Curves to Three-colour Work, &c. 



37 



7. Best Limits fw tlie Ink Absorptions. 



Assuming now that the absorptions should be abrupt, the next 

 question is, what limits should they have to give the best effects ? If 

 one is at A and the other at B in the accompanying diagram in 

 which equal heights of each colour would give white the yellow and 



blue inks will be complementary to the colour sensations, but they will 

 be very pale, for they reflect large quantities of all three colours. The 

 pink ink will be very dark, it will reflect only a very small amount of the 

 red sensation, and as the violet reflected is not equal to the red, it will 

 not be complementary to the green sensation. It could be made so 

 by shifting A further to the violet, so as to reduce the violet reflected 

 by the pink ink in fact, until the areas of the red and violet are equal. 

 It is obvious that this would give absurd inks ; the yellow and blue 

 would be so pale as to be mere tints, and the pink so dark as to appear 

 purple. 



If the absorptions are supposed to end at A' and B', the spectrum 

 will be much more evenly divided between the three colours, each 

 will be slightly pale, but now neither will be the complementary of the 

 colour sensation. For instance, if the yellow ink is to be conplement- 

 ary to the violet sensation, it should absorb either violet or violet and 

 equal amounts of red and green, measured by equal areas on the 

 above curves. But at A' (or at any other line between A and B) the 

 green sensation is evidently in excess. In the same way if the 

 absorption of the blue ink extends from the red end of the spectrum 

 beyond B, green is absorbed as well as red, and the ink cannot be 

 complementary to the red sensation. To make this ink comple- 

 mentary, it must have another absorption band somewhere in the 

 violet, and the difficulties we have been considering above will be 

 introduced, for there will then be a part of the spectrum absorbed by 

 more than one ink, namely, by the yellow and the blue, and in places 

 where these inks are printed side by side the absorption of this colour 

 Avill be twice as great as in those where they are on top of one 

 another. 



