THE EYE AS A SENSORY APPARATUS 247 



of these views; they assumed that any method of stimulating a 

 red-perceiving cone would give rise to red sensations; if red- and 

 green-perceiving cones were stimulated simultaneously the effect 

 in consciousness would be very different from that of stimulating 

 either one alone, the red cone and the green cone together, giving 

 yellow; and if all three sorts were stimulated at once in equal 

 amounts the effect would be a sensation of white. All our color 

 perceptions are supposed to be based on proper combinations of 

 stimuli acting on the groups of three cones. To explain some facts, 

 such as that pure red light as it becomes brighter and brighter 

 approaches and finally becomes white, the theory supposes that 

 no light stimulates only one cone; all three of the group are stim- 

 ulated by light of any color, and the effect in consciousness de- 

 pends on which is more strongly stimulated. 



It is easy to demonstrate that the color of a spot of light whose 

 retinal image is of such a size as to fall within the boundaries of a 

 single cone can be accurately distinguished. According to the 

 theory white light should seem to be one or the other fundamental 

 color under such circumstances, instead of looking white as it 

 actually does. 



When the theory was proposed it was thought that red-blindness 

 and green-blindness were entirely distinct forms of color blind- 

 ness. The theory fits that idea very well, since it supposes dis- 

 tinct red-perceiving and green-perceiving cones. Now that we 

 know that both red and green blindness are really forms of red- 

 green blindness in which neither red nor green gives normal color 

 sensations the theory does not agree at all with the facts in this 

 regard. The theory also fails to explain the distribution of color 

 vision over the retina or the fact that black is a true sensation. 

 It explains very well on the basis of fatigue the negative after- 

 images that one sees when the eyes are turned to a white surface 

 after looking at a colored body; for if one particular set of cones is 

 fatigued by looking steadily at any color, white light coming upon 

 the retina stimulates the unfatigued ones more powerfully than 

 the fatigued ones, and instead of the sensation of white which 

 follows equal stimulation of all cones, the complimentary color 

 to that one which fatigued the cones in the first place is seen. 

 The theory does not explain well the negative after-images seen 

 with closed eyes, nor does it explain the phenomena of contrast. 



