not or fuffers not, I call a Ray of Light.

D E F I N. IL

Refrangibility of the Rays of Lights is their 'Difpojition to be refracted or turned out of their IVay in paffing out of one tranfpare'nt Body or Medium into another. And a greater or lefs Re- frangibility of Rays, is their T>ifpoJition to be turned more or lefs out of their Way in like In- cidences on the fame Medium. Mathematicians ufually confider the. Rays of Light to be Lines reaching from the luminous Body to the Body illuminated, and the refradion of thofe Rays to be the bending or breaking of thofe lines in

their

[3 ]

their pading out of one Medium into another. And thus may Rays and Refradions be confi- dered, if Light be propagated in an inftant. But by an Argument taken from the JEquz^ tions of the times of the Edipfes of Ji^piter's Satellites it Teems that Light is propagated in time, {pending in its pallage from the Sun to us <about feven Minutes of time: And therefore I have chofen to define Rays and Refradions in fuch general terms as may agree to Light in both cafes.

D E F I N. III.

Re flexibility of Rays, is their T>ifpofition to be refleiiedor turned back into the fame Medium from any other Medium up07i whofe Surface they fall. And Rays are more or lefs re flexible^ which are turned back more or lefs eafily. As if Light pafs out of Glafs into Air, and by being inchned more and more to the common Surface of the Glafs and Air, begins at length to be totally re- lleded by that Surface ; thofe forts of Rays which at like Incidences are refleded moll co- pioufly, or by inclining the Rays begin fooneft to be totally refleded, are moft reflexible.

D E F I N. IV.

The Angle of Incidence is that Angle, which the Line defcribed by the incident Ray contains with the Perpendicular to the refleBing or re- fradiing Surface at the Toint of Incidence.

B X D E F I N.

C 4 ]

D E F I N. V.

The Angle of Reflexion or Refradiion^ is the Angle which the line defer ibed by the refleE^ed cr refraCied Ray containeth with the 'Perpendi- cular to the refleBing or refraUing Surface at the Toint of Incidence.

D E F I N. VI.

The Sines of Incidence, Refle:}iion, and Re fra- il ion, are the Sines of the Angles of Incidence^ Reflexion, and Refra^ion.

D E F I N. VII.

The Light whofe Rays are all alike Refran- gible, I call Simple , Homogeneal and Similar ; and that whofe Rays are feme more Refrangible than others, I call compound, Heterogeneal and 'Difflmilar. The former Light I call Homoge- neal, not becaufe I would affirm it fo in all re- fpeds ; but becaufe the Rays which agree in Re- frangibility, agree at leaft in all thofe their other Properties which I confider in the following Difcourfe.

D E F I N. VIII.

The Colours of Homogeneal Lights, I call Pri- mary, Homogeneal and Simple, and thofe of Heterogeneal Lights, Heterogeneal and Com- pound. For thefe are always compounded of the colours of Homogeneal Lights ; as will ap* pear in the following Difcourfe.

AXIOMS.

[5]

AXIOMS.

A X. I.

THE Angles- of Reflexion^ and RefraUton^ lie hi one and the fame Tlane with the Angle of Incidence.

A X. II.

The Angle of Reflexion is equal to the Angle of Incidence,

A X. III.

If the RefraBed Ray be returned diredily back to the Toint of Incidence ., it fhall be re- framed into the Line before defer ibed by the iu' cident Ray.

A X. IV.

Refra5fion out of the rarer Medium into the denfer, is made towards the Perpendicular^ that is., fo that the Angle of Refraction be lefs than the Angle of Incidence,

A X. V.

The Sine of Incidence is either accurately or very nearly in a given Ratio to the Sine of Re- fradi'ton.

Whence if that Proportion be known in any one Inclination of the incident Ray, 'tis known in all the Inclinations, and thereby the Refra- ftion in all cafes of Incidence on the fame refra- fting Body may be determined. Thus if the

B 3 B.efra^

1^1

-Refradion be made out of Air into Water, the Sine of Incidence of the red Light is to the Sine of its Refraction as 4 to 3. If out of Air into Gkfs, the Sines are as 17 to n. In Light of other Colours the Sines have other Proportions : but the difference is fo httle that it need feldom be confidered.

Suppofe therefore, that RS [in Fig. i.] repre- fents the Surface of llagnating Water, and that C is the point of Incidence in which any Ray coming in the Air from A in the Line A C is relieved or refrafted, and I would. know whither this Ray fhall go after Reflexion or Refradion : I eredt upon the Surface of the Water from the point of Incidence the Perpendicular C P and produce it downwards to Q, and conclude by the firfl Axiom , that the Ray after Reflexion and Re- fradion, lliall be found fomewhere in the Plane of the Angle of Incidence AC P produced. I let fall therefore* upon the Perpendicular C P the Sine of Incidence AD; and if the refleded Ray be defired , I produce AD to B f o that DB be equal to AD, and draw C B. For this Line CB Ihall be the refleded Ray ; the Angle of Reflexion BCPand its Sine BD being e- qual to the Angle and Sine of Incidence, as they ought to be by the fecond Axiom. But if the' refraded Ray be defired, I produce A D to H, fo that D H may be to A D as the Sine of Re- fradion to the Sine of Incidence, that is (if the Light be red) as 3 to 4; and about the Center C and in the Plane ACP with the Radius C A defcribing a Circle A BE, I draw Parallel to the Perpendicular C P Q, the Line H E cutting the

Circum-

C7]

Circumference in E, and joyningCE, this Line C E iliall be the Line of the refrafted Ray. For if E F be let fall perpendicularly on the Line PQ, this Line EF fliall be the Sine of Re- fraction of the Ray CE, the Angle ofRefradtion being E C Q ; and this Sine E F is equal to D H, and confequently in Proportion to the Sine of Incidence A D as 3 to 4.

In like manner, if there be a Prifm of Glafs (that is a Glafs bounded with two Equal and Parallel Triangular ends, and three plain and well poliflied Sides, which meet in three Parallel Lines running from the three Angles of one end to the three Angles of the other end) and if the Refradion of the Light in paffing crofs this Prifm be defired : Let ACB [in Fi^. i.] reprefent a Plane cutting this Prifm tranveruy to its three Parallel lines or edges there where the Light pafleth through it, and let D E be the Ray inci- dent upon the firlt lide of the Prifm AC where the Light goes into the Glafs ; and by putting the Proportion of the Sine of Incidence to the Sine of Refradion as 17 to 11 find EF the firii refracted Ray. Then taking this Ray for the Incident Ray upon the fecond fide of the Glaf? B C where the Light goes out, find the next refra6led Ray EG by putting the Proportion of the Sine of Incidence to the Sine of Re- fraftion as 11 to 17. For if the Sine of Inci- dence out of Air into Glafs be to the Sine of Refradion as 17 to 11, the Sine of Incidence out of Glafs into Air mud on the contrary be to the Sine of Refradion as 11 to 17, by the third! Axiom.

B 4 Much

[8]

Much after the fame manner, if ACBD [in Fig. 3.'] reprefcnt a Glafs fpherically Convex on both fides (ufually called a Lefis, fuch as is a Burn- ing-glafs, or Spedacle glafs, or an Objed glafs of a Telefcope) and it be required to know how Light falling upon it from any lucid point Q fliall be refraded, let Q M reprefent a Ray falling upon any point M of its firll fpherical Surface A C B, and by ere61jng a Perpendicular to the Glafs at the point M, find the firft re- frafted Ray M N by the Proportion of the Sines 17 to 11. Let that Ray in going out of the Glafs be incident upon N, and then find the fecond refraded Ray N ^ by the Proporti- on of the Sines 11 to 17. And after the fame- manner may the Refradion be found when the Lens is Convex on one fide and Plane or Con- cave on the other, or Concave on both fides.

A X. VI.

TJomogeneal Rays which flow from fever al "Joints of any ObjeBy and fall perpendicularly or almoft perpendicularly on any reflecting or refr ail- ing ^lane or fpherical Surface^ Jhall afterwards diverge fromfo many other Toints, or be Parallel to fo many other Lines y or converge to fomojiy other T'oints, either accurately or without any fenfible Error. And the fame thing will happen^ if the Rays be reflected or refradted fucceflively by two or three or more ^lane or Spherical Surfaces.

The Point from which Rays diverge or to which they converge may be called their Focus. And the Focus of the incident Rays being gi- ven, that of the refleded or refraded ones may

be

en

be found by finding the Rcfra^lion of any two Rays, as above; or more readily thus.

Caf. !.• Let A C B [in Fig. 4.] be a receding or refracting Plane, and Q the Focus of the incident Rays, and Q ^ Ca perpendicular to that Plane. And if this perpendicular be produced to ^, fo that ^ C be equal to Q C, the point q, fliall . be the Focus of the refleded Rays. Or if f G be taken on the fame fide of the Plane with Q C and in Proportion to Q C as the Sine of Incidence to the Sine of Rcfra6tion, the point q fliall be the Focus of the refraded Rays.

Caf. 2. Let AC B [in Fig. 5-.] be the refleding Surface of any Sphere whofe Center is E. Bi- feft any Radius thereof (fuppofe EC.) in T, and if in that Radius on the fame fide the poinc T you take the Points Q and ^, fo that TQ, TE, and T ^, be continual Proportionals, and the point Q be the Focus'of the incident Rays, the pointy lliall be the Focus of the refleded ones.

Caf I. Let ACB [in Fig. 6.] be the refrading Surface of any Sphere whofe Center is E. In any Radii^s thereof E C produced both ways . take E T and C t equal to one another and fe^ verally in fuch Proportion to that Radius as the leder of the Sines of Incidence and Re- fradion hath to the difference of thofe Sines. And then if in the fame Line you find any two Points Q and q, fo that TQ be to ET as E t to t q^ taking t q the contrary way from t which TQ lieth from T, and if the Point Q be the Focus of any incident Rays, the Point q fliiill be the Focus of the refraded ones.

And 3

[lO]

And by the fame means the Focus of the Rays after two or more Reflexions or Refrac- tions may be found.

Caf.^. Let ACBD [in Fig. 7.] be any refrac- ting Lens, fpherically Convex or Concave or Plane on either fide, and let CD be its Axis (that is the Line which cuts both its Surfaces perpendicularly, and palTes through the Centers , of the Spheres, ) and in this Axis produced let F and/be the Foci of the refraded Rays found as above, when the incident Rays on both fides the Lens are Parallel to the fame Axis ; and upon the Diameter F/bifeded in L, defcribe a Circle. Suppofe now that any Point Q be the Focus of any incident Rays. Draw Q E cutting the faid Circle in T and t, and therein take t q in fuch Proportion to ^ E as ^^ E or T E hath to T Q. Let t q lye the contrary way from t which TQ doth from T, and q fhall be the Focus of the refrafted Rays without any fenfible Error, pro- vided the Point Q be not fo remote from the Axis, nor the Lens fo broad as to make any of the Rays fall too obliquely on the refratling Surfaces.

And by the like Operations may the refleding or refrading Surfaces be found when the two Foci are given, and thereby a Lens be formed, which fhall make the Ravs flow towards or from what place you pleafe.

So then the meaning of this Axiom is, that if Rays fall upon any Plane or Spherical Surface or Lens, and before their Incidence flow from or towards any Point Q, they ihall after R«- fiexion or Refradion flow from or towards the

Point

[ " ]

Point q found by the foregoing Rules. And if the incident Rays flow from or towards feveral points Q, the refleded or refraded Rays ihall flow from or towards fo many other Points q found by the fame Rules. Whether the reflect- ed and refraded Rays flow from or towards the Point q is eafily known by the fituation of that Point. For if that Point be on the fame fide of the refle61ing or refrading Surface or Lens with the Point Q, and the incident Rays flow from the Point Q, the refleded flow towards the Point q and the refraded from it ; "and if the incident Rays flow towards Q, the reflected flow from q^ and the refraded to'wards it. And the contrary happens when q is on the other fide of that Surface.

A X. VII.

Wherever the Rays which come from all the Joints of any Obje£i meet again in fo many Totnts after they have been made to converge by Reflexion or RefraBion, there they will make a TiBure of the Object upon any white Body on which they fall.

So if PR [in Fig. 3.] reprefent any Objed: with- out Doors, and A B be a Lens placed at a hole in the Window-fliut of a dark Chamber, where- by the Rays that come from any Point Q of that Objeft are made to converge* and meet a- gain in the Pointy; and if a Sheet of white Pa- per be held at q for the Light there to fall up- on it: the Pidure of that Objed PR will ap- pear upon the Paper in its proper lliape and Co- lours.

C 12 ]

lours. For as the Light which comes from the Point Q goes to the Point ^, fo the Light which comes from other Points P and R of the Objeft, will go to fo many other correfpondent Points / and r (as is manifell by the fixth Axiom ;) fo that every Point of the Objed fhall illuminate a coirefpondent Point of the Pidure, and there- by make a Pidure like the Objed in Shape and Colour, this only excepted, that the Pidure fhall be inverted. And this is the reafon of that vulgar Experiment of calling the Species of Ob- jcds from abroad upon a Wall or Sheet of white Paper in a dark Room.

In like manner, when a Man views any Objeft PQR, [in Fig.. 8.] the Light which comes from the feveral Points of the Objeft is fo refraded by the tranfparent skins and humours of the Eye, (that is by the outward coat EFG called the Tunica Cornea^ and by the cryflalline hu- mour A B which is beyond the Pupil m k) as to converge and meet again at fo many Points in the bottom of the Eye, and there to paint the Pidure of the Objed upon that skin (called the Tunica Retina) with which the bottorn of the Eye is covered. For Anatomifts when they have taken off from the bottom of the Eye that out* ward and moil thick Coat called the T)ura Ma- ter, can then fee through the thinner Coats, the Piftures of Objeds Hvely painted there- on. And thefe Pidures propagated by Mo- tion along the Fibres of the Optick Nerves in- to the Brain, are the caufe of Vifion. For ac- cordingly as thefe Piftures are perfed or im- perfed, the Objed is feen perfedly or imperfed-

I 13 ]

ly. If the Eye be tinged with any colour (as in the Difeafe of the Jaund'tfe) {o as to tinge the Figures in the bottom of the Eye with that Colour, then all Objeds appear tinged with the fame Colour. If the humours of the Eye by old Age decay, fo as by Ihrinking to make the Cornea and Coat of the Cryfialline humour grow flatter than before, the Light will not be re- fracted enough, and for want of a fuHicient Re- fra61ion will not converge to the bottom of the Eye but to fome place beyond it, and by con- fequence paint in the bottom of the Eye a con- fufed Pidure, and according to the indiflind- nefs of this Pidure the Objed will appear con- fufed. This is the reafon of the decay of fight in old Men, and iliews why their Sight is mend- ed by Spedacles. For thofe Convex glafles fup- ply the defed of plumpnefs in the Eye, and by encreafmg the Refradion make the Rays con- verge fooner fo as to convene diftindly at the bottom of the Eye if the Glafs have a due de- gree of convexity. And the contrary happens in fhort-fighted Men whofe Eyes are too plump. For the Refradion being now too great, the Rays converge and convene in the Eyes before they come at the bottom; and therefore the Pidure made in the bottom and the Vifion caufed thereby will not be diitind, unlefs the Objed be brought fo near the Eye as that the place where the converging Rays convene may be removed to the bottom, or that the plump- nefs of the Eye be taken off and the Refradi- ons diminiflied by a Concave-glafs of a due de- gree of Concavity, or laftlv that by Age the

Eye

C 14]

Eye grow flatter till it come to a due Figure: For ihort-fighted Men fee remote Objeds beft in Old Age, and therefore they are accounted to have the moit lading Eyes.

A X.. VIIL

An ObjeEi feen by Reflexion or RefraBton.^ af fears in that place from whence the Rays af- ter their laft Reflexion or RefraBion diverge in- falling on the SpeEiators Eye.

If the Objed A [in Fig. 9.] be feen by Reflexion of a Looking-glafs m n^ it Ihall appear, not in its proper place A, but behind the Glafs at a^ from whence any Rays AB, AC, AD, which flow from one and the fame Point of the Objed:, do after their Reflexion made in the Points B, C, D, di- verge in going from the Glafs to E, F, G, where they are incident on the Spectator's Eyes. For thefe Rays do make the fame Pidure in the bottom of the Eyes as if they had come from the Objed really placed at a without the inter- pofition of the Looking glafs ; and all Vifion is made according to the place and ihape of that Pidure.

In like manner the Obje6l D [in Fig. x.] feen through a Prifm, appears not in its proper place D, but is thence tranflated to fome other place d fituated in the laft refraded Ray F G drawn backward from F to ^.

And fo the Objed Q [in Fig. 10.] feen through the Lens AB, appears at the place q from whence the Rays diverge in pafling from the Lens to the F/ye. Now it is to be noted, that the Image of

the

C 15]

the Objeft at q is fo much bigger or leiTer than the Object it felf at Q, as the diibnce of the Image at q from' the Lens A B is bigger or lefs than the ditlance of the Objedl at Q from the. fame Lens. And if the Objed be feen through two or more fuch Convex or Concave-glalTes, every Glafs fliall make a new Image, and the Objed lliall appear in the place and of the big- nefs of the lalt Image. Which confideration un- folds the Theory of Microfcopes and Telefcopes. For that Theory confifts in almoft nothing elfe than the defcribing fuch GlaHes as fhall make the laft Image of any Objedt as diitind and large and luminous as it can conveniently be made.

I have now given in Axioms and their Ex- plications the fumm of what hath hitherto been treated of in Opticks. For what hath been ge- nerally agreed on I content my felf to alTume under the notion of Principles, in order to what I have farther to write. And this may fuffice for an Introduction to Readers of quick Wit and good Underltanding not yet verfed in Op- ticks : Although thofe who are already acquain- ted with this Science, and have handled GlafTes, will more readily apprehend what followeth.

TROTO'

[ I«]

L

PROPOSITIONS.

TROT. I. Theor. I.

IGHTS 'uuhkh differ in Colour ^ differ alfi in degrees of Refrangibility.

The Proof by Experiments.

Exper. I. I took a black oblong ftifF Paper terminated by Parallel Sides, and with a Per- pendicular right Line drawn crofs from one bide to the other, diflinguiOied it into two e- qiial Parts. One of thefe parts I painted with a red colour and the other with a blew. The Paper was very black, and the Colours intenfe and thickly laid on , that the Phsenomenon might be more confpicuous. This Paper I view'd through a Prifm of folid Glafs, whofe two Sides through which the Light paifed to the Eye were plane and well poliilied, and contained an Angle of about fixty degrees : which Angle I call the refracting Angle of the Prifm. And whilft I view'd it, i held it and the Prifm before a Window in fuch manner that the Sides of the Paper were parallel to the Prifm, and both thofe Sides and the Prifm were parallel to the Horizon, and the crofs Line was alfo parallel to it : and that the Light which fell from the Window upon the Paper made an Angle with the Paper, equal to that Angle which was made witn the fame

Paper

[ 17 .]

Paper by the Light reflefted from it to theE^^e, Beyond the Prifm was the Wall of the Chamber under the Window covered over with black Cloth, and the Cloth was involved in Darknefs that no Light might be refledkd from thence, which in palhng by the edges of the Paper to the Eye, might mingle it felf with the Light of the Paper, and obfcure the Phsenomenon there- of . Thefe things being thus ordered, I ^ound that if the refrafting Angle of the Prifm be turned upwards, fo that the Paper may feem td be lifted upwards by the Refradion, its blue half will be lifted higher by the Refraction than its red half But if the refrading Angle of the Prifm be turned downward , fo that the Paper may feem to be carried lower by the Refra- dion, its blue half, will be carried fomething lower thereby than its red half Wherefore iri both cafes the Light which comes from the blue half of the Paper through the Prifm to the Eye, does in like Circumllances fuffer a greater Refradion than the Light which comes from the red half, and by- contequence is more refrangible.

llluftration. In the eleventh Figure, M-N reprefents the Window, and DE the Paper terminated with parallel Sides DJ and HE, and by the tranfverfe Line FG diftinguiCied into two halfs , the one D G of an intenfely blue Colour, the other FE of an intenfely red. And B A C f ^ ^ reprefents the Prifm whofe refrading Planes K^ b a, and h.C c a meet in the edge of the refrading Angle A ^?, This edge A a being upward^ is parallel both lo

G the

[i8]

the Horizon and to the parallel edges of the Paper D J and HE, and the tranfverfe Line FG is perpendicular to the Plane of the Window. Av\di d e reprelents the Image of the Paper feen hy Hefi'a(^ion upwards in fuch manner that the blue half DG is carried higher to ^^ than the red half F E is to fe, and therefore fuffers a greater Fvefradion. If the edge of the refradl- ing Angle be turned downward, the Image of the Paper will be refraded downward , fuppofe to ^ £, and the blue half will *be refraded lower to «^>' than the red half is to cp g.

Exper. z. About the aforelaid Paper, whofe two halfs were painted over with red and blue, and which was lliff like thin Paltboard, I lapped feveral times a flender thred of very black Silk, in fuch manner that the feveral ' rts of the thred might appear upon the Colours like fo many black Lines drawn over them, or like long and llender dark Shadows call: upon them. I might have drawn black Lines with a Pen, but the threds were fmaller and better defined. This Paper thus coloured and lined I fet againft a Wall perpendicularly to the Horizon, fo that one of the Colours might Hand to the right hand, and the other to the left. Clofe before the Paper at the confine of the Colours below I placed a Candle to illuminate the Paper Itrong- ly: For the Experiment was tried in the Night. • The flame of the Candle reached up to the lower edge of the Paper, or a very little higher. Then at the diftance of fix Feet and one or two Inches from the Paper upon the Floor I ereded a glafs Lens four Inches and a quarter broad,

which

r 1^3

which might colleft the Rays coming from the feveral Points of the Paper, and make them con- verge towards fo many other Points at the fame diftance of fix Feet and one or two Inches on the other fide of the Lens, and fo form the I- mage of the coloured Paper upon a white Pa- per placed there, after the fame manner that a Lens at a hole in a Window cafts the Images of Objeds abroad upon a Sheet of white Paper in a dark Room. The aforefaid white Paper, e- reeled perpendicular to the Horizon and to the Rays which fell upon it from the Lens, I moved fometimes towards the Lens, fometimes from it, to find the places where the Images of the blue and red parts of the coloured Paper appear- ed moff diftinc^t. Thofe places I eafily knew by the Images of the black Lines which I had made by winding the Silk about the Paper, For the Images of thofe fine and {lender Lines (which by reafon of their blacknefs were Hke Shadows on the Colours) were confufed and fcarce vifi- ble, unlefs when the Colours on either fide of each Line were terminated mofl diflindly. No- ting therefore , as diligently as I could , the places where the Images of the red and blue halfs of the coloured Paper appeared moft di- ftind:, I found that where the red half of the Paper appeared diflindl, the blue half appeared confufed , fo that the black Lines drawn upon it could fcarce be feen ; and on the contrary, where the blue half appeared moft diflinft, the red half appeared confufed , fo that the black Lines upon it were fcarce vilible. And between the two places where thele Images appeared

C 2. diflina

F 20 ]

diftin^t there was the diftance of an Inch and a half: the diftance of the white Paper from the Lens, when the Image of the red half of the coloured Paper appeared mod diftind, being greater by an Inch and an half than the diitance of the fame white Paper from the Lens, when the Image of the blue half appeared moft di- ftin^l. In like Incidences therefore of the blue and red upon the Lens, the blue was refrafted more by the Lens than the red, fo as to con- vefge fooner by an Inch and a half, and there- fore is more refrangible.

Illuftrat'ton. In the twelfth Figure, D E fig- nifies the coloured Paper, DG the blue half, FE the red half, MN the Lens, HJ the white Paper in that place where the red half with its black Lines appeared diltindl, and h i the fame Paper in that place where the blue half appear- ed dillind:. The place h i was nearer to the Lens MN than the place H J by an Inch and an half.

Scholium. The fame things fucceed, notwith- flanding that fome of the Circumitances be va- ried : as in the firil Experiment when the Prifm and Paper are any ways inclined to the Hori- zon, and in both when coloured Lines are drawn upon very black Paper. But in the De- fcription of thefe Experiments, I have fet down fuch Circumitances by which either the PhaB- nomenon might be render'd more confpicuous, or a Novice might more eafily try them, or by which I did try them only. The fame thing I have often done in the following Experiments : Concerning all which this one Admonition may

fuffice.

[21 ]

faffice. Now from thefe Experiments it fol- lows not that all the Light of the blue is more Refrangible than all the Light of the red : For both Lights are mixed of Rays diiierently Re- frangible, fo that in the red there are i'ome Hays not lefs Pvcfrangible than thofe of the blue, and in the blue there are fome Rays not more Re- frangible than thofe of the red : But thefe Rays in proportion to the whole Light are but few, and ferve to diminifh the Event of the Expe- riment, but are not able to deilroy it. For if the red and blue Colours were more dilute and weak, the diftance of the Images would be lefs than an Inch and a half ; and if they were more intenfe and full, that didance would be greater, as will appear hereafter. Thefe Experiments may fuffice for. the Colours of Natural Bodies. For in the Colours made by the Rerraftion of Prifms this Propofition will appear by the Ex- periments which are now to follow in the next Proportion.

T ROTAl. Theor.il

The Light of the Sun conjifts of Rays differently Refrangible,

The Proof by Experiments.

Ex{er. 3. TN a very dark Chamber at a round X. hole about one third part of an Inch broad, made in the Shut of a Windoiv I placed a Glafs Prifm, whereby the Beam of the Sun's Light which came in at that hole might

C 3 be

[ 22 ]

be refrafled upwards toward the oppofite Wall of the Chamber, and there form a colour'd I- mage of the Sun. The Axis of the Prifm (that is the Line palling through the middle of the Prifm from one end of it to the other end pa- rallel to the edge of the Refrafting Angle) was in this and the following Experiments perpen- dicular to the incident Rays. About this Axis I turned the Prifm llowly, and faw the refra- &d Light on the Wall or coloured Image of the [Sun firit to defcend, and then to afcend. Between the Defcent and Afcent when the I- mage feemed Stationary, I ftopp'd the Prifm, and fix'd it in that pollute, that it fliould be moved no more. For in that poflure the Re- fractions of the Light at the two fides of the lefrafting Angle, that is at the entrance of the Rays into the Prifm, and at their going out of it, were equal' to one another. So alfoin other Experiments, as often as I would have the Re- fradions on both fides the Prifm to be equal to one another, I noted the place where the Image of the Sun formed by the refraded Light flood Hill between its two contrary Motions, in the common Period of its progrefs andregrefs; and when the Image fell upon that place, I made fall the Prifm. And in this Polture, as the molt convenient, it is to be underllood that all the Prifms are placed in the following Experiments, unlefs where fome other poflure is defcribed. The Prifm therefore being placed in this po- flure, I let the refrafted Light fall perpendicu- larly upon a Sheet of white Paper at the oppo- |ite Wall of the Chamber, and obferved the Fi-

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