Page:Proceedings of the Royal Society of London Vol 69.djvu/50

42 colours left in the reflected light also will vary with the depth. This will not matter much when a large number of colours twelve or fourteen are used, as the average amount of red and blue put on by the successive inks will be nearly constant ; but with only one ink of each colour the effect is very marked.

This effect is greatest when the curve for the ink is a gradual one, for as the quantity of ink is increased in an arithmetical progression, the absorption of any given colour will increase in a geometrical pro- gression, assuming, of course, that the ink is transparent. Thus, unless the distributing apparatus of the machine is very perfect indeed, it will be impossible with such an ink to obtain uniform impressions.

Were the ink one with abrupt absorption, that is, one which Avas very transparent up to a certain colour, and then nearly opaque, slight variations in the quantity of ink would have very little effect, and one of the greatest difficulties would at once disappear.

12. Effi'd on the Purity of the Colour of an Abrupt Absolution.

It might perhaps be supposed that inks which reflected most light in the parts of the spectrum for which they are to be transparent, and gradually reflect less up to the part they are to absorb, would be nearer approximations to monochromatic inks than those whose absorp- tions begin abruptly. That this is not the case can be easily seen if we compare the two red-absorption inks opposite. In the second we have replaced the reflected light ABD by BEG. Now Maxwell's curves show that the proportion of red to green in BEG is much greater than it is in ABD, so that the absorption of the red is not so complete in II as

it is in I, while the transparency to green is less. Thus II would not be so good an ink as I. When, for instance, this and the violet absorption ink are printed, which ought to leave green only, there would be some red left, and this in greater amount than with the ink I. This would