Color Optical Illusions, 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.

Fig. 69. - By combining these
stereoscopically the effect of
metallic lustre (similar to graphite
in this case) is obtained.

[Editor's Note - This optical illusion may be more easily seen on a printout than on your computer screen. In any case, it can take upto a couple of minutes for your eyes to combine the two images.]

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 optical illusions of reversible perspective and others in Chapter V in this connection.

An interesting result of retinal rivalry can be experienced by combining two stereoscopic pictures, one black and one white. 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 Fig. 2 and Fig. 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.