584 THE PEINCIPLES OF SCIENCE. [CHAP. 



persons might have conceived this theory ; in fact, any 

 person regarding refraction as a quantitative effect must 

 see that different parts of the spectrum have suffered 

 different amounts of refraction. But the power of Newton 

 is shown in the tenacity with which he followed his theory 

 into every consequence, and tested each result by a simple 

 but conclusive experiment. He first shows that different 

 coloured spots are displaced by different amounts when 

 viewed through a prism, and that their images come to a 

 focus at different distances from the lense, as they should 

 do, if the refrangibility differed. After excluding by many 

 experiments a variety of indifferent circumstances, he fixes 

 his attention upon the question whether the rays are 

 merely shattered, disturbed, and spread out in a chance 

 manner, as Grimaldi supposed, or whether there is a con- 

 stant relation between the colour and the refrangibility. 



If Grimaldi was right, it might be expected that a 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 con- 

 stant refrangibility, so that a second prism would merely 

 bend it more or less, but not further disperse it in any con- 

 siderable 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 refrangibility varies with the colour. For his further 

 satisfaction, he sometimes refracted the light a third or 

 fourth time, and he found that it might be refracted up- 

 wards or downwards or sideways, and yet for each colour 

 there was a definite amount of refraction through each 

 prism. He completed the proof by showing that the sepa 

 rated rays may again be gathered together into white light 

 by an inverted prism, so that no number of refractions 

 alters the character of the light. The conclusion thus ob- 

 tained serves to explain the confusion arising in the use of 

 a common lense ; he shows that with homogeneous light 

 there is one distinct focus, with mixed light an infinite 



