Page:Proceedings of the Royal Society of London Vol 69.djvu/36

28 curves, and thus we should have to print it with the red and green inks. Now the original yellow was equivalent to the addition of a red and green light, so that it ought to be brighter when both colours are there than when only one. The reverse would be the case if printed as above, and, of course, as a white object reflects all the colours, it would be printed with all three inks. Now, though red, green, and violet lights when added give white, red, green, and violet inks super- posed on white paper certainly will not make white.

In fact, inks produce colour by absorption. Thus, instead of succes- sive inks adding to the light, they each reduce it.

Suppose, then, we proceed the other way, and instead of looking for those parts of the picture where red is reflected and printing with red ink, we print those parts where no red is reflected with an ink which absorbs red, but reflects all the other colours. This will be a bright bluish-green ink. In the same way we will print those parts which reflect no green with an ink which absorbs that colour ; this will lie a bright magenta or almost a pink. And those parts which reflect no violet we will print with a yellow ink. Now a yellow object reflects red and green but no violet. Hence it will have to be printed with the violet absorption ink only that is, the yellow and with neither of the others. A red object reflects neither green nor violet, and will be printed with the pink and yellow inks, which will leave red only. As a yellow object is printed with only the one ink, but a red one with two inks, the yellow will be the brighter, and this is as it should be.

As the inks are to absorb red, green, and violet respectively, they will, roughly speaking, be complementary to those colours, that is, when added to them either by a double-image prism, or by reflection from a clear glass surface, they should give white. Still, comple- mentary is a very vague description of a colour, and it must be our aim presently to define it more precisely.

The above theory is due to Mr. Ives.

3. Application of Maxwell'* Curves.

As we said at first, these curves were derived by adding three monochromatic lights, and are only strictly true of such colours. If, however, we use lights compounded of or, at least, capable of being matched by one of these monochromatic colours and white, we shall still obtain a result very nearly as perfect as with the pure colours. It can easily be seen, however, that if any other colour than white were combined with the pure colours the result would be spoilt.

All colours, including white, can be matched both in hue and luminosity* by combining the three monochromatic colours, red, green, and violet, in proportions given by Maxwell's curves. It is easy to


 * See Note 1, p. 27.