On the different Refr eligibility of the Rays of Light. 1 69 



knowledge, at the same time stating, that, as Mr. Wingate 

 had declined publishing, and I had in the mean time obtained 

 five additional examples of the new species, I had drawn up 

 a paper from my own materials, which I had sent in to the 

 Linnaean Society, with all the drawings I possessed on the 

 subject. ' Mr. Selby's paper, I have no doubt, will be worthy 

 his high and deserved reputation ; and the more information 

 we obtain of so interesting an addition to our native Fauna^ 

 the better. 



I regret sincerely the necessity of referring to private cor- 

 respondence; but the contents of letters have been quoted 

 against me, and self-justification must be my apology. 



I am, Gentlemen, yours, &c. 

 Ryder Street, St. James's, Aug. 10, 1830. Wm. YarreLL. 



XXVII. An Attempt to explain theoretically the different Re- 

 frangibility of the Rays of Light, according to the Hypothesis 

 of Undulations. By the Rev. J. Challis, Fellow of Trinity 

 College Cambridge, and of the Cam. Phil. Soc* 



r rHE object of this communication is to follow up an idea 

 ■*■ advanced by Dr. Young, to explain theoretically the dif- 

 ferent refrangibility of the rays of light. (Lectures on Natural 

 Philosophy, vol. ii. p. 623.) His notions on this subject have 

 not met with the attention they deserve, probably because they 

 are vague, and are not supported by mathematical calculation. 

 There is much plausibility in his leading idea, viz. that the ve- 

 locity ofpropagation of the aethereal undulations which traverse 

 any medium, is modified by the vibrations of the material 

 atoms of the medium, and differently according to the different 

 frequency of the undulations : but the precise manner of the 

 modification is probably not such as he supposes, and is open 

 to further consideration. 



For the object proposed, it will be necessary to attend to the 

 manner in which a series of undulations is reflected when they 

 encounter an obstacle. Suppose the fluid to be such that 

 the pressure is proportional to the density, p = a 2 {\ -J- s) ; 

 and the portion of it we consider, to be included in a slen- 

 der cylindrical tube. Let two series of undulations, equal 

 in every respect, and generated under circumstances exactly 

 alike, be propagated in opposite directions along the tube. By 

 the principle of the coexistence of small vibrations, these un- 

 dulations will be propagated in their respective directions with- 



* Communicated by the Author. 

 N.S. Vol.S. No. 45. Sept. 1830. Z -out 



