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300 OPTICAL PROJECTION shown by the prism to be generally of a most composite character. Another experiment may be made. Go up to within a yard or two of the screen, and hold up a large piece of coloured glass in the rays. It will be seen that in some rays it casts a perfectly black shadow — it is opaque to light of that colour— but in other rays it is more or less transparent. With more colouring matter, absorption increases, and it is instructive to prepare a few cells of wedge shape, or wider one end than the other, and fill them with coloured solutions. Placed as in fig. 166, it will be found that often the apparent colour changes when a thicker cell is used; a colour only slightly obstructed by a thin layer, being completely obstructed by a thicker layer. These experiments may be varied ad libitum. That the surf ace -colours of bodies are largely transparent colours, or remainders left after other colours have been absorbed by a layer of the surface molecules, is shown similarly, by holding large flowers with good masses of colour in the rays of the spectrum ; or the coloured prints sold with floral magazines will answer very well. Each colour, in some rays, will appear bright and natural; in others dull grey, and in others black. The monochromatic lights mentioned further on have the same effect. 171. Transmitted and Reflected Colours.—Surface colours are, however, not all due to transparency, and hence there is sometimes a great difference between the colours a substance reflects and transmits. This is specially true of metallic and semi-metallic bodies, and may be shown by a film of gold-leaf mounted as a slide. It appears green by transmitted light. Deflect the lantern parallel with the screen, condense all the light from the condensers through the open nozzle on its surface, at an angle of 45°, l and focus the reflecting surface with the loose lens, and it is yellow. The aniline dyes used for 1 Exactly as the soap-film, fig. 179, p. 827.