Stranica 166 [166]
OCR
100 LIGHT AND WATER necessary to point out that the mere mechanical mixture of these colours on the palette would produce a totally different hue to that given by their true mixture, such as the general tone of the rippled water, as seen from a little distance, caused by the union of the several colours reflected.* But the true resultant tone can be given in the same way by viewing a ' On page 66 we referred to the absorption of rays of certain definite wave-lengths in passing through coloured substances, such as red glass. In the same way light passing through two glasses of different colour undergoes two absorptions. If we look through a blue and yellow glass placed one over the other we see green. The blue glass, besides transmitting blue light, allows a certain amount of green light to pass; the yellow glass also transmits some green light; but the blue rays are cut off by the yellow glass and the yellow rays by the blue glass. It is thus only the green rays which are able to escape the twofold absorption and so reach the ~ eye. Similarly with a mixture of blue and yellow pigments the resulting colour is green; the greater proportion of the incident rays penetrates to a small depth below the surface before being reflected, and of these rays all except the green are absorbed by the combined action of the blue and yellow particles of which the mixture is composed. | But blue light and yellow light when mingled do not produce green, but w/zfe. ‘This can be shown by throwing beams of yellow and blue light on to the same white surface in a darkened room, or more simply by spinning a disc coloured partly yellow and partly blue. "Take two of Maxwell’s cardboard discs, [ELO each with a radial slit, one of the discs being b painted with ultramarine and the other with pale : \ sive) (not orange) chrome yellow. Adjust the discs so that half of one disc is concealed behind the other Fig. 28. —as shown in Fig. 28, where a—a indicates the radius at which the blue disc passes under the yellow disc, and 4—é the radius where the yellow disc passes under the blue—on rotating the compound disc, so that the eye
