ON LIGHT. 301 



viz., nil on the one hand, and reduplication on the 

 other. 



(83.) The vibrations by which light and musical sounds 

 (to which light is analogous} are conveyed are so exceed- 

 ingly minute, and the shock conveyed by each separately 

 to our nerves, in consequence, so small, that it requires 

 a continued series of them to impress our senses. The 

 first few vibrations therefore which run on " uninterfered 

 with " produce no sensation, and are as if they existed 

 not. And thus we see how it may happen, that in the 

 case of a complete opposition of phase two equal musical 

 sounds may produce silence, and two equal rays of light 

 complete and continued darkness ; that a perfect coinci- 

 dence of phase has the effect of doubling the sensation ; 

 and the intermediate states, a greater or less intensity as 

 the case may be, short of that limit. 



(84.) Let us now proceed to apply our principle (that 

 of "superposed and INTERFERING VIBRATIONS") to the 

 matter in hand. Suppose a series of equal and equidis- 

 tant light-waves (such as a ray of homogeneous light is 

 in this theory always understood to mean) to fall perpen- 

 dicularly upon a plate of any transparent medium. A 

 certain very small per-centage of it will be reflected back 

 by the first surface that is to say, a series of similar 

 undulations, but of much less intensity or " amplitude," 

 will be propagated back from the point of incidence. 

 The remainder of the total movement thus subdivided 

 will pass on, and, arriving at the second surface, again a 

 very nearly equal series (the per-centage being the same, 

 and the total incident light having suffered very little 



