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Page:Popular Science Monthly Volume 4.djvu/597

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579
MODERN OPTICS AND PAINTING.

to the preceding, though, at first sight, the connection is not very evident. Any color, if very luminous, seems paler than it really is. This simple piece of apparatus, where a bat-wing gas-flame is placed between a sheet of card-board and a plate of stained glass, will serve for experimental demonstration. The glass is red, and the paper seen through it appears of a deep-red hue, but the gas-flame itself, being much more luminous than the paper, does not look red at all; its tint is orange (Fig. 9). Replacing the red glass by green,

Fig. 9.
PSM V04 D597 Colors through flame experiment.jpg

we have the paper appearing with a deep-green hue, while the flame seems greenish-yellow. Let us see, if we can explain these curious changes of tint by Young's theory. The red glass used in the first experiment transmits to the eye only red light, or light capable of stimulating mainly the red nerves; but, if we increase its intensity beyond a certain point, its action on the red nerves begins to flag, and we soon have a state of things where a further increase of the red light produces no effect at all on the red nerves, they being already stimulated up to the maximum point. But, according to our theory, this red light has all along been acting, to some extent, on the green, and to a less extent on the violet nerves; and, as we add to its intensity, it acts still more powerfully on them, so that especially the green nerves come more and more into play, and a green is added to the original red sensations; the result, of course, is the sensation of orange.

The explanation of the tint obtained in the other experiment is quite similar. The green nerves are first stimulated up to their maximum point by green light of a certain strength, a further increase of its intensity brings into play the other two sets of nerves, particularly the red, and the tint quite naturally becomes greenish-yellow. You remember that, in a previous experiment, we found that a mixture of much green with a little red light gave a greenish-yellow. The nerves for violet light always lag behind the others, as will afterward be shown by a particular experiment.

The general effect, then, of a very bright illumination on natural objects is to cause their colors to appear paler than they otherwise