AS PERCEIVED BY THE EYE. 295 



tion of the colours of the spectrum and their mixtures, and subsequent calcula- 

 tion by the method used in the experiments with coloured papers. In this way 

 I hope to determine the relative positions in the colour-diagram of every ray Of 

 the spectrum, and its relative intensity in the solar light. The spectrum will 

 then form a curve not necessarily circular or even re-entrant, and its peculiari- 

 ties so ascertained may form the foundation of a more complete theory of the 

 colour-sensation. 



On the relation of the pure rays of the Spectrum to the three assumed Elementary Sensations. 



If we place the three elementary colour-sensations (which we may call, after 

 Young, red, green, and violet) at the angles of a triangle, all colours which the 

 eye can possibly perceive (whether by the action of light, or by pressure, dis- 

 ease, or imagination), must be somewhere within this triangle, those which lie 

 farthest from the centre being the fullest and purest colours. Hence the colours 

 which lie at the middle of the sides are the purest of their kind which the eye 

 can see, although not so pure as the elementary sensations. 



It is natural to suppose that the pure red, green, and violet rays of the spec- 

 trum produce the sensations which bear their names in the highest purity. But 

 from this supposition it would follow that the yellow, composed of the red and 

 green of the spectrum, would be the most intense yellow possible, while it is the 

 result of experiment, that the yellow of the spectrum itself is much more full in 

 colour. Hence the sensations produced by the pure red and green rays of the 

 spectrum are not the pure sensations of our theory. Newton has remarked, that 

 no two colours of the spectrum produce, when mixed, a colour equal in fulness 

 to the intermediate colour. The colours of the spectrum are all more intense 

 than any compound ones. Purple is the only colour which 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 ob- 

 servation 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 impres- 

 sion ; 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 



VOL. XXI. PART II. 4 K 



