SENSATION. 25 



moved. The contrast at once appears clearly, and its dis- 

 appearance tlirougli continued fixation can be accurately 

 watched. 



Brief mention of a few other cases of contrast must suf- 

 fice. The so-called mirror experiment consists of placing 

 at an angle of 45° a green (or otherwise colored) pane of 

 glass, forming an angle with two white surfaces, one hori- 

 zontal and the other vertical. On each white surface is a 

 black spot. The one on the horizontal surface is seen through 

 the glass and appears dark green, the other is i^eflected 

 from the surface of the glass to the eye, and appears by 

 contrast red. The experiment may be so arranged that we 

 are not aware of the presence of the green glass, but think 

 that we are looking directly at a surface with green and red 

 spots upon it ; in such a case there is no deception of judg- 

 ment caused by making allowance for the colored medium 

 through which we think that we see the spot, and therefore 

 the psychological explanation does not apply. On exclud- 

 ing successive contrast by fixation the contrast soon disap- 

 pears as in all similar experiments.* 



Colored shadoivs have long been thought to afi'ord a con- 

 vincing proof of the fact that simultaneous contrast is 

 psychological in its origin. They are formed whenever an 

 opaque object is illuminated from two separate sides by 

 lights of different colors. When the light from one source 

 is white, its shadow is of the color of the other light, and 

 the second shadow is of a color complementary to that of 

 the field illuminated by both lights. If noAv we take a tube, 

 blackened inside, and through it look at the colored shadow, 

 none of the surrounding field being visible, and then have 

 the colored light removed, the shadow still appears colored, 

 although ' the circumstances which caused it have disap- 

 peared.' This is regarded by the psychologists as con- 

 clusive evidence that the color is due to deception of judg- 

 ment. It can, however, easily be shoAvn that the persistence 

 of the color seen through the tube is due to fatigue of the 

 retina through the prevailing light, and that when the 

 colored light is removed the color slowly disappears as the 



* See Hering : Archiv. f . d. ges. Physiol , Bd. xli. S. 358 ff. 



