THE EYE AS A SENSORY APPARATUS 281 



two colors most commonly confounded, red and green, are those 

 used in maritime and railroad signals. Persons attach such dif- 

 ferent names to colors that a decision as to color blindness can- 

 not be safely arrived at by simply showing a color and asking its 

 name. The best plan is to take a heap of worsted of all tints, 

 select one, say a red, and tell the man to put alongside it all those 

 of the same color, whether of a lighter or a darker shade; if red 

 blind he will select not only the reds but the greens, especially 

 the paler tints, as well as the grays and browns. This test, which 

 is almost universally used, was devised by the Swedish physiologist, 

 Holmgren. 



After-images and Contrasts. These are well-marked visual 

 phenomena, and have to be taken into account in attempting to 

 explain the mechanism of color vision. After-images are visual 

 sensations which remain after the withdrawal of the stimulus. 

 They are best seen after looking at bright objects, or fixedly for 

 several seconds at the same object. After-images are of two 

 sorts, positive and negative. Positive after-images are always 

 the same color as the object looked at; if one looks for an instant 

 at an incandescent filament and then shuts his eyes he perceives 

 a positive after image of the filament. This is due, probably, to 

 the persistence of the chemical process in the retina after the light 

 which causes it is withdrawn. Negative after-images, instead of 

 being the color of the object looked at, are always of its compli- 

 mentary color; if a red paper is looked at fixedly for several 

 seconds and the eyes then turned to a white wall, a bluish green 

 after-image is seen, instead of a red one. Negative after-images 

 can also be seen by closing the eyes after looking fixedly at a 

 bright object for some seconds. 



Contrasts are effects produced by bringing side by side different 

 colors ; blue appears bluer when near yellow than when near other 

 shades or the same shade of blue. Red and green heighten each 

 other in a similar way. If a large black square and a large white 

 square are placed side by side the black square looks blacker on 

 the edge next the white than elsewhere, and the white looks 

 whiter next the black than elsewhere. 



Theories of Color Vision. A theory of color vision to be ac- 

 ceptable must explain first the fundamental facts of color per- 

 ception, our ability to distinguish innumerable shades of color, 



