Composition of White Light. 61 



the two prisms mentioned in a former experiment, the paper 

 A was illuminated at the same time with the red light from 

 the one, and a deep violet light from the other. By this 

 mixture the paper assumed a rich purple colour. The circle 

 A was then viewed through a prism at some distance, and 

 the appearance exhibited was two circles, R & V, the circle 

 R, nearer to the paper being red, and the more remote one, 

 V, violet. The prism in this case refracted the red and 

 violet light, mingled in the circle A, through different 

 angles ; the red being least refrangible was removed to R, 

 and the more refrangible violet light carried so far as V. — 

 (See Popular Account of Newton's Optics, published by the 

 Society for the diffusion of Useful Knowledge, Section 29.) 



I am aware that Newton calls the light with which these 

 experiments were made homogeneal ; but, as it was sepa- 

 rated from white light by the prism, in the usual manner, 

 it could not have been so, more than the other part of the 

 same colour from which it was taken, and with which it 

 must have corresponded ; if, then, one of the prismatic 

 colours be homogeneal, the rest must be the same. 



If, in each colour, the different degrees of refrangibility 

 were innumerable, no means could be adopted which would 

 render any breadth of refracted light, however small, homo- 

 geneous ; but in the experiment last quoted, in which it 

 does not appear that any particular precaution was taken 

 with this view, the illuminated paper was an inch in dia- 

 meter, and yet the action of the prism produced no elonga- 

 tion, which must have been the case if the light employed 

 had possessed different degrees of refrangibility. 



Several other experiments of Newton's might be quoted 

 in opposition to this part of his own theory ; indeed, he so 

 frequently neglects his particular views on this part of the 

 subject, that if they were not so expressly stated at the com- 

 mencement, it might be doubtful which side of the question 

 he meant to support. I am persuaded that many persons, 

 who having derived their knowledge of optics from our 

 popular treatises, which allude to this important distinction 

 in a very cursory manner, are not aware of its existence ; 

 so unnecessary is it to the explanation of the experiments 

 usually given in these works, that its introduction would 

 clog it with difficulties instead of removing them. 



