Page:Proceedings of the Royal Society of London Vol 69.djvu/47

Rh tion of the .added white, uucl the luminosity of the resultant colours in each case. I have worked out the curves for two interesting positions of the absorption limits.

8. Choice of Inks.

The mere appearance of an ink, even when a rubbing on paper is taken, is not much guide, as the eye is not suited for selecting the inks by their hue ; for they are double-colour inks, and we know that the eye cannot at all judge of the composition of a yellow or a blue. Yellow, for instance, might be spectrum .yellow, or a mixture of any part of the red end of the spectrum with almost any part of the green. In other words, the eye cannot distinguish between yellows of a great variety of compositions. But the eye is a fairly good judge of the three primary colours at least, of their hue so that if the inks be printed in pairs a fair estimate can be made. The violet produced by the pink and blue inks should be almost pure, containing only a small percentage of white, as the red and green are almost equal. The green should very closely match the green sensation. But the red will contain some green, and should about match the hue of A. 67-4.

But the only true test is the spectroscopic one. Examined spcctro- scopically

1. The inks should each absorb one band of the spectrum in the

red, green, and violet respectively.

2. Must transmit the remainder unaffected.

3. Must absorb the whole between them.

4. For half-tone work the absorptions must end abruptly, especially

in the case of the pink and blue inks.

5. The limits of the absorptions should be so chosen that the added

white may be distributed through the spectrum as nearly as possible proportionally to the luminosity of the spectrum, and that

6. The luminosity of the resulting colours in matching the spectrum

should be as high as possible.

7. The inks will not be complementary to the colour sensations.

For (3) the absorptions must meet, for (2) they must not overlap ; thus (2) and (3) lead to (4).

The above conditions are, I believe, sufficient. I have examined a large number of inks supplied by different firms to see how far they are realised by those at present in use.

As the orange, yellow, green, and blue-green parts of the spectrum are far brighter than the rest, it is here that any conditions laid down must be specially observed. In the blue and violet and in the deeper red it is not so important.