354 Prof. Challis on the Resistance of the Luminiferom 



hypothesis of oscillations now proposed." — (P. 407.) This 

 theory is accordingly distinguished from that commonly main- 

 tained, by the substitution of oscillations of the individual atoms 

 about axes for their vibrations in space. Here I take occasion 

 to remark, for the sake of preventing misapprehension, that 

 where in my former communications I have spoken of the oscil- 

 latory theory of light (which ought rather to have been called 

 the vibratory theory), I made no reference to the particular views 

 adduced above, intending only to signify by those terms a theory 

 which takes account of the motions of the discrete atoms of the 

 sether, and is in this respect opposed to the hydrodynamical 

 theory. In referring now to this modification of the usual 

 theory, my object is the same that I had in view when referring 

 in my last communication to the theory of elliptically-polarized 

 light, — namely, to exhibit the actual state of the theory of light 

 maintained by the mathematicians of the present day, and to 

 show by their own admissions that it requires hypothesis to be 

 added to hypothesis to support it. I have no doubt in my own 

 mind that it involves the initial and radical defect of treating by 

 common difi'erential equations, questions which demand the 

 more comprehensive analysis of pa?iial diflPerential equations. 

 It is this mode of treatment that necessitates the adoption of 

 hypotheses, respecting the molecular constitution of the aether, 

 which at best must be precarious, and which absolutely admit of 

 no verification, if, as there is reason to conclude from certain 

 analogies of light to sound, the phsenomena of light result from 

 the dynamical action of a continuous Amd. As the laws of sound 

 can be investigated only by means of partial difierential equa- 

 tions, it seems scarcely possible that this instrument of research 

 can be dispensed with in investigating the laws of light. 



In confirmation of the above views, I proceed now to give an 

 explanation of the non-resistance of the sether to the motions of 

 the heavenly bodies, by regarding it as a fluid substance the 

 movement of which is ascertained by the integration of partial 

 differential equations. For this purpose it is not necessary, nor 

 indeed should I consider it allowable, to make any other hypo- 

 theses than those which I have already made, viz. that the con- 

 stituent atoms of the bodies which move in the sether are hard, 

 inert, and spherical ; and that the sether itself is a continuous 

 and highly elastic fluid, varying in pressure in proportion as it 

 varies in density. The question consequently resolves itself into 

 the determination of the resistance of the sether to the motion 

 of a minute spherical body. This problem I have considered in 

 a communication to the Philosophical Magazine for July 1832, 

 (p. 40), and I do not yet see any reason for treating it in a different 

 manner. A solution of the same problem is also given in the 



