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66 LIGHI AND WATER substance, placed so as to intercept a beam of white light, only allows light of certain wave-lengths to pass, which give the sensation of colour to the eye. Thus in looking through a piece of red glass we see red, because all (or nearly all) the rays except the red ones are absorbed.* By experiment with the prism it will at once be seen that the red glass does not convert white light into red, but that it produces the change by subtraction of some of its constituents. Again, in the case of most opaque coloured objects, part of the white light falling on the object is reflected at the surface unchanged, while the remainder penetrates to a small depth below the surface before being reflected, and on its passage through a thin layer of the coloured substance suffers absorption of some of its component parts and emerges as coloured light. It is evident then that in the case of the red glass we Shall see no colour unless light pass through the glass. Place a piece of coloured glass ona dull black cloth and it appears black; but lay it on a sheet of | white paper and its colour at once becomes visible. The paper reflects light from its surface through the glass, which the black cloth does not. So it is with water; if a white stone is dropped into a clear lake, the deeper it goes the more it shows the real colour of the water—generally a greenish-blue—as the light reflected from it has to travel farther through the ' This is.true of the red glass used by photographers to exclude actinic rays; but it is not necessary that all light of other colours should be absorbed by the medium to give the sensation of red. If the complementary colour alone is absorbed we see red, though this red is actually far from being pure.
