Prof. Challis on the Undulatory Theory of Light. 335 



from observation by no larger amount, unless the principle. .of 

 the reasoning are fundamentally correct. I may here state that 

 he point of°no velocity mightW been taken on tbeaxis o 

 motion instead of being at a great distance from it «" 

 the motion contiguous to the axis may be ^PP^.^XLh 

 two equal sets of longitudinal and transverse motions ; and each 

 set might be treated^ndependently of the other. By conduct- 

 ing thf reasoning in this way I obtained the same results as by 



^ Ift^TbjeSd that when the effect of the development of 

 heat on the rate of propagation is taken into account the 

 mathematical result is contradicted by exper.men t I reply as 

 Mows —It is evident, from the mntua! relation of the longitu- 

 dina and transverse motions above described, that we have had 

 under consideration a case of free expansions and contractions 

 due to successive generations and fillings up of a partial vacuum 

 SwitTs admitted, I believe, that experiment has decided that 

 n such a case there is no change of temperature. Consequently 

 the rate of p opagation remains unaffected. The case of develop- 

 Snt or absorptmn of heat when air is suddenly let mto, orabs- 

 "acted torn, closed spaces, and when, in consequence, mrk* 

 don ha uo analogy to this. Upon the whole I seem entitled 

 to conclude that I have at length succeeded m solving the diffi- 

 cult probkm of determining mathematically the rate of propaga- 

 tion Va continuous elastic fluid. The results obtained are 

 essential to the undulatory theory of polarization 



(2) I proceed, in the next place, to advert to a commum 

 cation I made to the Philosophical Magazine or January 857 

 entitled " On the Transmutation of Rays of Light In tne 

 course of the article I have enumerated various inferences re- 

 adng to phenomena of light, which had been deduced by means 

 of the analysis I had applied to the undulatory theory; and to 

 one of these, which is numbered (4) in the order of the series I 

 Zh now to call attention. That deduction is expressed in the 

 3K* terms — " When the sether in motion suffers dirtnrb- 

 in e by en oTntering atoms actually or relatively at rest, and 

 the original motion ii a simple series of vibrations ot the usual 

 Joe o?is compounded of several such motions with parallel axes 

 and different value, of m, X, and c, the result of the disturbance 

 may in either case consist of an indefinite number of separate 

 motions having their axes in various directions, and having 

 TalueTof l, X, and e altogether different from the values of these 

 quantities n the original motion." Further on I remark ha 

 "when the circumstances of the disturbance are as supposed n 

 (4) light may produce new light, which may differ trom the 

 ir ginal light in intensity, colour composition, and direction of 



