36 Dr. B. S. Clay. On the Application of 



to entirely spoil the result, and this addition would be produced by a 

 displacement of the dots of about 1 /300th of an inch. 



It is obvious, then, that in process work the absorptions of the inks 

 must not overlap. On the other hand, if we are to be able to pro- 

 duce a black, the absorptions must meet. Thus in process work (in 

 the case of the blue and pink inks at least) the absorptions must just 

 meet without overlapping, and therefore they must end abruptly. It 

 will not matter so much in the case of the yellow and pink inks, for 

 the part of the spectrum where their absorptions meet is of far inferior 

 luminosity. 



In collotype printing this difficulty would not arise in the same way. 

 Here the tint is produced by the strength of the colour, the lighter 

 shades being produced by covering the whole surface with a thin 

 layer of ink. In this case there would be no uncertainty due to 

 imperfect registering. But at the same time the ink curves for a thick 

 layer of ink are very different to those for a thin layer, as is shown by 

 the measurements I have added of the curves of different depths of the 

 same ink. Thus the proportions in which the three inks should be 

 combined to match a certain spectrum colour will vary with the 

 luminosity of that colour. 



To illustrate this. Suppose an object all one colour, such as a cast, 

 to be illuminated by a monochromatic light such as a sodium flame, 

 and a photograph taken, the result ought to be a picture in shades of 

 yellow. But if the curves of the same ink when printed to different 

 depths are different so that the proportions of the colour sensations 

 that it reflects are different, the proportion in which it is combined 

 with the other two inks to match a bright yellow will not be the same 

 as that required to match a dull yellow of the same wave-length. 

 Thus if the filters are adjusted to give the exposures on the three 

 plates that will reproduce that hue in, say, the high lights, it will not 

 be correct in the half-tones, and will be still worse in the shadows. 

 The ideal ink for this process would be one which would absorb the 

 colours in the same relative proportions, whatever the depth to which 

 it was printed. The curve would be one of perfect transparency up to 

 the absorption band, then a sudden drop (for the full colour), say, to 

 about 2 p'er cent., to remain at this height till the end. The curve 

 should have no rounded corners. 



It will be seen that the sharpness of the drop depends on the depth 

 to which the colour is printed ; thus for " process " work, where it is 

 always printed to the same strength, the absorption can be made 

 abrupt by using a very full tint. But if this is to be done, the ink 

 must be a very transparent one to the part of the spectrum it is not 

 supposed to absorb, for, as we have seen, it is very important that the 

 same part of the spectrum should not be absorbed by more than one 

 colour. 



