Note 15 — Chapter III · SIGHT.

The ordinary mixing of pigments is not an addition, but rather, as Helmholtz has shown, a subtraction, of lights. To add one color to another we must either by appropriate glasses throw differently colored beams upon the same reflecting surface; or we must let the eye look at one color through an inclined plate of glass beneath which it lies, whilst the upper surface of the glass reflects into the same eye another color placed alongside—the two lights then mix on the retina; or, finally, we must let the differently colored lights fall in succession upon the retina, so fast that the second is there before the impression made by the first has died away. This is best done by looking at a rapidly rotating disk whose sectors are of the several colors to be mixed.