AND THE THEOEY OF LIGHT. 55 



liow this can be done. Transparent or partially trans- 

 parent bodies composed of different plates or laminae not 

 parallel divide the incident light into different sections ; 

 some reflecting light to the spectator — others from him, 

 or, which is the same thing, producing the effect of 

 darkness. 



80. The intensity of these shadows may be conceived as 

 being analogous to the lengths and tensions of musical 

 strings ; and I have little doubt they are so : but I have no 

 data on which to found a precise analogy, and all specula- 

 tion is foreign to my plan. Even were I to resort to the 

 analogy of sound, there are other difficulties than those 

 mentioned J and not the least is the difference of the 

 instruments by which we see and hear, I expect, however, 

 that the discoveries which have been made in light will 

 lead to new methods of research in several other sciences ; 

 and consequently I have no greater desire to follow out 

 the analogy with sound than to apply these discoveries to 

 other sciences. As, however, a comparison has frequently 

 been instituted between the two phenomena of light and 

 sound, I wish to show that if the analogy is to hold good, 

 a negative term must also be introduced into the science 

 of colour. 



81. None of the theories of light deny to it motion. 

 We may, therefore, assume that light is ether in motion, 

 or the agitation of a circumambient ether. 



82. Where there is reflection there is light; where there 

 is no reflection there is darkness. The most convenient 

 substances for representing these two different states are 

 white paper and black velvet, or lamp-black used as a pig- 

 ment. It is with these substances that my experiments 

 were made : but a bit of Bristol board having various 

 figures painted black on it, or cut out of it, I found to 

 serve equally well ; for, there being no reflection from the 

 spaces cut out, they appear black. 



