[15] 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.
[16] Martin: op. cit.
[17] Martin, pp. 525-8.
[18] In teaching the anatomy of the ear, great assistance will be yielded by the admirable model made by Dr. Auzoux, 56 Rue de Vaugirard, Paris, described in the catalogue of the firm as "No. 21—Oreille, temporal de 60 cm., nouvelle édition," etc.
[19] This description is abridged from Martin's 'Human Body'.
[20] Martin: op. cit.
[21] Martin: op. cit.
[22] Martin: op. cit.
[23] Martin: op. cit.
[24] Martin: op. cit.