30 Dr. R S. Clay. On the Application of 



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 



