Page:Proceedings of the Royal Society of London Vol 69.djvu/44

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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.