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
