AND THE THEORY OF LIGHT, 31 



of colour^ but it would be doing more : it would be proving 

 the homogeneous nature of light ; it would be proving that 

 the luminous ether no more requires three or seven, or 

 any other number of primary colours or waves of different 

 values, to produce all the modifications of light, than the 

 air requires seven or eight constituent principles to pro- 

 duce all the modulations of the gamut. The ancients 

 appear to have thought that shadow or darkness was a 

 positive principle, and so far as it is a sensation it is ; and 

 Newton seems to have experimented under that idea, or 

 how could he have thought of reflecting light on shadow ? 

 At all events, the ancients considered that they saw 

 some connection between shadow and colour, and Newton 

 set himself to prove that they were in no way connected. 

 It did not occur to Newton, v/hen experimenting with his 

 rays of light (derived from the spectrum) on the " confines 

 of shadow," as he describes it, that he was not in any way 

 altering the constituent elements of his rays of light. He 

 was not reducing their momentum, nor was he making 

 them alternate with shadow. Wherever change of colour 

 is observed in such phenomena as we have been describing, 

 attentive consideration of the whole circumstances will 

 show us that there are two planes necessary to produce 

 the two sensations of light and no-light. In a luminous 

 atmosphere the shadow must be produced by one ray or a 

 portion of a ray, and illuminated by another. In refracting 

 substances, as will be shown by and by, there are two 

 planes at difierent angles, one reflecting light, the other 

 no-light; for how otherwise could the intervals of motion 

 in the vibrations of the luminous ether be perceived ? But 

 although there are, apparently, two difl'erent processes in 

 nature, they are virtually one. My experiments are an 

 attempt to produce the same effect by motion, so as to 

 imitate nature in these two processes. 



50. What, then, is a shadow? or, what is darkness? 



