20 MR. J. SMITH ON THE ORIGIN OF COLOUR 



sing this to be done — " that the effect would be the same 

 as if we took away the yellow light from white, and sub- 

 stituted the indigo which would be left." This I cer- 

 tainly will not attempt to deny. I would, however, ask. 

 Did any man ever perform such an experiment on light; 

 or could any one perform, or even understand, it ? Who 

 ever saw the white light after the yellow was taken from 

 it ? or, who knows what the light of a candle is after the 

 abundant yellow is taken from it? Is it the light of the 

 sun, or what ? But, according to Dr. Young's theory, 

 there is no yellow to take from white. Even allowing, 

 for the sake of argument, that the yellow — which I sup- 

 pose is some proportion of red and green, according to 

 Dr. Young's hypothesis — can be taken from the white, 

 are we, therefore, to conclude that indigo would be left? 

 We are thus compelled to make two suppositions, unless 

 we can, prove by experiment that such would be the case. 

 It may be fairly asked, Is common daylight made up of 

 only two colours, yellow and indigo ? Of course Dr. Young 

 supposes that it is, for his argument here would require 

 it to be so ; whereas he says in another part of his work : 

 " We may consider white light as a mixture of red, green 

 and violet — only in the proportion of two parts red, four 

 green and one violet, with respect to the quantity or 

 intensity of sensations produced." If white light is com- 

 posed of red, green and violet in these or any other pro- 

 portions, how can we take yellow from it and leave violet ? 

 We might be able to compose yellow of certain propor- 

 tions of these colours ; but how we could take yellow from 

 white and leave*indigo, on such a supposition, is not easily 

 to be comprehended, unless we make the bold supposition 

 that all the red plus all the green is equal to yellow, which 

 we are nowhere told is the case, and cannot prove. Yellow 

 and indigo must be by this theory compound, and not 

 primary, colours, and consequently could not be taken or 



