21 8 HISTORY OF SCIENCE. 



same order, which, beginning from the most refracted portion, was 

 violet, indigo, blue, green, orange, red. Newton found it at first " a 

 very pleasing divertisement to view the vivid and intense colours pro- 

 duced thereby," but when he began to study the spectrum more atten- 

 tively he was much struck with its great length as compared with its 

 width. By all the laws of refraction then understood, the image on the 

 screen should have been circular, whereas he found the sides of it rec- 

 tilinear, and only in the extremities could a semicircular form be recog- 

 nized. He tried, first, whether the thickness of the glass, or the size 



of the aperture in the window, or veins and defects in the glass, could 

 be the cause of the elongation of the image. By substituting prisms 

 of various angles and formed of various materials, reducing and en- 

 larging the aperture in the shutter, and by other experiments, he found 

 that the oblong image was not occasioned by any of these causes, and 

 then he began to suspect that the received law of refracdon (p. 155) 

 could not be quite accurate ; but this supposition had to be abandoned 

 when he found that the length of the image was unaffected by a con- 

 siderable change in the position of the prism. The removal, one after 

 another, of these possible causes of the elongation of the image, at 

 length led to the suggestion of an experimtntum crucis. By receiving 

 the image on a board having a small hole in it, he was able to inter- 

 cept all the light except such as fell upon this hole, through which such 

 light as passed was made to traverse a second prism placed benind the 

 board before reaching the wall. By turning the first prism a little way 

 about its axis, any part of the coloured image could be made to send 

 its light through the opening in the board, and the observer could de- 

 termine by the place on the wall how it was refracted by the second 

 prism. Newton thus found, that in passing through the second prism, 

 the violet light was more refracted than blue, blue than the yellow, and 



