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LIGHT: COLOUR 301 red ink, poured over glass, dried, and treated in the same way t are red by transmitted light, mostly yellow-green by reflected light. Thin silver deposited on glass appears blue by trans- mitted light, though it apparently reflects all the rays. 172. Complementary Colours.—The primary meaning of complementary colours has been illustrated already by the two coloured images of the slit, produced by deflecting part of a spectrum by a wedge-prism, before re-uniting it by a lens into a white image of the slit on the screen (see p. 286). Two complementaries of this kind contain between them the whole spectrum of white light, and may be varied ad libitum by the wedge-prism, and also by using a quartz plate and double-image prism as in Chapter XXII. But much less than the whole spectrum will produce white, which may be shown by cutting out a kind of comb in black card, the teeth and spaces being about half an inch wide. Held in the spectral rays proceeding from the re-uniting lens, this cuts out half of them, but the image of the slit is still white. And it can be shown that even pairs of colours produce white, by re-uniting the spectrum with a lens, either spherical or cylindrical, of not less than 14 inches focus and 6 inches diameter, so as to have the spectrum ' spread ' considerably before re-uniting it on the slit image. Then close up to the lens must be fixed a slip of wood with a deep saw-cut along its top surface, in which black cards with apertures cut in them can be made to stand and be adjusted at pleasure. A slit in the red, one rather wider in the green, and a broad band in the violet, will give a white of three colours. A rather narrow slit in the red, and one double the width in greenish-blue, will give a white of two colours. A narrow slit in orange-yellow (just the red side of the D line) and a rather broad band in full blue, also give white. And the eye is here also deceived, for it cannot distinguish (except in brightness) between these two-colour whites, and that consuming the whole spectrum. Nor can the eye distinguish—as may be shown in the same