VOL. VI.] PHILOSOPHICAL TRANSACTIONS. 6rJ 



tions conspire, must press and beat the contiguous air more violently tl::;ii un 

 the other, and there excite a reluctancy and reaction of the air proporticjnably 

 greater. And for the same reason, if the rays of light should possibly be glo- 

 bular bodies, and by their oblique passage out of one medium into another ac- 

 quire a circulating motion, they ought to feel the greater resistance from the 

 ambient aether, on that side where the motions conspire, and thence be con- 

 tinually bowed to the other. But notwithstanding this plausible ground of 

 suspicion, when I came to examine it, I could observe no such curvity in thtni. 

 And besides (which was enough for my purpose) I observed, that the difTercnte 

 between the length of the image and diameter of the hole, through which llio 

 light was transmitted, was proportionable to their distance. 



The gradual removal of these suspicions, at length led me to the experiment 

 tum crucis, which was this : I took two boards, and placed one of them close 

 behind the prism at the window, so that the light might pass through a small 

 hole, made in it for the purpose, and fall on the other board, which I placed 

 at about 12 feet distance, having first made a small hole in it also, for some of 

 that incident light to pass through. Then I placed another prism behind this 

 second board, so that the light trajected through both the boards, might pass 

 through that also, and be again refracted before it arrived at the wall. This 

 done, I took the first prism in my hand, and turned it to and fro slowly about 

 its axis, so much as to make the several parts of the image, cast on the second 

 board, successively pass through the hole in it, that I might observe to what 

 places on the wall the second prism would refract them. And I saw, by the 

 variation of those places, that the light tending to that end of the image, to- 

 wards which the refraction of the first prism was made, did in the second prism 

 suffer a refraction considerably greater then the light tending to the other end. 

 And so the true cause of the length of that image was detected to be no other, 

 than that light consists of rays differently refrangible, which, without any re- 

 spect to a difference in their incidence^ were according to their degrees of re- 

 frangibility, transmitted towards divers parts of the wall. 



When I understood this, I left off my aforesaid glass works ; for I saw, that 

 the perfection of telescopes was hitherto limited, not so much for want of 

 glasses truly figured according to the prescriptions of optic authors, (which all 

 men have hitherto imagined,) as because that light itself is a heterogeneous 

 mixture of differently refrangible rays. So that, were a glass so exactly figured, 

 as to collect any one sort of rays into one point, it could not collect those also 

 into the same point, which having the same incidence upon the same medium 

 are apt to suffer a different refraction. Nay, I wondered, that seeing the dif- 

 ference of refrangibility was so great, as I found it, telescopes should arrive to 



VOL. I. 4 R 



