6i24^^ Prof. Challis on the TVansmutation of Bays of Light, 



Helmholtz (published in Poggendorff's Annalen in 1852), which 

 are proper for decomposing hglit that might have such an origin, 

 but not proper for discovering that origin. In further explana- 

 tion of these views, it should be stated that I am led by the 

 theory explained at the beginning of this communication to 

 admit that the new light due to change of refrangibility is not 

 necessarily dispersed in all directions, but may in certain sub- 

 stances deviate but little from the direction of the original light. 

 This, however, is a point which experiment must decide. 

 Jo The remark that the phsenomenon of change of refrangibility 

 *^ may be set aside in speculating on the sensation of colour," is 

 altogether irrelevant, the question being not about sensation of 

 colour, but as to whether or not a ray of prismatic purity is com- 

 pounded. From the fact of change of refrangibility by absorbing 

 media, it might fairly be argued that light admits of a resolution 

 different from that effected by the prism. It was therefore an 

 important preliminary to my theory of the composition of colours 

 to establish that this is not the case. Professor Stokes has said 

 that "were a change of colour made out, it would be a point of 

 the utmost importance to consider in reference to any physical 

 theory of light." This importance equally attaches itself to a 

 change of refrangibility, and on this account I endeavoured to 

 explain the fact by reference to the mathematical theory which is 

 stated more at length at the beginning of the present communi- 

 cation. Professor Stokes has shown that he felt the necessity of 

 some theory by speculating largely on the subject in arts. 226 to 

 236 of his paper, but has not attempted to give a mathematical 

 theory. 



The other remarks relate chiefly to composition of colours. 

 Professor Stokes does not admit that a composition of blue and 

 yellow lights can produce green. Referring to an experiment I 

 made with a blue ribbon and a yellow ribbon put in contact and 

 held to the light, which appeared to give a green colour, he says, 

 '^the superposition of two coloured glasses or ribbons by no 

 means gives the effect of the mixture of the two colours;" and 

 then adduces an argument which does not apply to the ribbons, 

 because light from one is received through the interstices of the 

 other. He makes no allusion to an experiment by Sir John 

 Herschel on which I chiefly relied, in which blue and yellow 

 lines are drawn on paper close together and alternating with each 

 other. Unquestionably in this instance the eye receives a mix- 

 ture of blue and yellow lights. I have repeated the experiment 

 and find the result to be green. The green is remarkably con- 

 trasted with blue, if the blue lines be extended beyond the yellow. 



I did not question the fact that " the production of white 

 by a mixture of blue and yellow is by no means confined to 



