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

I noted the place where the Paper was, when the Letters of the Book, illuminated by the fulleft red Light of the folar Image falling up- on it, did cait their Species on that Paper moil diltindly : And then I flay'd till by the Motion of the Sun and confequent. Motion of his Image on the Book, all the Colours from that red to the middle of the blue pafs'd over thofe Let- ters; and when thofe Letters were illuminated by that blue, I noted again the place of the Pa- per when they caft their Species moil diftindlly upon it : And I found that this lail place of the Paper was nearer to the Lens than its former place by about two Inches and an half, or two and three quarters'. So much fooner therefore did the Light in the violet end of the Image by a greater Refradion converge and meet, than the Light in the red end. But in trying this the Chamber was as dark as I could makp it. For if thefe Colours be diluted and Weak- ned by the mixture of any adventitious Light, the diflance between the Places of the Paper will not be fo great. This diliance in the fe- cond Experiment where the Colours of natural Bodies were made ufe of, was but an Inch and an half, by reafon of the imperfection of thofe Colours. Here in the Colours of the Prifm , which are manifeltly more full, intenfe, and live- ly than thofe of natural Bodies, the diltance is two Inches and three quarters. And were the Colours' Hill more full, I queftion not but that the diftance would be confiderably greater. For the coloured Light of the Prifm, by the interr fering'of the Circles defcribed in the fecond

Figure

t45]

Figure of the fifth Experiment, and alfo by the Li'ght of the very bright Clouds next the Sun's Body intermixing with thefe Colours, and by the Light fcattered by the inequalities in the Polifli of the Prifm, was fp very much com- pounded, that the Species which' thofe faint and dark Colours, the indigo and violet, caft upon the Paper were not diitinft enough to be well obferved.

Exfer. 9. A Prifm, whofe two Angles at its Bafe were equal to one another and half right ones, and the third a right one, I placed in a beam of the Sun's Light let into a dark Cham« ber through a hole in the Window-fliut as in the third Experiment. And turning the Prifm flowly about its Axis until all the Light which went through one of its Angles and was refra- cted by it began to be refleded by its Bafe, at which till then it wxnt out of the Glafs, I ob- ferved that thofe Rays which had fufFered the greateit Refraftion were fooner refleded than the reft. I conceived therefore that thofe Rays of the refle^fted Light, which were molt re- frangible , did firft of all by a total Reflexion become more copious in that Light than the reft, and that afterwards the reft alfo, by a to- tal Reflexion, became as copious as thefe. To try this, I made the reflefted Light pafs through another Prifm, and being refracted by it to fall afterwards upon a fheet of white Paper placed at fome diflance behind it, and there by that Refraction to paint the ufual Colours of the Prifm. And then caufing the firft Prifm to be turned about its Axis as above, I obferved that

when

[4^ ]

when thofe Rays which in this Prifm had fuf- fered the greateit Refraction and appeared of a blue and violet Colour began to be totally* re- fleded, the blue- and violet Light on the Paper which was moil refrafted in the fecond Prifm received a fenfible increafe above that of the red and yellow, which was leait refraded ; and afterwards when the reft of the Light which was green, yellow and red began to be totally reflected in the firft Prifm, the Light of thofe Colours on the Paper received as great an in- creafe as the violet and blue had done before. Whence 'tis manifeft, that the beam of Light refleded by the Bafe of the Prifm , being aug- mented firft by the more refrangible Rays and afterwards by the lefs refrangible ones, is com- pounded of Rays differently refrangible. And that all fuch reflefted Light is of the fame na- tiire with the Sun's Light before its Incidence on the Bafe of the Prifm, no Man ever doubt- ed : it being generally allowed, that Light by fuch Reflexions fufTers no alteration in its Mo- difications and Properties. I do not here take notice of any Refractions made in the fides of the firft Prifm, becaufe the Light enters it per- pendicularly at the firft fide , and goes out per- pendicularly at the fecond fide, and therefore fufFers none. So then, the Sun's incident Light being of the fame Temper and Conftitution with his emergent Light, and the laft being compounded of Rays differently refrangible, the firft mufl be in like manner compounded.

Illuflration. In the twenty firft Figure, ABC is the firft Prifm, B C its Bafe, B and C its

equal

[47 ]

equal Angles at the Bafe, each of 4^ Degrees, A its re&ngular Vertex,- F M a beam of the Sun's Light let into a dark Room through a hole F one third part of an Inch broad, M its Incidence on the Bafe of the Prifm, MG a lefs refrafted Ray, MH a more refraded Ray, MN the beam of Light refleded from the Bafe, V X Y the fecond Prifm by which this beam in pafling through it is refrafted, N t the lefs re- fraded Light of this beam, and N / the more refrafted part thereof. When the firft Prifm ABC is turned about its Axis according to the order of the Letters ABC, the Rays MH e- merge more and more obliquely out of that Prifm , and at length after their moft oblique Emergence are refleded towards N, and going on to p do increafe the number of the Rays N /. . Afterwards by continuing the Motion of the firft Prifm, the Rays M G are alfo refleded to N and increafe the number of the Rays N t. And therefore the Light MN admits into its Compofition, firft the more refrangible Rays, and then the lefs refrangible Rays, and yet af- ter this Compofition is of the fame nature with the Sun's immediate Light FM, the Reflexion of the fpecular Bafe B C caufmg no alteration therein.

Exfer. 10. Two Prifms, which were alike in fliape, I tied fo together, that their Axes and oppofite Sides being parallel, they compofed a Parallelopiped. And, the Sun Hiining into my dark Chamber through a little hole in the Win- dow-lhut, I placed that Parallelopiped in his beam at fome diftance from the hole, in fuch a

poiiure 3

[48 ]

poftiire that the Axe§ of the Prifms might be perpendicular to the incident Rays , and that thofe Rays being incident upon the firft Side of one Prifm, might go on through the two contiguous Sides of both Prifms, and emerge out of the lalt Side of the fecond Prifm. This Side being parallel to the firlt Side of the firil Prifm , caufed the emerging Light to be paral- lel to the incident. Then, beyond thefe two Prifms I placed a third, which might refrad that emergent Light, and by that Refradion cafl the ufual Colours of the Prifm upon the oppofite Wall, or upon a ilieet of white Paper held at a convenient diftance behind the Prifm for that refraded Light to fall upon it. After * this I turned the Parallelopiped about its Axis, ^ and found that when the contiguous Sides of the two Prifms becam.e fo oblique to the inci- dent Rays that thofe Rays began all of them to be reflected , thofe Rays which in the third Prifm had fuffered the greateft Refradion and painted the Paper with violet and blue, were ^ firit of all by a total Reflexion taken out of the 1 tranfmitted Light, the reft remaining and oa the Paper painting their Colours of green, yel- low, orange, and red as before ; and afterwards by continuing the Motion of the two Prifms, the refl: of the Rays alfo by a total Reflexion vanifhed in order, according to their degrees of Refrangibility. The Light therefore which emerged out of the two Prifms is conipound- ed of Rays differently refrangible , feeing the more refrangible Rays may be taken out of it while the lefs refrangible remain. But this

Light

[ 49 ]

Light being trajedled only through the parallel Superficies of the two Prifms, if it fuffer'd any change by the Refraftion of one Superficies it loll that impreflion by the contrary 'Refracaon of the other Superficies, and fo being reflor'd to its priftine Conititution became of tl>e fame nature and condition as at firfl before its Inci- dence on thofe Prifms; and therefore, before its Incidence, was as much compounded of Rays differently refrangible; as afterwards.

Illnftration. In the twenty fecond Figure ABC and BCD are the two Prilms tied toge- ther in the form of a Parallelepiped, their Sides B C and C B being contiguous, and their Sides A B and CD parallel. And HJK is the third Prifm, by which the Sun's Light propagated through the hole F into' the dark Chamber, and there pafTmg through thofe fides of the Prifms AB, BC, CB and CD, is refra^ed at O to the white Paper P T, falling there partly upon P by a greater Refradtion, partly upon T by a lefs Refradion, and partly upon R and other in- termediate places by intermediate Refradions. By turning the Parallelepiped ACBD about its Axis, according to the order of the Letters A, C, D, B> at length when the contiguous Planes B Cand C B become fufliciently oblique to the Rays F M, which are incident upon them at M, there will vanifli totally out of the refrafted Light OPT, firfl of all the mofl refrafted Rays O P, (the refl O R and O T remaining as be- fore) then the Rays O R and other intermedi- ate ones, and laftly, the leaft refradled RaysOT, For when the Plane B C becomes fuiiiciently

E oblique

C 50]

oblique to the Rays incident upon it, thofe Rays will begin to be totally refleded by it to- wards N ; and firll the moft refrangible Rays will be totally reflefted (as was explained in the preceding Experiment) and by confeqUence mutt fiiit dilappear at P, and afterwards the reft as they are in order totally refleded to N, they mutt difappear in the fame order at R and T. So then the Rays which at O fuf- fer the greateft Refradion, may be taken out of the Light M O whilll the retl of the Rays remain in it, and therefore that Light M O is compounded of Rays differently refrangi- ble. And becaufe the Planes A B and C D are • parallel, and therefore by equal and con- trary Refradions dettroy one anothers Ef- feds, the incident Light F M mult be of the fame kind and nature with the emergent Light M O, and therefore doth alfo confitt of Rays differently refrangible. Thefe two Lights F M and M O, before the mott refrangible Rays are feparated out of the emergent Light M O, a- gree in colour, and in all other properties fo far as my obfervation reaches, and therefore are defervedly reputed of the fame nature and contlitution , and by confequence the one is compounded as well as the other. But after the moft refrangible Rays begin to be totally retledted, and thereby feparated out of the e- mergent Light M O, that Light changes its co- lour from white to a dilute and faint yellow, a pretty good orange, a very full Ked fuccef- fively and then " totally vaniilies. For after the mott refrangible Rays which paint the Paper at

P with.

[51.]

P with a purple Colour, are by a total Refle- xion taken out of the beam of Light M O, the relt of the Colours which appear on the Paper at R and T being mix'd in the Light M O compound there a faint yellow, and after the blue and part of the green which appear on the Paper between P and R are taken away, the rell which appear between R and T (that is the yel- low, orange, red and a little green) being mix- ed in the beam M O compound there an orange ; and when all the Rays are by Reliexion taken out of the beam M O, except the Icalt" refran- gible, which at T appear of a full red, their Colour is the fame in that beam M O as after- wards at T, the Refraftion of the Prifm HJ K ferving only to feparate the differently refran- gible Rays, without making any alteration in their Colours, as lliall be more fully proved hereafter. All which confirms as well the firlt Propofition as the fecond.

Scholium. If this Experiment and the former be conjoined and made one by applying a fourth Prifm VXY [in Fig. 22.] to refrad the refled:- ed beam MN towards /^/, the conclufion will be clearer. For then the Light N p which in the fourth Prifm is more refraded, will become fuller and flronger when the Light O P, which in the third Prifm H J K is more refraded, va- nifties at P ; and afterwards when the lefs re- fraded Light O T vanillies at T, the lefs re- fraded Light N t will become encreafed whilft. the more refraded Light at p receives no far- ther encreafe. And as the trajeded beam M O in vaniihing is always of fuch a Colour as ought

Ex to

C.52]

to refult from the mixture of the Colours which fall upon the Paper P T, fo is the reflecSted beam MN always of fuch a Colour as ought to refult from the mixture of the Colours which fall upon the Paper p t. For when the moft refrangible Rays are by a total Reflexion taken out of the beam M O, and leave that beam of. an orange Colour, the Excefs of thofe Rays in the refleded Light, does not only make the violet, indigo and blue at p more full, but alfa. makes the beam MN change from the yellowifli Colour of the Sun's Light, to a pale white in- clining to blue, and afterward recover its yel- lovvifn Colour again, fo foon as all the reft of the tranfmitted Light MOT is refleded-

Now feeing that in all this variety of Expe- riments, whether the Trial be made in Light reflected, and that either from natural Bodies, as in the firlt and fecond Experiment, or fpe- cular, as in the ninth ; or in Light refraded, and that either before the unequally refraded Rays are by diverging feparated from one an- other, and lofing their whitenefs which they have altogether, appear feverally of feveral Co- lours, as in the fifth Experiment ; or after they are ieparated from one another, and appear co- lour'd as in the fixth, feventh, and eighth Ex- periments ; or in Light trajeded through paral- lel Superficies, deftroying each others ElFefts, as in the tenth Experiment ; there are always .found Rays, which at equal Incidences on the fame Medium fuffer unequal Refraffions, and that without any fplitting or dilating of fingle Rays, or contingence in the inequality of the

Refra«

C 53 ]

Refraflions, as is proved in the fifth and fixth Experiments. And feeing the Rays wliich dif- fer in Refrangibility may be parted and forted from one another, and that either by Refra- ction as in the third Experiment , or by Re- flexion as in the tenth, and then the feveral forts apart at equal Incidences fuffer unequal Refractions, and thofe forts are more refrafted than others after feparation, which were more refrafted before ir, as in the fixth and folio w- iing Experiments, and if the Sun's Light be tra- ijefted through three or more crofs Prifms fuc- [ceffively, thofe Rays which in the^firfl Prifiu iare refraCled more than others, are in all the following Prifms refracted more than others *iii the fame rate and proportion, as appears by ithe fifth Experiment ; it's manifeil that the iSun's Light is an htrerogeneous mixture of jRays^ fome of which are conltantly more re- frangible than others, as was propofcd.

'P ROT. III. Theor. III.

The Suits Light confifls of Rays differing hi Re- Jiexibility^ and thofe Rays are 7nore reflexible than others ijuhich are more refrangible.

HIS is manifefl by the ninth and tenth Experiments : For in the ninth Experi- ment, by turning the Prifm about its Axis, un- til the Rays within it which in going out into the Air were refraCled by its Bafe, became fo ^oblique to that Bafe, as to begin to be totally

E 3 reflected

[54]

refleded thereby; thofe Rays became firft of all totally refleded, which before at equal In- cidences with the reft had fuffered the greateft Refradion. And the fame thing happens in the Reflexion made by the common Bafe of the two Prifms in the tenth Experiment.

TROT.W, Pro B.I.

To fe^arate from one another the heterogeneous' Rays of compound Light.

' I ^ H E heterogeneous Rays are in fome mea- j fure feparated from one another jby the Retradion of the Prifm in the third Experi- ment, and in the fifth Experiment by taking a- way the Penumbra from the redilinear fides of the coloured Image, that feparation in thofe ve- , ry redilinear fides or ftraight edges of the I-' mage becomes perfed. But in all places be- tween thofe rectilinear edges, thofe innumera- ble Circles there defcribed, which are feveral- ly illuminated by homogeneal Rays, by interfe- ring with one .another, and being every where commix'd, do render the Light fumciently compound. But if thefe Circles, whilft their centers keep their diftances and pofitions, could be made lefs in diameter, their interfering on0!| with another and by confequence the mixture of the heterogeneous Rays would be propor- tionally diminilh'd. In the twenty third Figure let A G, B H, C J, D K, E L, F M be the Cir- cles which fo many forts of Rays flowing froni

C 55]

the fame difque of the Sun, do in the third Experiment illuminate ; of all which and in- numerable other intermediate ones lying in a continual Series between the two reftilinear and parallel edges of the Sun's oblong Image PT, that Image is compos'd as was explained in the fifth Experiment. And let a g-i b h, c /, d'k^ e /, fm be fo many lefs Circles lying in a like continual Series between two parallel right Lines ^/and g m with the fame diitances be- tween their centers , and illuminated by the fame forts of Rays, that is the Circle a g with the fame fort by which the correfponding Cir- cle A G was illuminated , and the Circle b h with the fame fort by which the correfponding Circle BH was illuminated, and the relt of the Circles f /, d k^ e I, f m refpeftively, with the fame forts of Rays by which the feveral corre- fponding Circles C J, D K, EL, F M were il- lutainated. In the Figure P T compofed of the greater Circles, three of thofe Circles A G, B H, CJ, are fo expanded into one another, that the three forts of Rays by which thole Circles are illuminated, together with other innumerable forts of intermediate Rays, are mixed at Q R in the middle of the Circle B H. A-nd the like mixture happens throughout almoll: the whole length of the Figure P T. But in the Figure / t compofed of the lefs Circles, the three lefs Circles ag^ bh, ci, which anfwer to thofe three greater, do not extend into one another ; nor are there any where mingled fo much as any two of the three forts of Rays by which thofe

E 4 ' Circles

I S6-\

Circles are illuminated, and which in the Figure P T are all of them intermingled at B H.

Now he that ihall thus confider it, will eafily underftand that the mixture is diminifhed in the fame proportion with the Diameters of the Circles. ' If the Diameters of the Circles whilil their Centers remain the fame, be made three times lefs than before, the mixture will be alfo three times lefs ; if ten times lefs, the mixture will be ten times lefs, and fo of other propor- tions. That is, the mixture of the Rays in the greater Figure V T will be* to their mixture in the lefs p t^ as the Latitude of the greater Fi- gure is to the Latitude of the lefs. For the Latitudes of thefe Figures are equal to the Di- ameters of their Circles. And hence it eafily follows, that the mixture of the Rays in the re- frafted Spectrum / ^ is to the mixture of the Rays in the dired and immediate Light of the Sun, as the breadth of that Spedrum is to the difference between the length and breadth qf the fame Speftrum.

So then, if we would diminifh the mixture of the Rays, we are to diminifli the Diameters of the Circles. Now thefe would be diminifli- ' ed if the Sun's Diameter to which they anfwer could be made lefs than it is, or (which comes to the fame purpofe) if without doors, at a great diltance from the Prifm towards the Sun, lome opake Body were placed, with a round hole in the middle of it, to intercept all the Sun's Light, excepting fo much as coming from the middle of his Body could pafs througl

[57]

that hole to the Prifm. For fo the Circles A G, BH and the reft, would not any longer anlVer to the whole Difque of the Sun, but only to that part of it which could be feen from the Prifm through that hole, that it is to the appa- rent magnitude of that hole view'd from the Prifm. But that thefe Circles niay anfwer more diftinftly to that hole, a Lens is to be placed by the Prifm to caft the Image of the hole, (that is, every one of the Circles A G, B H, &c.) di- {lindly upon the Paper at P T, after fuch a manner as by a Lens placed at a Window the Species of Objects abroad are caft diftinftly up- on a Paper within the Room, and the redili- near Sides of the oblong folar Image in the fifth Experiment became diftinft without any Pen- umbra. . If this be done it will not be necelfa- ry to place that hole very far off, no not beyond the Window. And therefore inftead of «that hole, I ufed the hole in the Window-fhut as follows.

Exper. II. In the Sun's Light let into my darkned Chamber through a fmall round hole in my Window-lhut, at about ten or twelve Feet from the Window, I placed a Lens, by which the Image of the hole might be diitinc^t- ly caft upon a Sheet of white Paper, placed at the diftance of fix, eight, ten or twelve Feet from the Lens. For according to the diife- rence of the Lenfes.I ufed various diftances, which I think not worth the while to* defcribe. Then immediately after the Lens I placed a Prifm, by which the trajeded Light might be refraded either upwards or fideways^ and there^

■ ' ■ ' ' by

C 58 ]

l)y the round Image which the Lens alone did caft upon the Paper might be drawn out into a long one with parallel Sides, as in the third Ex- periment. This oblong Image I let fall upon another Paper at about the fame diitance from the Prifm as before, moving the Paper either towards the Prifm or from it, until I found the juft diftance where the redilinear Sides of the Image became moft diitind:. For in this cafe the circular Images of the hole which compofe that Image after the fame manner that the Cir- cles a g^ b h, c i, 8cc. do the Figure /> f [in Fig. 23.] were terminated moll diitindly without any Penumbra, and therefore extended into one another the lealt that they could, and by confequence the mixture of the heterogeneous Rays was now the leait of all. By this means I ufedto form an oblong Image (fuch as is/ t) lin-Fig. 13, and 24.] of circular Images of the hole (fuch as are a g, b h^ c i, &c.) and by u- fmg a greater or lefs hole in the Window-fliut, I made the circular Images ag^ b hi ^ /, &c. of which it was formed, to become greater or lefs at pleafure, and thereby the mixture of the Rays in the Image/ 1 to be as much or as little as I defired-

Illuftration. In the twenty fourth Figure, F reprefents the circular hole in the Window- fhut, M N the Lens whereby the Image or Spe- cies of that hole is caft diilindly upon a Paper at'J, ABC the Prifm whereby the Rays are at their emerging out of the Lens refracted from J towards another Paper at / t, and the round Image at J is turned iato an oblong Image / t

falling

C 59 ]

falling on that other Paper. This Image/ t con- fifls of Circles placed one. after another in a re- dilinear order, as was fufficiently explained in the fifth Experiment ; and thefe Circles are e- qual to the Circle J , and confequently anfwer in magnitude to the hole F ; and therefore by diminifhing that hole they may be at pleafure diminifhed, whilft their Centers remain in their places. By this means I made the breadth of the Image / ^ to be forty times, and fometimes fixty or feventy times lefs than its length. As for inflance, if the breadth of the hole F be one tenth of an Inch, and M F the diftance of the Lens from the hole be 12 Feet ; and if/ B or/ M the di- Itance of the Image / t from the Prifm or Lens be 10 Feet, and the refradling Angle of the Prifm be 61 Degrees, the breadth of the I- mage / t will be one twelfth of an Inch and the length about fix Inches, and therefore the length to the breadth as 71 to i, and by confe- quence the Light of this Image 71 times lefs compound than the Siin's dired Light. And Light thus far fimple and homogeneal, is fuffi- cient for trying all the Experiments in this Book about fimple Light. For the compofition of heterogeneal Rays is in this Light fo little that jt is fcarce to be difcovered and perceiv'd by Senfe, except perhaps in the indigo and vio- let. For thefe being dark Colours, do eafily fuffer a fenfible allay by that little fcattering Light which ufes to be refraded irregularly by the inecjualities of the Prifm.

Yet inttead of the circular hole F, 'tis better to fubititute an oblong hole fhap^ like a long

Paral-

I 60 1

parallelogram with its length parallel to the Prifm ABC. For if this hole be an Inch or two long, and but a tenth or twentieth part of an Inch broad, or narrower : the Light of the Image / t will be as fimple as before, or fimpler, and the Image will become much broader, and therefore more fit to have Experiments tried in its Light than before.

Inltead of this parallelogram hole may be fub- ftituted a triangular one of equal Sides, whofe Bafe for initance is about the tenth part of an Inch," and its height an Inch or more. For by this means, if the Axis of the Prifm be parallel to the Perpendicular of the Triangle, the Image £t [in Fig. 15-.] will now be form'd of equi- crural Triangles a g, bhy c i^ dk, e /, fm^ &c. and innumerable other intermediate ones an- fwering to the triangular hole in Ihape and big- nefs, and lying one after another in a continual Series between two parallel Lines a f and g m, Thefe Triangles are a little intermingled at their Bafes but not at their Vertices, and therefore the Light on the brighter fide /?/of the Image where the Bafes of the Triangles are, is a little compounded, but on the darker fide gm is al- together uncompounded , and in all places be- tween the fides the Compofition is proportio- nal to the diflances of the places from that ob- fcurer fide g m. And having a Spedrum / t of fuch a Compofition, we may try Experiments either in its flronger and lefs fimple Light near the fide ^/, or in its weaker and fimpler Light pear the other fide g nty as it Ihall feem mofl convenient. •

But

[61 ]

But in making Experiments of this kind the Chamber ought to be made as dark as can be, left any foreign Light mingle it felf with the Light of the Spedrum/r, and render it com- pound ; efpecially if we would try Experiments in the more fimple Light next the fide g m of the Spedrum ; which being fainter, will have a lefs proportion to the foreign Light, and fo by the mixture of that Light be more troubled and made more compound. The Lens alfo ought to be good, fuch as may ferve for opti- cal ufes, and the Prifm ought to have a large Angle, fuppofe of 65- or 70 Degrees, and to be well wrought, being made of Glafs free from bubbles and veins, with its Sides not a Httle convex or concave, as ufually happens, but truly plane, and its Polifli elaborate, as in working Optick-glalTes, and not fuch as is ufually vi^roughc with Putty, whereby the edges of the Sand- holes being worn away, there are left all over the Glafs a numberlefs cornpany of very little convex polite Rifmgs like Waves, The edges alfo of the Prifm and Lens fo far as they may make any irregular Refradion, muft be covered with a black Paper glewed on. And all the Light of the Sun's beam let into the Chamber ' which is ufelefs and unprofitable to the Experi- ment, ought to be. intercepted with black Pa- per or other black Obftacles. For otherwife the ufelefs Light being reflec^ted every way in the Chamber, will mix with the oblong Spe- ftrum and help to difturb it. In trying thefe' things fo much diligence is not altogether ne- cellary, but it will promote the fuccefs of the

Expe-

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Experiments, and by a very fcrupulous Examin- er of things deferves to be applied. It's difficult to get Glafs Prifms fit for this purpofe, and therefore I ufed fometimes prifmatick VefTels made with pieces of broken Looking-glafTes, and filled with Rain Water. And to increafe the Refraftion, I fometimes impregnated the Water itrongly with S ace bar urn Saturnt.

T ROT.Y. Theor.IV.

Homogeneal Light is refracted regularly with" out any dilatation fplitting or /battering 'of the Rays^ and the confufed Vifion of Objedis feen through re framing Bodies by heterogeneal Light arijes from the different Refrangibility of fever al forts of Rays.

THE firfl part of this Propofition has been already fufficiently proved in the fifth Ex- periment, and will farther appear by the Experi- ments which follow.

Exper, II. In the middle of a black Paper I made a round hole about a fifth or fixth part of an Inch in diameter. Upon this Paper I caufed the Speftrum of homogeneal Light defcribed in the former Propofition, fo to fall, that fome part of the Light might pafs through the hole of the Paper. This tranfmitted part of the Light I refrafted with a Prifm placed behind the Paper, and letting this refrafted Light fall perpendicularly upon a white Paper two or three Feet dillant from the Prifm, I found that

the

C ^3 3

the Speftrum formed on the Paper by this Light was not oblong, as when 'tis made (in the third Experiment) by refrading the Sun's compound Light, but was (fo far as I could judge by my Eye) perfectly circular, the length being no greater than the breadth. "Which fliews that this Light is refraded regularly without any Di- latation of the Rays.

Exper. 13. In the homogeneal Light I placed . a Paper Circle of a quarter of an Inch in diameter, and in the Sun's unrefrafted heterogeneal white Light I placed another Paper Circle of the fame bignefs. And going from the Papers to the diltance of fome Feet, I viewed both Circles through a Prifm. The Circle illuminated by the Sun's heterogeneal Light appeared very ob- long as in the fourth Experiment, the length being many times greater than the breadth ; but the other Circle illuminated with homogeneal Light appeared circular and didindly defined as when 'tis view'd with the naked Eye. Which proves the whole Propofition.

Exper. 14. In the homogeneal Light I placed Flies and fuch like minute Objeds, and view- ing them through a Prifm, I faw their parts as diltindly defined as if I had viewed them with the naked Eye. The fame Objefts placed in the Sun's unrefraded heterogeneal Light which was white I viewed alfo through a Prifm, and faw them molt confufedly defined, fo that I could not diltinguifli their fmaller parts from one another. I placed alfo the Letters of a fmall print one while in the homogeneal Light and then in the heterogeneal, and viewing them

through

[^4]

through a Prifm, they appeared in the latter cafe fo confufed and indillinft that I could not read them ; but in the former they appeared fo diitind that I could read readily, and thought I favv them as diflin^l as when I view'd them with my naked Eye. In both cafes I view'd the fame Objeds through the fame Prifm at the fame diftance from me and in the fame fitua- tion. There was no difference but in the Light by which the Objefts were illuminated, and which in one cafe was limple and in the other compound, and therefore the diftinft Vifion in the former cafe and confufed in the latter could arife from nothing elfe than from that difference of the Lights. Which proves the whole Pro- pofition.

And in thefe three Experiments it is farther very remarkable, that the Colour of homoge- neal Light was never changed by the Refra- dion.

T ROT. VL Theor.V.

The Sine of Incidence of every Ray conjidered a* party is to its Sine of Refra5iion in a gtveti Ratio,

"^H AT every Ray confider'd apan*t is con-

llant to it felf in fome degree of Refran-^

gibility, is fufficiently manifell out of what has been laid. Thofe Rays which in the firft Re- fraftion are at equal Incidences moft refrafted,^ are alfo in the following Refradions at equa!

Inci-

r<^5]

Incidences tnofl refrafted ; and fo of the leaft refrangible, and the reft which have any mean degree of Refrangibility, as is manifeft by the fifth, fixth, feventh, and eighth, and ninth Ex- periments. And thofe which the firft time at like Incidences are equally refrafted, are again at like Incidences equally and uniformly refra- ded , and that whether they be refradled be- fore they be feparated from one another as in the fifth Experiment, or whether they be re- fraded apart, as in the twelfth, thirteenth and fourteenth Experiments. The Refradion there- fore of every Ray apart is regular* and what Rule that Refradtion obferves we are now to iliew.

The late Writers in Opticks teach, that the Sines of Incidence are in a given Proportion to the Sines of Refradion, as was explained in the fifth Axiom : and fome by Initruments fit- ted for meafuring of Refradions, or otherwife experimentally examining this Proportion, do acquaint us that they have found it accurate. But whillt they , not underftanding the diffe- rent Refrangibility of feveral Rays, conceived them all to be refrafted according to one and the fame Proportion, 'tis to be prefumed that they adapted their meafures only to the middle of the refraded Light ; fo that from their mea- fures we may conclude only that the Rays which have a mean degree of Refrangibility, that is thofe which when feparated from the reft appear green, are refraded according to a given Proportion of their Sines. And there- fore we are now to fhew that the like given

F Pro-

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