Page:Proceedings of the Royal Society of London Vol 69.djvu/49

Rh and (supposing for the moment the plate to be equally sensitive to all colours) will be proportional to the final opacity of the plate. If the filter curves have been arranged so as to match the spectrum in luminosity as well as hue, then the resulting print will reflect the sum of light which is equivalent in ray composition to the sum of this band of spectrum colour, and will therefore match it in dominant hue. But if the spectrum has been matched in hue only, the resultant colour will be the sum of the colours in this band mixed in the wrong proportions, and will not have the same dominant hue as the original.

If the inks are neither perfectly transparent or perfectly opaque to each colour, so that there are no parts of the spectrum to which they are only partially transparent, and if also the absorptions do not overlap, the resultant colour will be that left after successive subtraction by the three inks, and will be the same as would result from the successive addition of the three complementary lights in triple projection; and then the filters can be found by determining the dominant hue of the complementary colour,

The curves for the filters can also be calculated in this case by finding what percentage area the inks should cover so as to transmit light whose composition, in sums of the colour sensations, is everywhere that given by Maxwell's curves. This is how I calculated the curves for the hypothetical inks.

But when we deal with practical inks these conditions are far from being fulfilled, and the filters cannot be found by dealing with the complementary lights. For, as we have seen, since the same colour is absorbed, at least to some extent, by more than one ink, the arithmetical law of absorption will not hold, and the colour resulting from the printing with the inks is not that produced by the combination of the complementary lights. So that in practice we shall have to adopt some other method, which must be experimental, and must be founded upon impressions of the actual inks.

One of the greatest practical difficulties in printing with only three colours, and especially with such strong colours as red and blue, is to ensure that the quantity of ink shall not vary from impression to impression. Usually, very slight variation in the amount of ink is accompanied by a change in the whole tone of the picture. This is because with most inks the curve of absorption is different for different depths of colour, and thus the proportion of the three fundamental