Page:Proceedings of the Royal Society of London Vol 69.djvu/38

30 with generally the green in excess of that required to combine with the red to form white. Thus with such bands for our primary colours, we should have not the true sensation colours, but those colours with an addition of either green or white. The white will not matter much, but the green will, of course, spoil the effect.

With these bands more light is used, and both the grey produced by the original printing and the final colours will be much brighter ; thus the picture could be examined in a less powerful light.

Lastly, imagine the bands still wider ; in fact, allow them just to touch, so that they divide up the whole spectrum between them ; and suppose each ink to exactly absorb one of these bands. The first printing in neutral colour is no longer necessary, and we shall see the full white of the card. Also the picture will no longer require a strong illumination. But we shall now have our primary colours mixed with large amounts of other colours. The red will be mixed with green, the violet will be diluted with green and red (see Abney's curve, loc. <iL, p. 283), and the green with both red and green in pro- portions which may or may not form white.

Xote. It is possible to arrange that the red and violet in the middle

band shall make white, but it is not possible to so arrange that the red band shall be diluted with white, for there is no violet at the red end of the spectrum. Thus the blue ink cannot be made the comple- mentary of the red sensation if it is to leave a red band of any reasonable width, unless it has an absorption at the violet end of the spectrum, and this introduces a very serious fault, as the same colour would then be absorbed by more than one ink. (See 6.)

It will now be obvious that if we print these inks in amounts the complementaries of the Maxwell's curves (as in the theoretical case above) the colours we produce will be far from correct. The red will be diluted with green and so on.

If the red could be diluted with Avhite instead of green the picture would be much improved ; in fact, it would be almost as good as if the colours were pure. And fortunately this is quite possible, not how- ever by modifying the inks but by altering the proportions in which they are. printed, i.e., by varying the curves to suit them.

Suppose, for instance, we want to print a colour to match the red sensation. If we print with the green and the violet absorption inks the pink and yellow ones we shall leave the red end of the spectrum, which the curves show to excite the red and green sensa- tions. We want the red only, or if we cannot obtain that we must be content with red and white. This we can obtain with our inks by printing the violet absorption ink the yellow rather less fully. The violet and green then, with a part of the red will form white, and leave red only. So with any colour, if we cannot obtain it pure, we can always match it when diluted with white. The amount of white