Page:Proceedings of the Royal Society of London Vol 69.djvu/51

Rh mean that to match green more violet would have to be left to form white with the excess red, and the dilution of the green with white would be greater.

13. Summary of Reasons for using Inks with Abrupt Absorption.

i. The colour will not be affected by slight variations of register.

ii. Variations of the amount of ink in the several impressions will not be so important.

iii. The colours will be much purer, being mixed with less white.

iv. The curves for the filters can be found if desired by determining the complementary light, and working as Abney has described in his paper in the ' Photographic Journal ' already mentioned.

This last will, however, only be possible if the curves are nearly 100 per cent, up to the absorption in each case, and so far I have found no set of inks, nor even a single ink for which this is the case.

By theoretical inks I merely mean inks which are perfectly trans- parent until the absorption commences, and then are perfectly opaque. With such inks we can calculate the percentage area to be covered to match any given colour by considering the absorptions they produce.

I drew on a large scale the curves found by Abney for the ray composition of the normal spectrum, in which equal ordinates form white. I drew vertical lines where the ink absorptions should termi- nate, and with a planimeter I found the areas of the curves up to these lines. Thus I obtained the ray composition of the inks.

Thus, taking the vertical lines at wave-lengths 49 and 59, I found the following areas. From the violet end of the spectrum up to A. 49

Violet 122-4 or Violet 119-4

Green 4-8 Green 1-8

Red ....:.... 3-0 Red 0-0

White 3-0

This will be the part of the spectrum absorbed by the yellow ink. From A 49 to A 59 the areas were

Violet 25-8 or Violet 0-0

Green 128-0 Green 102-2

Red 79-5 Red 53-7

White 25-8