EXPERIMENTS ON COLOUR, AS PERCEIVED BY THE EYE. 151 



must be produced by combination. The experiments of Helmholtz lead to the 

 same conclusion ; and hence it would appear that we can find no part of the 

 spectrum which produces a pure sensation. 



An additional, though less satisfactory evidence of this, is supplied by the 

 observation of the colours of the spectrum when excessively bright. They then 

 appear to lose their peculiar colour, and to merge into pure whiteness. This 

 is probably due to the want of capacity of the organ to take in so strong an 

 impression ; one sensation becomes first saturated, and the other two speedily 

 follow it, the final effect being simple brightness. 



From these facts I would conclude, that every ray of the spectrum is capable 

 of producing all three pure sensations, though in different degrees. The curve, 

 therefore, which we have supposed to represent the spectrum will be quite within 

 the triangle of colour. All natural or artificial colours, being compounded of 

 the colours of the spectrum, must lie within this curve, and, therefore, the colours 

 corresponding to those parts of the triangle beyond this curve must be for ever 

 unknown to us. The determination of the exact nature of the pure sensations, 

 or of their relation to ordinary colours, is therefore impossible, unless we can 

 prevent them from interfering with each other as they do. It may be possible 

 to experience sensations more pure than those directly produced by the spec- 

 trum, by first exhausting the sensibility to one colour by protracted gazing, and 

 then suddenly turning to its opposite. But if, as I suspect, colour-blindness be 

 due to the absence of one of these sensations, then the point D in diagram (2), 

 which indicates their absent sensation, indicates also our pure sensation, which 

 we may call red, but which we can never experience, because all kinds of 

 light excite the other sensations. 



Newton has stated one objection to his theory, as follows: "Also, if only 

 two of the primary colours, which in the circle are opposite to one another, be 

 mixed in an equal proportion, the point Z" (the resultant tint) "shall fall upon 

 the centre " (neutral tint) ; " and yet the colour compounded of these two shall 

 not be perfectly white, but some faint anonymous colour. For I could never yet, by 

 mixing only two primary colours, produce a perfect white." This is confirmed by 

 the experiments of Helmholtz ; who, however, has succeeded better with some 

 pairs of colours than with others. 



In my experiments on the spectrum, I came to the same result ; but it 

 appeared to me that the very peculiar appearance of the neutral tints produced 



