54 MR. J. SMITH ON THE ORIGIN OF COLOUR 



because they both result from the vibrations of a circum- 

 ambient ether, until these laws are investigated. Assum- 

 ing, however, the analogy between sound and light, and 

 arguing on the assumption, it follows that the theory of 

 light requires a negative principle to carry out the ana- 

 logy. Sound requires an active element to put inert 

 matter in motion; colour requires an inert or opposing 

 element to modify the motion of an active agent. 



jj. When we know the length of a string, which when 

 struck under certain conditions produces the key note in 

 music, we can estimate the lengths of the other strings 

 which will produce the notes in the entire scale. These 

 have been studied for many ages, and can be very accu- 

 rately calculated, although the calculation is still subject 

 to a rectification, determined by trial or experiment. Could 

 we estimate the value of a shadow, and did we know the 

 value of the illuminating ether, the same thing might be 

 done with light. 



78. Every known substance reflects light, whether it be 

 opaque or transparent. It could not be seen otherwise. 

 That portion of the light which is reflected must cast a 

 shadow, and where can that shadow fall but in the first 

 place within the transparent substance? and if the sub- 

 stance is a refracting substance a portion of the shadow 

 will appear coloured under certain conditions as will be 

 shown by and by. As the distance from the refracting 

 substance increases the light decreases, while the shadow 

 or penumbra which has been cast by the reflected light 

 increases; they are thus coordinate elements, as will be 

 presently shown, in the formation of colour. 



79. But darkness, or the effect of shadow, may be pro- 

 duced by reflection as well as by the interposition of a 

 solid body. It is chiefly by reflection that darkness is pro- 

 duced in nature, at least such darkness as I am speaking 

 of. The geometrician will have no difficulty in conceiving 



