which we can measure the saturation of color or its exact place in the chromatic circle; hence, if we have no undoubted external standard at hand with which to compare our colors, we are easily deceived. A slip of paper, of a pale but very decided blue-green hue, was placed on a sheet of paper of the same general tint, but somewhat darker and more intense or saturated in hue. The small slip now appeared pure gray, and by no effort of the reason or imagination could it be made to look otherwise. In this experiment no undoubted pure gray was present in the field of view for comparison, and, in point of fact, the small slip did actually approach a pure gray in hue more nearly than the large sheet; hence the eye instantly accepted it for pure gray. The matter did not, however, stop here: a slip of pure gray paper was now brought into the same green field, but, instead of serving as a standard to correct the illusion, it assumed at once the appearance of a reddish-gray. The pure gray slip really did approach reddish-gray more than the green field surrounding it, and hence was accepted for this tint.
It has been stated above that the effects produced by simultaneous contrast are due not to retinal fatigue, but to deception of the judgment; now, as the effects of simultaneous contrast are identical in kind with those generated by successive contrast, it is evident that the statement needs some proof. This can be furnished with the aid of a beautiful experiment with colored shadows. In making this experiment, we allow white daylight to enter a darkened room through an aperture. A, arranged in a window, as indicated in Fig. 13. At M we set up a rod
and allow its shadow to fall on a sheet of white cardboard or on the white wall of the room. It is evident, now, that the whole of the cardboard will be illuminated with white light, except those portions occupied by the shadow, 1. We then light the candle at C (Fig. 14); its light will also fall on the cardboard screen, and will then cast the