THE UNDULATORY THEORY. 493 



pagated. Yet the theory suggested no reason why 

 the velocity should be different for different colours : 

 for, by mathematical calculation, vibrations of all 

 degrees of rapidity (in which alone colours differ) 

 are propagated with the same speed. Nor does 

 analogy lead us to expect this variety. There is no 

 such difference between quick and slow waves of 

 air. The sounds of the deepest and the highest bells 

 of a peal are heard at any distance in the same 

 order. Here, therefore, the theory was at fault. 



But this defect was far from being a fatal one. 

 For though the theory did not explain, it did not 

 contradict, dispersion. The suppositions on which 

 the calculations had been conducted, and the ana- 

 logy of sound, were obviously in no small degree 

 precarious. The velocity of propagation might 

 differ for different rates of undulation, in virtue of 

 many causes which would not affect the general 

 theoretical results. 



Many such hypothetical causes were suggested 

 by various eminent mathematicians, as solutions of 

 this conspicuous difficulty. But without dwelling 

 upon these conjectures, it may suffice to notice that 

 hypothesis upon which the attention of mathema- 

 ticians was soon concentrated. This was the hypo- 

 thesis of finite intervals between the particles of 

 the ether. The length of one of those undulations 

 which produce light, is a very small quantity, its 

 mean value being l-50,000th of an inch ; but in the 

 previous investigations of the consequences of the 



