Opticks (1721, 3rd Edition) — part 2 of 18

[23 3

gure and Dimenfions of the Solar Image form- ed on the Paper by that Light. This Image was Oblong and not Oval, but terminated with two Reftilinear and Parallel Sides, and two Se- micircular Ends. On its Sides it was bounded pretty dillinftly, but on its Ends very confufed- ly and indiftinftly , the Light there decaying and vanifliing by degrees. The breadth of this Image anfvvered to the Sun's Diameter, and was about two Inches and the eighth part of an Inch, including the Penumbra. For the Image was eighteen Feet and an half diitant from the Prifm, and at this diflance that breadth if di- miniflied by the Diameter of the hole in the Window- fliut, that is by a quarter of an Inch, fubtended an Angle at the Prifm of about half a Degree, which is the Sun's apparent Diame- ter. But the length of the Image was about ten Inches and a quarter, and the length of the Re- dilinear Sides about eight Inches ; and the re- fracting Angle of the Prifm whereby fo great a length was made, was 64 degrees. With a lefs Angle the length of the Image was lefs, the breadth remaining the fame. If the Prifm was turned about its Axis that way which made the Rays emerge more obliquely out of the fecond refradling Surface of the Prifm, the Imnge foon became an Inch or two longer, or more ; and if the Prifm was turned about the contrary way, fo as to make the Rays fall more oblique- ly on the firll refrading Surface, the Image foon became an Inch or two fliorter. And there- fore in tr} ing this Experiment, I was as curi- ous as I could be in placing the Prifm by the

C 4 above-

[ 24]

above-mention'd Rule exadlly in fuch a poijure that the Refradions of the Rays at their emer- gence out of the Prifm might be equal to that at their incidence on it. This Prifm had fome Veins running along within the Glafs from one end to the other, which fcattered fome of the Sun's Light irregularly, but had no fenfible ef- fe6l in increafing the length of- the coloured Spedlrum. For I tried the fame Experiment V/ith other Prifms with the fame Succefs. And particularly with a Prifm which feemed free from fuch Veins, and whofe refrafting Angle was 62,4 Degrees, I found the length of the Irpage 9-+- or 10 Inches at the diftance of 18 4- Feet from the Prifm, the breadth of the hole in the Window-iliut being i of an Inch, as be- fore. And becaufe it is eafy to commit a mi- itake in placing the Prifm in its due poilure, I repeated' the Experiment four or five times, and always found the length of the Image that which is fet down above. With another Prifm of clearer Glafs and better Pohili, which feem- ed free from Veins, and whofe refra<^ling Angle was 63 4 Degrees , the length of this Image at the fame diliance of 18 4 Feet was alfo about 10 Inches, or ro4. Beyond thefe Meafures for about a 7 or 7 of an Inch at either end of the Spe- «ftrum the Light of the Clouds feemed to be a little tinged with red and violet, but fo very faintly, that I fufpec^ted that tincture might ei- ther wholly or in great meafure arife from fome Rays of the Spe61rum fcarrered irregularly by fome inequalities in the Su!.>ltaace and poliih of the Glafsj- and therefore I did not include it in ' ' . • thefe

[25]

thefe Meafures. Now the different Magnitude of the hole in the Window- Ihut, and different thicknefs of the Prifm where the Rays paffed through it, and different inchnations of the Prifm to the Horizon, made no fenfible chan- ges in the length of the Image. Neither did . the different matter of the Prifms make any: for in a Vcffel made of poliilied Plates of Glafs cemented together in the lhap& of a Prifm and filled with Water, there is the like Succefs of the Experiment according to the quantity of the Refraftion. , It is farther to be obferved, that the Rays went on in right Lines from the Prifm to the Image, and therefore at their ve- ■ ry going out of the Prifm had all that Inclina- tion to one another from which the length of the Image proceeded, that is the Inclination of more than two degrees and an half And yet according to the Laws of Opticks vulgarly re- ceived, they could not poffibly be fo much incli- ned to one another. For let E G [in Fig. 13.'] re- prefent the Window-fliut, Fthe hole made there- m through which a beam of the Sun's Light was tranfmitted into the darkned Chamber, and ABC a Triangular Imaginary Plane whereby the Prifm is feigned to be cut tranfverily through the middle of the Light. Or if you pleafe, let ABC reprefent the Prifm it felf, looking 4i- redly towards the Speftator's-Eye with its near- er end: And let XY be the Sun, MN the Pa- per upon which the Solar Image or Speftrum is caft, and PT the Image it felf whofe fides to- . wards v and w are Redilinear and Parallel, and ends towards P and T Semicircular. YKHP

and

[20

and X L J T are two Rays, the firfl of which comes from the lower part of the Sun to the higher part of the Image, and is refraded in the Prifm at K and H, and the latter comes from the higher part of the Sun to the lower part of the Image, and is refradled at L and J. Since the Refradions on both fides the Prifm are e- qual to one another, that is the Refradion at K equal to the Refradion at J, and the Refra- ftion at L equal to the Refradion at H, fo that the Refradions of the incident Rays at K and L taken together are equal to the Refradions of the emergent Rays at H and J taken together: it follows by adding equal things to equal things, that the Refradions at K and H taken together, are equal to the Refradions at J and L taken together, and therefore the two Rays being e- qually refraded have the fame Inclination to one another after Refradion which they had before, that is the Inclination of half a Degree anfwering to the Sun's Diameter. For fo great was the IncUnation of the Rays to one another before Refradion. So then, the length of the Image P T would by the Rules of Vulgar Op- ticks fubtend an Angle of half a Degree at the Prifm, and by confequence be equal to the breadth v w, and therefore the Image would be round. Thus it would be were the two Rays X L J T and Y K H P, and all the reft which fprip the Image V w T v, alike refrangible. And therefore feeing by Experience it is found that the Image is not round but about five times longer than broad, the Rays which go-^ ing to the upper end P of the Image fufFer the*

greateft

[ 27 3

greatefl Refra6lion, mufl be more refrangible than thofe which go to the lower end T, un- lefs the inequality of Refradion be cafual.

This Image or Speftrum P T was coloured, being red at its leall refraded end T, and vio- let at its moft refraded end P, and yellow green and blue in the intermediate Spaces. Which agrees with the firit Propofition, that Lights which differ in Colour do alfo differ in Refrangibility. The length of the Image in the foregoing Experiments I meafured from the fainted and outmoft red- at one end, to the faintelt and outmod blue at the other end, ex- cepting only a little Penumbra, whofe breadth fcarce exceeded a quarter of an Inch, as was faid above.

Exper, 4. In the Sun's beam which was pro- pagated into the Room through the hole in the Window-fliut, at the dillance -of fome Feet from the hole, I held the Prifm in fuch a po-» dure that its Axis might be perpendicular to that beam. Then I looked through the Prifm upon the hole, and turning the Prifm to and fro about its Axis to make the Image of the hole afcend and defcend, when between its two contrary Motions it feemed Stationary, I llopp'd the Prifm that the Refradions of both fides of the refrading Angle might be equal to each other, as in the former Experiment. In this Situation of the Prifm viewing through it the faid hole, I obferved the length of its re- fraded Image to be many times greater than its breadth, and that the moft refraded part thereof appeared violetj the leaft refraded red, ■' ^ • . the.

E 28 ]

the middle parts blue green and yellow in or- der. The fame thing happen'd when I remo- ved the Prifm out of the Sun s Light, and look- ed through it upon the hole (hining by the Light of the Clouds beyond it. And yet if the Refraftion were done regularly according to one certain Proportion of the Sines of Inci- dence and Refradion as is vulgarly fuppofed , the refradted Image ought to have appeared round.

So then, bythefe two Experiments it appears that in equal Incidences there is a confiderable inequality of Refraftions. But whence this in- equaUty arifes, whether it be that fome of the incident Rays are refraded more and others lefs, conftantly, or by chance, or that one and the fame Ray is by Refradion difturbed, fliatrer'd, dilated , and as it were fplit and fpread into many diverging Rays, as Grimaldo fuppofes, *does not yet appear by thefe Experiments, but will appear by thofe that follow.

Exper. s- Confidering therefore, that if in the third Experiment the Image of the Sun fliould be drawn out into an oblong form, ei- ther by a Dilatation of every Ray, oc by any o- ther cafual inequality of the Refradions, the fame oblong Image would by a fecond Refra- dion made fideways be drawn out as much in breadth by the like Dilatation of the Rays, or o- ther cafual inequality of the Reffadions fide- ways, I tried what would be the efFeds of fuch a fecond Refradion. For-tliis end I ordered . all things as in the third Experiment, and then placed a fecond Prifm immediately after the

firft

C 29 ]

firft in a crofs Pofition to it, that it might again refraft the beam of the Sun's Light which came to it through the firll Prifm. In the fiiit Prifm this beam was refraded upwards, and in the fecond Tideways. And I found that by the Re- fr^idion of the fecond Prifm the breadth of the Image was not increafed , but its fuperior part which in the firfl Prifm fufFered the greater Refradion and appeared violet and blue, did again in the fecond Prifm fuffer a greater Re- fradion than its inferior part, which appeared red and yellow, and this without any Dilatation of the Image in breadth.

Illuftrat'ion. .Let S [in Ftg. 14.] reprefent the Sun, F the hole in the Window, ABC the firll Prifm, D H the fecond Prifm, Y the round Image of the Sun made by a direft beam of Light when the Prifms are taken away, PT the oblong Image of the Sun made by that beam pafRng through the firft Prifm alone when the fecond Prifm is taken away, and p t the Image made by the crofs Refradions of both Prifms together. Now if the Rays, which tend to- wards the feveral Points of the round Image Y were dilated and fpread by the Refradion of the firft Prifm, fo that they fliould not any lon- ger go in fmgle Lines to fmgle Points, but that every Ray being fplit, fhattered, and changed from a Linear Ray to a Superficies of Rays di- verging from the Point of Refradion, land ly- ing in the Plane of the Angles of Incidence and Refradion, they fhould go in thofe Planes to fo many Lines reaching almoft from one end of the Image P T to the other, and if that Image

fliouid

[30]

fhould thence become oblong : thofe Rays and their feveral parts tending towards the feveral Points of the Image P T ought to be again di- lated and fpread fideways by the tranfverfe Refradion of the fecond Prifm, fo as to com- . pofe a four fquare Image, fuch as is reprefent- ed at TT^. For the better underllanding of which, let the Image P T be diltinguiflied into five e- qual parts PQK, KQRL, LRSM, MSVN, K V T. And by the fame irregularity that the orbicular Light Y is by the Refraction of the firft Prifm dilated and drawn out into a long Image P T, the Light PQK which takes up a fpace of the fame length and breadth with the Light Y ought to be by the Refradion of the fecond Prifm dilated and drawn out into the long Image -n- qkp^ and the Light KQR L in- to the long Image kqr I, and the Lights LRSM, MSVN, NVT, into fo many other long I- mages Irsm, tnsvn^ nvf\ and all thefe long Images would compofe the four fquare Image 5r7. Thus it ought to be were every Ray dila- ted by Refraflion, and fpread into a triangular Superficies of Rays diverging from the Point of Refradfion. For the fecond Refradion would fpread the Rays one way as much as the firft doth another, and fo dilate the Image in breadth as much as the firft doth in length. And the fame thing ought to happen, were fome Rays cafually refradfed more than others. But the Event is otherwife. The Image P T was not made broader by the Refraction of the fecond Prifm, but only became oblique, as 'tis reprefented at / /, its upper end P being by

th^

[ 31 3

the Refraftion tranflated to a greater diftance than its lower end T. So then the Light which went towards the upper end P of the Image, was (at equal Incidences) more refrafted in the fecond Prifm thaji the Light which tended towards the lower end T, that is the blue and violet, than the red and yellow ; and therefore was more refrangible. The fame Light was by the Refradion of the firfl Prifm tranflated far- ther from the place Y to which it tended before Refradion ; and therefore fuffered as well in the firlt Prifm as in the fecond a greater Refra- ftion than the reft of the Light, and by con- fequence was more refrangible than the refl, even before its incidence on the firft Prifm.

Sometimes I placed a third Prifm after the fecond, and fometimes alfo a fourth after the third, by all which the Image might be often, refraded fideways: but the Rays which were more refrafted than the reft in the firft Prifm were alfo more refra£led in all the reft, and that without any Dilatation of the Image fideways : and therefore thofe Rays for their conftancy of a greater Refradion are defervedly reputed more refrangible.

But that the meaning of this Experiment may more clearly appear, it is to be confidered that the Rays which are equally refrangible do fall upon a circle anfwering to the Sun's Difque. For this was proved in the third Experiment. By a Circle I underftand not here a perfe6t geo- metrical Circle, but any orbicular Figure whofe length is equal to its breadth, and which, as to fenfe, may feem circular. Let therefore A G 6 [in

[ 32 ]

[in Fig. If.] reprefent the Circle which all the moil refrangible Rays propagated from the whole Difque of the Sun, would illuminate and paint upon the oppofite \yall if they were a- lone ; E L the Circle which all the leaft refran- gible Rays would in like rnanner illuminate and paint if they were alone; B H, C J, D K, the Circles which fo many intermediate forts of Rays would fucceffively paint upon the Wall, if they were fmgly propagated from the Sun in fucceffive order, the reft being always in- tercepted ; and conceive that there are other intermediate Circles without number, which innumerable other intermediate forts of Rays would fucceffively paint upon the Wall if the Sun fliould fucceffively emit every fort apart. And feeing the Sun emits all thefe forts at once, they muft all together illuminate and paint in- numerable equal Circles, of all which, being according to their degrees of Refrangibility placed in order in a continual Series, that ob- long Speftrum P T is . compofed which I de- fcribed in the third Experiment. Now if the Sun's circular Image Y [in Fig. 14, 15-.] which is made by an unrefradled beam of Light was by any Dilatation of the fmgle Rays, or by any other irregularity in the Refradion of the firlt Prifm , converted into the oblong Speftrum , FT: then ought every Circle AG, BH, CJ^ ^c. in that Spedrum, by the crofs Refraftion of the fecond Prifm again dilating or otherwife fcattering the Rays as before, to be in Hke man- ner drawn out and transformed into an oblong Figure, and thereby the breadth of the Image

FT

r 33 3

P T would be now as much augmented as the length of the Image Y was before by the Refra- dion of the firil Prifm ; and thus by the Refra- dions of both Prifms together would be form- ed a four fquare Figure/ tt t% as I defcribed a- bove. Wherefore lince the breadth of the Spe- drum P T is not increafed by the Refraflion Tideways, it is certain that the Rays are not fplit or dilated, or otherways irregularly fcat- ter'd by that Refradion , but that every Circle is by a regular and uniform Refraftion tranfla- ted entire into another place , as the Circle A G by the greateft Refradion into the place a gy the Circle B H by a lefs Refradion into the place b h, the Circle C J by a Refradion ftill lefs into the place c /', and fo of the reft ; by which means a new Spedrum p t inclined to the former P T is in like manner compofed of Circles lying in a right Line ; and thefe Circles mull be of the fame bignefs with the former, becaufe the breadths of all the Spedrums Y, P T and p t 2X equal diflances from the Prifms are equal.

I conlidered farther, that by the breadth of the hole F through which the Light enters in- to the dark Chamber, there is a Penumbra made in the circuit of the Spedrum Y, and that Penumbra remains in the redilinear Sides of the Spedrums P T and p t. I placed there- fore at that hole a Lens or Objed-glafs of a Te- lefcope which might caft the Image of the Sun diftindly on Y without any Penumbra at all, and found that the Penumbra of the redilinear Sides of the oblong Spedrums P T and / ^ was

D alfo

[ 34.]

alfo thereby taken away, fo that thofe Sides ap- peared as diftinftly defined as did the Circum- ference of the firlt Image Y. Thus it happens if the Glafs of the Priims be free from Veins, and their Sides be accurately plane and well poUfhed without thofe numberlefs Waves or Curies which ufually arife from Sand-holes a little fmoothed in polifhing with Putty. If the Glafs be only well poHfhed and free from Veins and the Sides not accurately plane but a little Convex or Concave , as it frequently happens ; yet may the three Spedrums Y , P T and / t want Penumbras, but not in equal diftances from the Prifms. Now from this want of Pen- umbras, I knew more certainly that every one of the Circles was refrafted according to fome moil regular, uniform, and conitant law. For if there were any irregularity in the Refraclion, the right Lines A E and G L which all the Cir- cles in the Spedrum P T do touch, could not by that Refraftion be tranflated into the Lines a e and j: / as diftinft and ftraight as they were before, but there would arife in thofe tranflated . Lines fome Penumbra or Crookednefs or Un- dulation, or other fenfible Perturbation con- trary to what is found by Experience. What- foever Penumbra or Perturbation fhould be made in the Circles by the crofs Refradion of the fecond Prifm, all that Penumbra or Pertur- bation would be confpicuous in the right Lines a e and g I which touch thofe Circles. And therefore iince there is no fuch Penumbra or Perturbation in thofe right Lines there mud be none in the Circles. Since the diltahce between

thofe

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thofe Tangents or breadth of the Speftrum is not increased by the Refradions, the Diameters of the Circles are not increafed thereby. Since thofe Tangents continue to be right Lines, e- very Circle which in the firft Prifm is more or lefs refraded , is exadly in the fame propor- tion more or lefs refradted in the fecond. And feeing all thefe things continue to fucceed af- ter the fame manner when the Rays are again in a third Prifm, and again in a fourth refrad- ed fideways, it is evident that the Rays of one and the fame Circle, as to their degree of Re- frangibility continue always uniform and ho- mogeneal to one another, and that thofe of fe- veral Circles do differ in degree of Refrangibi- lity, and that in fome certain and conllant pro» portion. Which is the thing I was to prove.

There is yet another Circumitance or two of this Experiment by which it becomes itill more plain and convincing. Let the fecond Prifm D H [in Fig. i(5.] be placed not im- mediately after the firlt, but at fome diflance from it ; fuppofe in the mid- way between it and the Wall on which the oblong Spedrum P T is cait, fo that the Light from the firil Prifm may fall upon it in the form of an oblong Spedrum tt? parallel to this fecond Prifm, and be refraded fideways to form the oblong . .Spedrum / t upon the Wall. And you will find as before, that this Speftrum / r is inclined to that Spedrum P T, which the firfl Prifm forms"alone without the fecond; theblue ends P and/ being farther diftant from one another" than the red ones T and ^, and by confequence

D 2. that

c sn

that the Rays which go to the blue end ^r of the Image tt? and which therefore futfer the greateft Refraftion in the firlt Prifm, are again in the lecond Prifm more refraded than the reft.

The fame thing I try'd alfo by letting the Sun's Light into a dark Room through two ht- tle round holes F and (p [in Fig. 17.] made in the Window, and with two parallel Prifms ABC and ci/Sy placed at thofe holes (one at each) refrading thofe two beams of Light to the oppofite Wall of the Chamber, in fuch man- ner that the two colour'd hnages P T and MN which they there painted were joined end to end and lay in one ftraight Line, the red end T of the one touching the blue end M of the o- ther. For if thefe two refraded Beams were again by a third Prifm D H placed crofs to the two firit, refraded Tideways, and the Spedrums .thereby tranflated to fome other part of the Wall of the Chamber, fuppofe the Speftrum FT to pt and the Spedrum MN to ;i^ Ji, thefe tranflated Spedrums J^f and mn would not lie in one Itraight Line with their ends contiguous as before, but be broken off from, one another and become parallel, the blue end m of the I- niage mn being by a greater Refradion tran- flated farther from its former place MT, than the red end f of the other Image / t from the fame place M T; which* puts the Propofition pafl difpute. And this happens whether the third Prifm D H be placed immediately after the two firft, .or at a great diflance from them, fo that the Light refraded in the two

firit

[37]

firft Prifms be either white and circular, or cq- loured and oblong when it falls on the third.

Exper. 6. In the middle of two thin Boards I made round holes a third part of an Inch in diameter, and in the Window -fliut a much broader hole being made to let into my dark- ned Chamber a large Beam of the Sun's Light ; I placed a Prifm behind the Shut in that beam to refrad it towards the oppofite Wall, and clofe behind the Prifm I fixed one of the Boards, in fuch manner that the middle of the refradled Light might pafs through the hole made in it, and the reit be intercepted by the Board. Then at the dillance of about twelve Feet from the firit Board I fixed the other Board in fuch man- ner that the middle of the refraded Light which came through the hole in the firft Board and fell upon the oppofite Wall might pafs through the hole in this oiher Board, and the reit being intercepted by the Board might paint upon it the coloured Spectrum of the Sun. And clofe behind this Board I fixed another Prifm to re- fract the Light .which came through the hole. Then I returned fpeedily to the firlt Prifm, and by turning it flowly to and fro about its Axis, I caufed the Image which fell upon the fecond Board to move up and down upon that Board, that all its parts might fucceflively pafs through the hole in that Board and fall upon the Prifm behind it. And in the mean time, I noted the places on -the oppofite Wall to which that Light after its Refradion in the fecond Prifm did pafs ; and by the difference of the places I found that the Light which being mod refraded in the

Da • M

:^-

[ 38 ]

firft Prifm did go to the blue end of the Image, was again more refraded. in the fecond Prifm than the Light which went to the red end of that Image, which proves as v/ell the firlt Pro- portion as the fecond. And this happened whether the Axis of the two Prifms were pa- rallel, or inclined to one another and to the Horizon in any given Angles. • Ilhiftratmt. Let F [in Fig. i8.] be the wide hole in the Window (liut, through which the Sun lliines upon the firil Prifm ABC, and let the refrafted Light fall upon the middle of the Board DE, and the middle part of that Light upon the hole G made in the middle of that Board. Let this trajeded part of the Light fall again upon the middle of the fecond Board d e and there paint fuch an oblong coloured I- mage of the Sun as was defcribed in the third Experiment. By turning the Prifm ABC flow- ly to and fro about its Axis this Image will be made to move up and down the Board d e^ and by this means all -its parts from one end. to the other may be made to pafs fucceflively through the hole g which is made in the mid- dle of that Board. In the mean while another Prifm ^ ^ r is to be fixed next after that hole g to refrad the trajeded Light a fecond time. And thefe things being thus ordered, I marked the places M and N of the oppofite Wall upon which the refrafted Light fell, and found that whilit the two Boards and fecond Prifm re- mained unmoved, thofe places by turning the firil Prifm about its Axis were changed perpe- a tually. For when the lower part of the Light *

which

C 39 3

which fell upon the fecond Board d e was caft through the hole g it went to a lower place M on the Wall, and when the higher part of that Light was caft through the fame hole g^ it went , . to a higher place N on the Wall, and when a- ny intermediate part of the Light was cafl through that hole it went to fome place on the Wall between M and N. The unchanged Po- fition of the Holes in the Boards, made the In- cidence of the Rays upon the fecond Prifm to be the fame in all cafes. And yet in that com- mon Incidence fome of the Rays were more re- fracted and others lefs. And thofe were more refrafted in this Prifm which by a greater Re- fra6lion in the firlt Prifm were more turned out of the way, and therefore for their con- ftancy of being mxore refraded are defervedly called more refrangible.

Exper. 7. At two holes. made near one ano- ther in my Windovv-fiiut I placed two Prifms, one at each, which might cad upon the oppo- *fite Wall (after the manner of the third Expe- riment ) two oblong coloured Images of the Sun. And at a little diflance from the Wall I placed a long flender Paper with flraight and parallel edges, and ordered the Prifms and Pa- .per fo, that the red Colour of one Image might fall diredtly upon one half of the Paper, and the violet Colour of. the other Image upon the o- ther half of the fame. Paper ; fo that the Pa- per appeared of two Colours, red and violet, much after the manner of the painted Paper in the firit and fecond Experiments. Then with a black Cloth I covered the Wall behind

D 4 the

[40 ]

the Paper ; that no Light might be reflefled from it to didurb the Experiment, and view- ing the Paper through a third Prifm held pa- rallel to it, I favv that half of it which was il- luminated by the violet Light to be divided from the other half by a greater Refradion, e- • fpecially when I went a good way off from the Paper. For when I viewed it too near at hand, the two halfs of the Paper did not appear fully divided from one another, but feemed conti- guous at one of their Angles like the painted Paper in the firlt Experiment. Which alfo happened when the Paper was too broad,

Sometimes inilead of the Paper I ufed a white Thred, and this appeared through the Prifm divided into two parallel Threds as is repre- fented in the nineteenth Figure, where D G denotes the Thred illuminated with violet Light from D to E and with red Light from F to G, and e d f g are the parts of the Thred feen by Refraction. If one half of the Thred be con- Itantly illuminated with red, and the other half be illuminated with all the Colours fuccellively, (which may be done by caufmg one of the Prifms to be turned about its Axis whiht the other remains unmoved) this other half in view- ing the Thred through the Prifm, will appear in a continued right Line with the firfl half when ' illuminated with red , and begin to be a little divided from it when illuminated with orange*, and remove farther from it when illuminated with yellow, and itill farther when with green, and farther when with blue, and go yet farther off when illuminated with indigo, and fartheft

when

[41 ]

when with deep violet. Which plainly fliews, that the Lights of feveral Colours are more and more refrangible one than another, in this or- der of their Colom's, red, orange, yellow, green, blue, indigo, deep violet ; and fo proves as well the firlt Propofition as the fecond.

I caufed alfo the coloured Speftrums P T [in Fig. 17.] and MN made in a dark Cham- ber by the Refradions of two Prifms to lye in a right Line end to end, as was defcribed above in the fifth Experiment , and viewing them through a third Prifm held parallel to their length, they appeared no longer in a right Line, but became broken from one another, as they are reprefented at / ^ and mji, the violet end m of the Spedrum m n being by a greater Refra- ction tranflated farther from its former place M T than the red end t of the other Spedrum

I farther caufed thofe two Spedrums PT [in Fig. xo.] and M N to become co-incident in an inverted order of their Colours, the red end of each falling on the violet end of the 0- ther, as they are reprefented in the oblong Fi- gure P T M N ; and then viewing them through a Prifm DH held parallel to their lengtih, they appeared not co-incident as when view'd with the naked Eye, but in the form of two diftind Spedrums f t and m n croffing one another in the middle after the manner of the letter X. Which (hews that the red of the one Spedrum and violet of the other, which were co-incident at P N and M T, being parted from one another by a greater Refradion of the violet to f and m ' than

C 42 ].

then of the red to n and t^ do differ in degrees of Refrangibility.

I illuminated alfo a little circular piece of white Paper all over with the Lights of both Prifms intermixed, and when it was illumina- ted with the *red of one Spe6lrum and deep violet of the other, fo as by the mixture of thofe Colours to appear all over purple, I view- ed the Paper, firft at a lefs dillance, and then at a greater, through a third Prifm ; and as I went from the Paper, the refrafted Image there- of became more and more divided by the une- qual Refradion of the two mixed Colours^ and at length parted into two dillin6l Images, a red one and a violet one, whereof the violet was fartheit from the Paper, and therefore fuffered the greateit Refradion. And when that Prifm at the V/indow which call the violet on the Pa- per was taken away, the violet Image difap- peared ; but when the other Prifm was taken away the red vaniflied: which fliews that thefe two Images were nothing elfe than the Lights of the two Prifms which had been intermixed on the purple Paper, but were parted again by their unequal Refradions made in the third Prifm through which the Paper was view'd. This alfo was obfervable , that if one of the Prifms at the Window, fuppofe that which caft the violet on the Paper, was turned about its Axis to make all the Colours in this order, vio- let, indigo, blue, green, yellow, orange, red, fall fucceiiively on the Paper from that Prifm, the violet Image changed Colour accordingly, turning fucceiiively to indigo, blue, green, yel- low

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low and red, and in changing Colour came nearer and nearer to "the red Image made by the other Prilm, until when it was alfo red both Images became fully co-incident. • I placed alfo two Paper Circles very near one another, the one in the red Light of one Prifm, and the other in the violet Light of the other. The Circles were each of them an Inch in dia- meter, and behind them the Wall was dark that the Experiment might not be dilturbed by any Light coming from thence. Thefe Circles thus illuminated , I viewed through a Prifm fo held that the Refraftion might be made towards the red Circle, and as I went from them they came nearer and nearer together, and at length be- came CO incident ; and afterwards when I went IHll farther off, they parted agam in a contrary order, the violet by a greater Refradion being carried beyond the red.

Exfer, 8. In Summer when the Sun's Light ufes to be (trongeft, I placed a Prifm at the hole of the Window-ihut, as in the third Expe- riment, yet fo that its Axis might be parallel to the Axis .of the World , and at the oppofite Wall in the Sun's refrafted Light, I placed an open Book. Then going fix Feet and two In- ches from the Book, I placed there the above- mentioned Lens, by which the Light refledted from the Book might be made to converge and meet again at the diftance of fix Feet and two Inches behind the Lens, and there paint the Species of the Book upon a llieet of white Pa- per much after the m.anner of the fecond , Ex- periment. The Book and Lens being made faft,

Ino-

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