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 



