150 \PERIMENTS ON COLOUR, AS PERCEIVED BY THE I: 



supposed to terminate. It may be remarked, that the apparent colour of the 

 spectrum changes most rapidly at three points, which lie respectively in the 

 yellow, between blue and green, and between violet and blue. The wave-lengths 

 of the corresponding rays in W<r are in the proportions of three geometric 

 means between 1 and 2 very nearly. This result, however, is not to be con- 

 sidered established, unless confirmed by better observations than mine. 



The only safe method of completing Newton's construction is by an exami- 

 nation of the colours of the spectrum and their mixtures, and subsequent 

 calculation 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 peculiarities so ascertained may form the foundation of a more complete 

 theory of the colour-sensation. 



On the relation of tlte pure rays of the Spectrum to the three assumed Elementnrij 



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, 

 disease, 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 

 spectrum 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 nb 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 



