a disk cut like the last with open spaces, and armed with red, yellow, and blue glasses. You can predict the result beforehand: it must be white light with added red light—and, as you see, we actually do obtain a broad circular band of red light. Replacing this disk by one provided with glasses capable of transmitting red, green,
Fig. 5.
and violet light, we find that their mixture actually gives us white light. In all these experiments we have been content with the colored light furnished by stained glasses, but Helmholtz has pushed the investigation much further, and has obtained corresponding results by the use of the pure colored rays of the spectrum.
I called your attention some time ago to the typical mode of expressing the old theory by three intersecting circles of red, yellow, and blue; we have now again on the screen three intersecting circles; the colors are red, green, and violet, with white at the centre
Fig. 6.
(Fig. 6). It expresses in a condensed form some of the main points of the theory of Young and Helmholtz, and gives us at the same time some of the chief laws of Nature's palette, showing, in a kind of short-hand way, the changes which the tints of surfaces undergo
