Purkinje Effect.—The relative sensibility of the retina varies for different colors with a change in brightness; or it may be better to state that the relative sensations for various colors alters as the brightness values are reduced to a low intensity. For example, if a reddish purple (consisting of red and blue or violet rays) be illuminated in such a manner that the intensity of illumination, and consequently its brightness, may be reduced from normal to a low value (approximating moonlight conditions), it will be seen to vary from reddish purple to violet. In doing this its appearance changes through the range of purples from reddish to violet. This can be accomplished by orientation of the purple surface throughout various angles with respect to the direction of light or by reducing the illumination by means of screens.
In general the Purkinje effect may be described as an increasing sensibility of the retina for light of shorter wave-lengths (violet, blue, green) as the brightness decreases, or a corresponding decreasing sensibility for light of longer wave-lengths (yellow, orange, red). The effect may be seen on any colored surfaces at twilight illumination. A blue and a red flower, which appear of the same brightness before sunset will begin to appear unequal in this respect as twilight deepens. The red will become darker more rapidly than the blue if there are no appreciable changes in the color of the daylight. Finally all color disappears. It is better to perform this experiment under artificial light, in order that the spectral character of the illuminant may be certain to remain constant. In this case rheostats must not be used for dimming the light because of the attendant changes in color or quality of the light.
The Purkinje effect may be noticed by the careful observer and it is responsible for certain illusions. Apparently it cannot operate over one portion of the retina, while the remainder is stimulated by normal intensities of light.
Retinal Rivalry.—Many curious effects may be obtained by stimulating the two retinas with lights, respectively different in color. For example, it is interesting to place a blue glass before one eye and a yellow or red one before the other. The two independent monocular fields strive for supremacy and this rivalry is quite impressive. For a moment the whole field may appear of one color and then suddenly it will appear of the other color. Apparently the fluctuation of attention is a factor. Usually it does not seem to be possible to reach a quiescent state or a perfect mixture of the two colors in this manner. The dependence of one monocular field upon the other, and also their independence, are emphasized by this experiment. It is of interest to consider the illusions of reversible perspective and others in Chapter V in this connection.
Fig. 69.—By combining these stereoscopically the effect of metallic lustre
(similar to graphite in this case) is obtained.
One of the interesting results of retinal rivalry is found in combining two stereoscopic pictures in black and white with the black and white reversed in one of them. The apparently solid object will appear to possess lustre. The experiment may be tried with [Fig. 69] by combining the two stereoscopic pictures by converging or diverging the axes of the eyes as described in connection with [Figs. 2] and [3].
It will be noted that in order for two stereoscopic pictures, when combined, to produce a perfect effect of three dimensions their dissimilarity must be no more than that existing between the two views from the two eyes respectively. The dissimilarity in [Fig. 69] is correct as to perspective, but the reversal of white and black in one of them produces an effect beyond that of true third dimension. When the colors are so arranged in such pictures as to be quite different in the two the effects are striking. There is, in such cases, an effect beyond that of perfect binocular combination.
By means of the stereoscope it is possible to attain binocular mixture of colors but this is usually difficult to accomplish. The difficulty decreases as the brightness and saturation of the colors decrease and is less for colors which do not differ much in hue and in brightness. These effects may be studied at any moment, for it is only necessary to throw the eyes out of focus for any object and to note the results. Many simple experiments may be arranged for a stereoscope, using black and white, and various combinations of colors. For example, [Fig. 65] may be combined by means of double images (produced by converging or diverging the optical axes) so that the two inner squares are coincident. Actual observation is much more satisfactory than a detailed description.