46 MR. J. SMITH ON THE ORIGIN OF COLOUR 



from or sections of the great photospheric or ethereal 

 wave. We thus see the value of the negative element 

 when once it has been introduced, and the difficulty, the 

 impossibility of again separating it in any colorific ray. I 

 therefore hold to my argument, that light and shadow are 

 coordinate elements of colour. 



But perhaps I may be told, that if red and blue do not 

 make white, then red, blue and green, the colours which 

 we have now got, make white. Dr. Young says they do. 

 And if by combining them I could prove that I produced 

 purple, I might then be told that red, green, blue and 

 purple make white ; and so on ad infinitum. 



From this experiment, combined with the former, a good 

 resume of my whole argument may be obtained. For 

 although in the experiment with the two wafers we could 

 not but infer that green was a combination of two sensa- 

 tions, it did not show us that shadow took any part in the 

 formation of colour; it merely enabled us to draw the 

 inference, that the change of colour could not be accounted 

 for but on the supposition that there is a vibratory motion 

 in light. These experiments, I think, give us solid argu- 

 ments to conclude that light and shadow are coordinate 

 elements of colour. 



PART II. 



EXPEEIMENTAL. 



69. In my researches into the cause of colour I was 

 led — in order to satisfy the conditions of several prob- 

 lems — to introduce in considering the subject a negative 

 term, not in accordance with the commonly received 

 theories. Newton, in his theory of light, recognises only 

 positive terms, for he considered that he had demonstrated 



