230 THE PRINCIPLES OF SCIENCE. 



as Grimaldi supposed, or whether there is a constant 

 relation between the colour and the refrangibility. If 

 Grimaldi was right, it might be expected that any part 

 of the spectrum taken separately, and subjected to a 

 second refraction, would suffer a new breaking up, and 

 produce some new spectrum. Newton inferred from his 

 own theory that a particular ray of the spectrum would 

 have a constant refrangibility, so that a second prism 

 would merely bend it more or less, but not further dis- 

 perse it in any considerable degree. By simply cutting 

 off most of the rays of the spectrum by a screen, and 

 allowing the remaining narrow ray to fall on a second 

 prism, he proved the truth of this conclusion ; and then 

 slowly turning the first prism, so as to vary the colour 

 of the ray falling on the second one, he found that the 

 spot of light formed by the twice-refracted ray travelled 

 up and down, a palpable proof that the amount of refran- 

 gibility varied with the colour. For his further satisfac- 

 tion, he sometimes refracted the light a third or fourth 

 time, and he found that it might be refracted upwards or 

 downwards or sideways, and yet for each coloured light 

 there was a definite amount of refraction through each 

 prism. He completes the proof by showing that the 

 separated rays may again be gathered together into white 

 light by an inverted prism. So that no number of refrac- 

 tions alters the character of the light. The conclusion 

 thus obtained serves to explain the confusion arising in 

 the use of a common lense ; with homogeneous light he 

 shows that there is one distinct focus, with mixed light 

 an infinite number of foci, which prevent a clear view 

 from being obtained at any one point. 



What astonishes the reader of the ' Opticks ' is the 

 persistence with which Newton follows out the conse- 

 quences of a preconceived theory, and tests the one notion 

 by a wonderful variety of simple comparisons with fact. 



