REFRANGIBILITY. 



out of tlie fun's liglit, and looked through it upon the hole 

 ftiining by the liglit of the clouds beyond it. 



4. Cbnfidering that if the image of the fun fhould be 

 drawn into an oblong form, citlur by a dilatation ol every ray, 

 or by any other cafual inequality of the rcfracitions, the fame 

 oblong image would, by a fccond rcfrattiou, made fideways, 

 be drawn out as much in breadth, he plated a fecond prifm 

 immediately after the firll, in an oblique pofition with refpeft 

 to it, that it might again refrai't llie light of the beam of the 

 fun's light, which came to it through the lirfl prifm ; fo that 

 in the firil prifm, the beam would be refrafted upwards, and 

 in the fecond fideways. But he found that the breadth of 

 the image was not incrcafed by the refraction of the fecond 

 prifm, but only its upper part, which in the firfl prifm fuf- 

 fered the greatelt refradion, and appeared violet and blue, 

 did again, in the fecond prifm, fufler a greater refraftion, 

 than the lower part of it, which was red and yellow, and 

 this without any dilatation of the breadth of the image. 

 Thus let S {fg. 4.) reprefont the fun, F the hole in 

 the window, ABC the firll prifm, and D H the fe- 

 cond. If Y reprefcnt the round image of the fun, made 

 by a diredt beam of light, when the prifms are taken 

 away, P T will be the oblong image of the fun, made 

 by the fame beam pafling through the firit prifm only, and 

 p t will be the image made by the crofs refraftions of both 

 prifms together. Sometimes he placed a third prifm after 

 the fecond, and fonietimes a fourth after the third ; by 

 all which the image might be often refrafted fideways, but 

 the rays which were more refrafted than the reil in the firlt 

 prifm, were alfo more refrafted in all the others, and that 

 without any dilatation of the image fideways ; and, there- 

 fore, thofe rays, on account of their conftancy of a greater 

 refraftion, he called the more refrangible ones. He obferves, 

 in order to render the meaning of this experiment more evi- 

 dent, that all the rays, which are equally refrangible, fall 

 upon a circle anfwering to the fun's difc. Let, therefore, 

 A G {Jig. 5.) reprefentthe circle which all the inoft refran- 

 gible rays, tranfmitted from the whole difc of the fun, would 

 illuminate, and paint upon the oppofite wall, if they were 

 alone. Let E L be the circle which all the leaft refrangible 

 rays would, in like manner, illuminate, and paint, if they 

 were alone, and let B H, C .1, and D K, be the circles 

 which fo many intermediate kinds of rays would fucceflively 

 paint upon the wall, if they were fingly propagated from 

 the fun, the reft being always intercepted, and conceive that 

 there are other intermediate circles without number, which 

 other innumerable intermediate kinds of rays would fuccef- 

 fively paint upon the wall, if the fun fliould fucceflively 

 emit every kind by itfelf. Now fmce the fun emits rays of 

 all thefe kinds at once, they muft altogether illuminate and 

 paint innumerable equal circles, of all which, being ranged 

 according to their different degrees of refrangibihty, the 

 oblong image P T before defcribed is compofed. 



Now if the fun's circular image, Y, which is made by an 

 unrefrafted beam of light, was by any dilatation of the fiii- 

 gle rays, or by any other irregularity in the refraftion of the 

 firft prifm, converted into the oblong image P T, then 

 ought every circle in the image to be in like manner drawn 

 out into a fimilar oblong figure, contrary to the refult of 

 this experiment. 



He confidered farther, that by the breadth of the hole 

 through which the light enters into the dark chamber, there 

 is a penumbra made in the circumference of the image Y, 

 which is alfo vifible at the fides of the oblong images P T 

 and p t. He, therefore, placed at tiiat hole a lens, or ob- 

 jcft-glafs of a telefcopc, which might call the image of the 

 fun diftinftly on Y, without any penumbra at all ; and he 



found that the penumbra of the rcftilinear fides of the ob- 

 long images P T and p I was alfo thereby taken away, fo 

 that they were as diftinftly defined as the circumference of 

 -he firft image Y. 



There are fonie other circumftances attending this experi- 

 ment, by which the conclufion drawn from it is made ftill 

 more plain and convincing. 



Let the fecond prifm, D H, {Jig. 6.) be placed not im- 

 mediately after the ^rll, but at fome diftance from it, fo 

 that the light from the firft prifm may fall upon it in the 

 form of an oblong fpeftrum qr, parallel to this fecond 

 prifm, and may be refracled fideways, to form the oblong 

 image, pt, upon the wall, and it will be found that this 

 image, p t, is inclined to the image P T, which the firft 

 prifm would have formed without the fecond ; the blue 

 ends, P and p, being farther diftant from one another than 

 the red ones, T and t ; and, confequently, the rays which 

 go to the blue end, q, of the image q r, and which, there- 

 fore, fuffer the greatcft rcfraAion in the firft prifm, are 

 again, in the fecond prifm, more refradted than the reft. 



At two holes made near one another in his window- 

 ftiutter, he placed two prifms, one at each, which might 

 call upon the oppofite wall two oblong coloured images of 

 the fun ; and at a little diftance from the wall he placed a 

 long flender paper, with ftraight and parallel edges ; and he 

 placed the prifms and papers, fo that the red colour of one 

 image, at T, (^g. 7.) might fall directly upon one half of 

 the paper, and the violet colour, M, of the other image, 

 upon the other half of the fame paper. Then with a black 

 cloth he covered the wall behind the paper, that no light 

 might be reflefted from it to difturb the experiment ; and 

 viewing the paper through a third prifm, held parallel to it, 

 he faw that half of it which was illuminated by t4ie violet 

 light to be divided from the other half, by a greater refrac- 

 tion, efpecially when he retired to a confiderable diftsnce 

 from the paper. 



He farther caufed the two images, P T and M N l^g. 8.) 

 to coincide, in an inverted order of their colours, the red 

 end of each falling on the violet end of the other ; and- then 

 viewing them through a prifm D H, held parallel to their 

 length, they no longer appeared coincident, as when they 

 were viewed with the naked eye, but in the form of two 

 diftinft images, pt and mn, crofling one another in the 

 middle ; which fhews that the red of the one image, and the 

 violet of the other, which were coincident at P N and M T, 

 being parted from one anotlier by a greater refraftion of the 

 violet to p m, than that of the red to n and /, differ in de- 

 gree of refrangibihty. 



Having placed a prifm, whofe two angles at its bafe were 

 equal to one another, and half right ones, and the third a 

 right one, in a beam of the fun's light, admitted into the 

 room as before, he turned it flowly about its axis, till all 

 the light which went through one of its angles, and was re- 

 frafted by it, began to be reflefted by its bafe, (at which, 

 till then, it went out of the glafs,) and then he obferved 

 that thofe rays which had fuffered the greateft refraftion 

 were fooner reflefted than the reft. He imagined, therefore, 

 that thofe rays of the reflefted hght, which were moft re- 

 frangible, did firft of all, by a total refleftion, become more 

 copious in that light than the reft ; and that afterwards the 

 reft, alfo, by a total refleftion, became as copious as the^fe. 

 To try this, he made the reflefted liglit pafs through another 

 prifm, and, being refrafted by it, to fall afterwards upon a 

 ftieet of white paper, at fome diftance behind it, and there 

 to paint the ufual colours of the prifm. Then caufing the 

 firft prifm to be turned about its axis, he obferved, that 

 when thofe rays which, in this prifm, liad fuffered the 



greateft 



