Prof. Challis on the Transmutation of Rays of Light. 523 



As the directions of propagation of the transmuted rays depend 

 only on the circumstances of the disturbance, the new rays might, 

 consistently with this theory, be propagated only in directions 

 coinciding nearly with the direction of propagation of the original 

 rays. 



By (5) it might happen, consistently with the theory, that 

 light is not produced under the circumstances supposed in (4), 

 the values of \ for the transmuted rays not falling within the 

 limits proper for visibility ; or, that rays for which the values of 

 X do not fall within the limits of visibility give rise to visible rays. 



By (6), the transverse motion being always alike in all direc- 

 tions from an axis, the transmuted light can in no case be polar- 

 ized light. 



I proceed now to the consideration of the '' Remarks.'^ And 

 in the first place, I am quite ready to admit that Professor Stokes 

 is entitled, by his experimental acquaintance with the class of 

 phsenomena under consideration, to set me right in regard to 

 matters of fact, and the terms which appropriately describe them. 

 His remarks have led me to see that I have improperly used the 

 word " epipolic " to signify true internal dispersion in general, 

 of which the phsenomenon that the word strictly applies to is 

 only a particular phase. This misapplication of the term, 

 although the sense in which I use it is explained on its first in- 

 troduction, has not without reason caused some misapprehen- 

 sion of my views. I am not, however, prepared to admit that I 

 have " confused three distinct things,'^ not being able to perceive 

 good reasons for the distinctions that Professor Stokes makes. 

 In the very complete discussion of the various phsenomena which 

 is contained in his paper on Change of Refrangibility (Phil. 

 Trans. 1852, part 2. p. 463), I could not make out a distinction 

 between the second and third kinds of phsenomena, both being 

 apparently referred to true internal dispersion and change of 

 refrangibility ; and with respect to the first, 1 had reasons, partly 

 theoretical, for referring them to the same head. Change of 

 refrangibility appeared to me the single appropriate idea which 

 embraces all the phsenomena, so far as they are not attributable 

 to known causes. I am aware that several experimenters have 

 referred the first kind to known causes and to " subjective illu- 

 sion.^' But Sir David Brewster does not admit this explanation, 

 and I did not consider myself at liberty to disregard the opinion 

 on this point of so able and experienced an experimenter. It 

 must also be remembered, that the experimenters referred to had 

 no suspicion of a change of refrangibility, this idea having been 

 first admitted by Professor Stokes after his discovery that invi- 

 sible rays might give rise to visible. The absence of any such 

 conception is particularly apparent in the experiments of M. 



2 M 2 



