12 



THE POPULAR SCIENCE MONTHLY. 



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 judg- 

 ment ; now, as the effects of simultaneous contrast are identical in kind 

 with those generated by successive contrast, it is evident that the state- 

 ment needs some proof. This can be furnished with the aid of a beau- 

 tiful 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 



Fig. 14.— Shadows of Eod, using Daylight and Candle-Light. 



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 card- 

 board will be illuminated with white light, except those portions occu- 

 pied 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 



