590 



MANUAL OF PHYSIOLOGY. 



shades of color, we see, depend upon mixtures of different 

 strengths of these primary colors, so as to produce different 

 degrees of stimulation of each set of nerve terminals. 



The view that such special nerve apparatus do exist for red, 

 green and violet, is supported by the fact that the most anterior 

 or marginal part of the retina is incapable of being stimulated 

 by red objects which, therefore, look black when only seen by 

 this part of the retina. This inability to see red may extend 

 over the whole retina, as is found in some persons who may be 

 said to be red-blind. Moreover, if we investigate our negative 

 after images, after looking for a long time at a red object, we 



FIG. 232. 



Jt OY Gr. J31. V 



Diagram of the three Primary Sensations : 1 = red ; 2 = green ; 3 =- violet. The let- 

 ters below are the initials of the colors of the spectrum. The height of the shaded part 

 gives extent to which the several primary sensations are excited by different kinds of 

 light in the spectrum. 



find them to be greenish-blue ; that is to say, the nervous mech- 

 anism for receiving red impressions is fatigued, and consequently 

 those of its complementary color are easily stimulated. 



MENTAL OPERATIONS IN VISION. 



Our visual sensations enable us to perceive the existence, the 

 position, and the form of the various objects around us. For the 

 perfection of a visual perception much more is necessary than 

 the mere perfection of the dioptric media of the eye, and of the 

 retinal nerve mechanisms. Besides the changes produced in the 

 retina by the light, by means of which the optic nerve is stiinu- 



