386 HISTORY OF OPTICS. 



temporaries, shows how remote it was from the 

 then accepted opinions. There appears to have 

 been a general persuasion that the coloration was 

 produced, not by any peculiarity in the law of re- 

 fraction itself, but by some collateral circumstance, 

 some dispersion or variation of density of the 

 light, in addition to the refraction. Newton's dis- 

 covery consisted in teaching distinctly that the law 

 of refraction was to be applied, not to the beam of 

 light in general, but to the colours in particular. 



When Newton produced a bright spot on the 

 wall of his chamber, by admitting the sun's light 

 through a small hole in his window-shutter, and 

 making it pass through a prism, he expected the 

 image to be round ; which, of course, it would have 

 been, if the colours had been produced by an equal 

 dispersion in all directions; but to his surprize he 

 saw the image, or spectrum, five times as long as 

 broad. He found that no consideration of the dif- 

 ferent thickness of the glass, the possible uneven- 

 ness of its surface, or the different angles of rays 

 proceeding from the two sides of the sun, could be 

 the cause of this shape. He found, also, that the 

 rays did not go from the prism to the image in 

 curves ; he was then convinced that the different 

 colours were refracted separately, and at different 

 angles; and he confirmed this opinion by trans- 

 mitting and refracting the rays of each colour 

 separately. 



The experiments are so easy and common, and 



