INTERFERENCE OF LIGHT. 303 



Young had not this degree of prudence. He showed 

 at once that his theory would agree with the phenomena, 

 but without going beyond mere possibility. When at a 

 later period he arrived at real proofs of it, the public 

 had other prepossessions, which he was not able to over- 

 come. However, the experiment, whence our colleague 

 deduced so memorable a discovery, could not excite the 

 shadow of a doubt.* 



* In the retrospective glance which the author thus gives over the 

 progress of discovery previous to the period at which Dr. Young first 

 entered on the field, what we have chiefly to observe is, that up to 

 that date nothing like a connected view of the physical character of 

 this wonderful agent had been attained ; a few isolated speculations 

 had indeed been put forth respecting a theory of emitted molecules 

 on the one hand, and of waves in an ethereal medium on the other; 

 and a few experimental facts bearing on the choice between such hy- 

 potheses had been ascertained. 



The several distinct phenomena of common reflexion and refrac- 

 tion, of double refraction, of inflexion or diffraction, and of the 

 coloured rings, did not seem to be connected by any common princi- 

 ple; nor, even, separately considered, could it be said that they were 

 very satisfactorily explained. It was now the peculiar distinction of 

 Young to perceive, and to establish in the most incontestable manner, 

 a great principle of the simplest kind, which at once rendered the 

 wave hypothesis applicable to the two last-named classes of facts, and 

 thus directly connected them with the former. 



It is not always that we are enabled to trace the first rise and pro- 

 gress of the idea of a great discovery in the inventor's mind. We can- 

 not forbear from here noticing, that Dr. Young has left on record the 

 progress of the first suggestions which occurred to him on the subject 

 of interference. The first view which presented itself was that of the 

 analogies furnished by sound, which, as is well known, is conveyed by 

 means of waves propagated in air. And in the case of two sounds 

 differing a very little from the same pitch, produced at the same time, 

 we have, not a continuous sound, but beats, that is, alternations of 

 sound and silence ; the waves in the one case conspiring with and 

 reinforcing each other, in the other counteracting, neutralizing, and 

 destroying each other. 



I But in more special reference to light, Dr. Young's account of the 

 origin of his ideas is so clear and striking that we must give it in his 



