Chap, i.] Newton's Difcoveries on Colours. 165 



lours. Upon meafuring the coloured image, which 

 was made by the light admitted into a dark chamber 

 through a prifm, he found that its length was five 

 times greater than its breadth. So unaccountable a 

 circumftance induced him to try the effect of two 

 prifms j and he found that the light, which by the firft 

 prifm was diffufed into an oblong, was by the fecond 

 reduced to a circular form, as regularly as if it had 

 paffed through neither of them. After many conjec- 

 tures and experiments relative to the caufe of thefe 

 phenomena, he at length applied to them what he calls 

 the experimentum crucis. He took two boards, and 

 placed one of them clofe to the window, fo that the 

 light might be admitted through a fmall hole made in 

 it, and after paffing through a prifm might fall on the 

 other board, which was placed at about twelve feet 

 diftance, and in which there was alfo a fmall aperture, in 

 order that fome of the incident light might pafs through 

 it. Behind this hole, in the fecond board, he alfo 

 placed a prifm, fo that the light, after patting both the 

 boards, might fuffcr a fecond refraction before it 

 reached the wall. He then moved the firft prifm ia 

 fuch a manner as to make the feveral parts of the image 

 caft upon the fecond board pafs fucceffively through 

 the hole in it, that he might obferve to what places on 

 the wall the fecond prifm would refract them. The 

 confequence was, that the coloured light, which form<- 

 ed one end of the image, fuffercd a refraction confi- 

 derably greater than that at the other end ; in other 

 words, rays or particles of light of one colour were 

 found to be more refrangible than thole of another. 

 The true caufe, therefore, of the length of the image 

 was evident, fince it was proved by the experiment, 

 that light was not homogenial, but confided of diffe- 

 rent particles or rays, which were capable of different 

 M 3 degrees 



