Most of us know the result of turning a series of circles (as in [Fig. 11]) horizontally with the eye. The circles appear to revolve rapidly round their center, and in different directions. This is solely because the eyes become confused, giving one more proof, were it needed, that they are no more infallible than anything else on this wide earth.
Some very interesting experiments in color illusions can be made. So cunning is the deception played upon us by our eyes, it is extremely difficult to believe that some of the tints we see in the experiments are but imaginary.
Fig. 11.—Revolving circles.
Fig. 12.—The color top.
In [Fig. 12] you see a top which can be constructed of cardboard in this way. Take a postcard and cut a circle, upon which you draw a diameter as A B. Black the part A C B with India ink, and divide the other half into four equal portions by lightly penciling the radii G E, G D, and G F. Now, still using your India ink, make arcs in these four divisions in the same way as is shown in the figure.
Fig. 13.—To make black print red.
Having done this carefully and rubbed out the pencil radii when the ink has dried, put a pin through the center G from the back, so that the card can easily revolve whilst the pin-head prevents it from falling off. Your color top is now ready. Make it turn rapidly upon the pin; look closely at the card and what do you see? The inner circles become red and the outer ones blue! And yet you know perfectly well that the only colors really upon the card are black and white!