Maxwell's U Hi- vex to Three-colour Work, dx 27 



correct, it may be matched by a mixture of the monochromatic light 

 and white. Thus the colours on the screen will not be true spectrum 

 colours, but spectrum colours diluted with white. 



The amount of white, of course, depends upon the glasses used, and 

 in matching some parts of the spectrum there will be more white than 

 in others. Yet the result is very good ; indeed, without direct com- 

 parison the white would hardly be noticed. The beautiful pictures 

 produced and exhibited with such coloured glasses by Mr. Ives is a 

 very good proof of the fact that an admixture of white is of very small 

 consequence. 



Note 1. The above is not strictly accurate, as it is not possible to 

 match some parts of the spectrum even when using pure spectrum 

 colours. The match can only be made after white has been added. 

 The reason for this can be seen from the curves. Owing to the over- 

 lapping of the sensation curves, the green sensation is nowhere excited 

 alone, but is always accompanied by a small excitation of the red and 

 violet. Thus the spectrum green although really a pure colour 

 excites all three sets of sensations in the eye, i.e., it produces the 

 sensation green and white. Now, to match the yellow, we require to 

 excite only green and red, and this we cannot do using spectrum red 

 and green, for the green always excites to some extent the violet 

 sensation. This violet combines with proper proportions of green and 

 red to produce white, so that we can imitate the yellow when it is 

 mixed with a little white, but not the pure yellow alone. The same is 

 true of the other end of the spectrum where the red sensation should 

 only be slightly excited. 



We shall in what follows neglect this necessity for the addition of 

 white in speaking of colour matches as it only complicates the question 

 without materially affecting the results. (Where my statements 

 require modification in consequence I shall indicate it by a star (*).) 



Note 2. In Abney's paper above referred to, he shows that the third 

 sensation is probably not violet, but a blue, which is near the blue 

 lithium line in the spectrum. As I shall be using the word " blue " in 

 speaking of the double colour blue ink, to avoid confusion, I shall 

 throughout refer to the third colour sensation as " violet." I shall, 

 however, use Abney's curves, and by the term " violet " shall mean his 

 " blue sensation." 



2. Three-coliwr Printing. 



When we turn to printing, matters are not so simple. One's first 

 impulse would be to select those parts of the original picture in which 

 red light occurs, or which excite the red sensation, and print them with 

 a red ink, and so with the green and violet ; but a little consideration 

 will show that this would not be right. For instance, a yellow object 

 affects both our red and green sensations, according to Maxwell's 



