CHAPTER XIII. 



CONFIRMATION AND EXTENSION OF THE UNDULATORY 

 THEORY. 



AFTER the undulatory theory had been de- 

 veloped in all its main features, by its great 

 authors, Young and Fresnel, although it bore marks 

 of truth which could hardly be fallacious, there was 

 still here, as in the case of other great theories, a 

 period in which difficulties were to be removed, ob- 

 jections answered, men's minds familiarized to the 

 new conceptions thus presented to them; and in 

 which, also, it might reasonably be expected that 

 the theory would be extended to facts not at first 

 included in its domain. This period is, indeed, that 

 in which we are living; and we might, perhaps 

 with propriety, avoid the task of speaking of our 

 living contemporaries. But it would be unjust to 

 the theory not to notice some of the remarkable 

 events, characteristic of such a period, which have 

 already occurred; and this may be done very simply. 

 In the case of this great theory, as in that of 

 gravitation, by far the most remarkable of these 

 confirmatory researches were conducted by the 

 authors of the discovery, especially Fresnel. And 

 in looking at what he conceived and executed for 

 this purpose, we are, it appears to me, strongly 

 reminded of Newton, by the wonderful inventiveness 



