Prof. Challis on the Transmutation of Rays of Light, 531 



instrument which produces a spark five times the length of those 

 produced by any of Ruhmkorff^s arrangements ; but as he was 

 certainly the originator of these instruments, and therefore de- 

 serves nearly all the credit, I hope that mine and those pro- 

 duced by others will still be called RuhmkorflP^s coils. 



Yours obediently, 



C. A. Bentley. 



LXVII. On the Transmutation of Rays of Light ; with a Reply to 

 the ^' Remarks " of Professor Stokes in the December Number. 

 By Professor Challis*. 



BEFORE replying to the Remarks which Professor Stokes 

 has made on certain parts of my communication in the 

 November Nuriiber, entitled " A Theory of the Composition of 

 Colours/^ I am desirous of entering upon some theoretical consi- 

 derations by which the views there advanced, and the arguments 

 by which 1 support them, will be better understood. I commence 

 with saying, that, consistently with all I have hitherto written on 

 the undulatory theory of light, I shall suppose the sether to be a 

 continuous medium so constituted that variations of its pressure 

 are proportional to variations of its density, and shall admit for the 

 explanation of phsenomena of light only such properties of the me- 

 dium as may be deduced from this constitution by calculating on 

 hydrodynamical principles. Also for the present purpose it will 

 suffice merely to enunciate those deductions which bear upon the 

 phsenomena of light with which this communication is imme- 

 diately concerned, and to mention that proofs of them, which I 

 am prepared to maintain, have been given in articles contributed 

 at various times to this Journal. 



When a medium, constituted as above supposed, suffers a con- 

 tinuous vibratory disturbance by the motions of the atoms of any 

 extraneous substance, or when a state of continuous vibratory 

 motion of the sether suffers disturbance by the passive action of 

 atoms at rest, the consequences, as deduced by the analysis, are 

 the following : — 



(1) The state of the disturbed sether at each instant, both as 

 to velocity and condensation, results from the composition of an 

 indefinite number of separate motions, such that each has an 

 axis of symmetry parallel and transverse to which vibrations take 

 place according to a law expressed by the function 



msm— - (x—at-i-c). 



* Communicated by the Author. 

 Phil. Mag, S. 4. No. 82. Suppl Vol. 12. 2 M 



