A HISTORY OF SCIENCE 



ambient ether on that side where the motions con- 

 spire, 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 'twixt 

 the length of the image and diameter of the hole 

 through which the light was transmitted was pro- 

 portionable 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, placing one of them close 

 behind the prism at the window, so that the light 

 must pass through a small hole, made in it for the 

 purpose, and fall on the other board, which I placed at 

 about twelve feet distance, having first made a small 

 hole in it also, for some of the 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 hands 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 these 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 than the light tending 

 to the. other end. And so the true cause of the length 



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