A NEW THEORY OF LIGHT AND COLOURS. 463 



by the former computation, it was evident, that the difference of the 

 incidence of rays, flowing from divers parts of the sun, could not 

 make them, after a decussation, diverge at a sensibly greater angle, 

 than that at which they before converged; which being at most but 

 about 31 or 32 minutes, there still remained some other cause to be 

 found out, from whence it could be 2° 49'. 



Then I began to suspect whether the rays, after their trajection 

 through the prism, did not move in curve lines, and according to 

 their more or less curvity tend to divers parts of the wall. And it 

 increased my suspicion, when I remembered that I had often seen a 

 tennis ball, struck with an oblique racket, describe such a curve line. 

 For, a circular as well as a progressive motion being communicated 

 to it by that stroke, its parts on that side, where the motions con- 

 spire, must press and beat the contiguous air more violently than on 

 the other, and there excite a reluctancy and reaction of the air pro- 

 portionably greater. And for the same reason, if the rays of light 

 should possibly be globular bodies, and by their oblique passage out 

 of one medium into another acquire a circulating motion, they ought 

 to feel the greater resistance from the ambient aether, on that side 

 where the motions conspire, and thence be continually bowed to the 

 other. But notwithstanding this plausible ground of suspicion, when 

 I came to examine it, I could observe no such curvity in them. And 

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

 difference between the length of the image and diameter of the hole, 

 through which the light was transmitted, was proportionable to their 

 distance. 



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

 experimentum 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, towards 

 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 



