Page:Proceedings of the Royal Society of London Vol 69.djvu/46

38 Thus the blue ink should not be the complementary of the red sensa- tion. And in fact the ink will be far from complementary to the red. I find by taking the areas of Abney's curves up to B' with a planimeter, that it should very nearly match the green at about A 50-3.

The yellow ink would be made complementary to the violet sensa- tion by ending the absorption at A, but it would be very pale, for it would transmit a large amount of white, as there is a large proportion of the violet beyond A. The result will be an unnecessary amount of white in the final picture. The result will be better if the absorption reaches to A'. Thus this ink also should not le complementary to the violet sensation. The pink ink at the red end of the spectrum transmits red and green, and at the other end chiefly violet and green. If the violet is equal to the red it will be complementary to the green sensation. This can be achieved by moving A towards the violet end, but only at the expense of the yellow, which would be rendered very pale. So this ink also should not le complementary.

I have gone into this rather fully as it has so frequently been stated that the inks ought to be complementary to the colour sensations. In a very rough and general way this is true, but there is no advantage in such inks; on the contrary, they would, even if obtainable with abrupt absorptions, not be so good as others that are not comple- mentary. The only possible advantage in using complementary inks would arise in the case in which the absorptions were abrupt, for then the filters would be complementary to Maxwell's curves. But the advantage here is only one of theory, the printed result being, as we have seen on the preceding page, a picture with a very large excess of white. As in practice the filters would be adjusted by trial, even this advantage is illusory.

To summarise. The inks could only be complementary

1. If they had abrupt absorptions with limits near the ends of the

spectrum, when the colours would be in some cases mere tints ; or

2. If they have overlapping absorptions, or if some colours are

absorbed by more than one ink, both of which are bad, since the resultant absorption will follow a geometrical and not an arithmetical law.

The exact positions of the limits will, no doubt, finally depend on the fact that there will be very few pigments which sufficiently fulfil the conditions, and the blue and pink ones with abrupt absorptions near the yellow will not be numerous. Still it is theoretically interesting to determine the best positions, apart from the difficulty of finding inks to suit. This will be best done by calculating the curves for different absorption limits and estimating the amount and distribu-