494 HISTORY OF OPTICS. 



theory, it had been assumed that the distance from 

 each other, of the particles of the ether, which, by 

 their attractions or repulsions, caused the undula- 

 tions to be propagated, is indefinitely less than this 

 small quantity ; so that its amount might be neg- 

 lected in the cases in which the length of the undu- 

 lation was one of the quantities which determined 

 the result. But this assumption was made arbi- 

 trarily, as a step of simplification, and because it 

 was imagined that, in this way, a nearer approach 

 was made to the case of a continuous fluid ether, 

 which the supposition of distinct particles imper- 

 fectly represented. It was still free for mathema- 

 ticians to proceed upon the opposite assumption, of 

 particles of which the distances were finite, either 

 as a mathematical basis of calculation, or as a phy- 

 sical hypothesis; and it remained to be seen if, 

 when this was done, the velocity of light would still 

 be the same for different lengths of undulation, that 

 is, for different colours. M. Cauchy, calculating, 

 upon the most general principles, the motion of 

 such a collection of particles as would form an 

 elastic medium, obtained results which included the 

 new extension of the previous hypothesis. Pro- 

 fessor Powell, of Oxford, applied himself to reduce 

 to calculation, and to compare with experiment, the 

 result of these researches. And it appeared that, on 

 M. Cauchy's principles, a variation in the velocity 

 of light is produced by a variation in the length of 

 the wave, provided that the interval between the 



