282 Professor Powell on the Nature qftlit 



riods, by the principle of superposition, afforded the results of 

 circular or elliptic motion of the molecules. 



Still, in the primary assumption as to the particular mode in 

 which the vibrations may be occasioned, or the circumstances of 

 the medium on which they may depend, there was great latitude 

 of choice. Amid these primary assumptions, then, might it not 

 be possible to find some, which, leaving all the formerly esta- 

 blished conclusions unaffected, might involve consequences which 

 should include the explanation of other points to which the 

 theory had not yet been applied ? Such seems to have been the 

 inquiry which occurred to M. Cauchy, and he appears to have 

 been perfectly successful in the answer he has elicited. 



In fact, nothing can give any physical hypothesis a higher 

 character as a truly philosophical theory, than this capacity of 

 accommodating itself to new phenomena, — of adaptation to them, 

 that is, without any sacrifice of what has been already attained, 

 and without any capricious alteration of the first principles ori- 

 ginally assumed ; but merely from the circumstance that those 

 principles were in the first instance so cautiously or so happily 

 selected, and of so comprehensive a character, as really to include 

 more than they were supposed capable of doing. 



The idea possibly presented itself to its first inventor under 

 one very simple and limited aspect ; but it was in itself much 

 more extensive. He regarded it perhaps only with i-eference to 

 a single application, while it was in reality far more fertile than 

 he imagined. 



Such is, in fact, precisely the view in which the undulatory 

 theory of light presents itself to us, when we compare it in its ordi- 

 nary form, and in that which M. Cauchy has given it in his pro- 

 found " Memoire sur la Dispersion de la Lumiere," Paris 18S0. 

 He assumes a medium in which waves are propagated, whose 

 motions will accord with those of the common theory. He sup- 

 poses two molecules slightly disturbed by any cause (either agi- 

 tation produced by the peculiar action of the luminous body, or 

 source of light, or the attraction of neighbouring particles de- 

 pending on tlieir previous disturbance), whence he investigates 

 expressions for the forces thus brought into action, and then, by 

 the integration of a differential equation, finds the motions. But 



