276 HUMAN PHYSIOLOGY 



1. Total, in which all color sensations are absent, all objects 

 appearing colorless. 



2. Partial. 



Partially color-blind persons have but two kinds of color sensa- 

 tions : 



(a) The more general form, the red-green blindness, in which 

 only blue and yellow give rise to sensations, while, red and green 

 appear colorless. 



(b) The blue-yellow blindness, in which only the red and green 

 give rise to sensations, while blue and yellow appear colorless. 



The red-green blind can be divided into two groups which are 

 separated from each other by typical differences in the lack of 

 color sensations. For the one red or bluish green are colorless, 

 for the other purple-red and green. 



Color-blindness has been explained either by the lack of certain 

 primary sensations or by changes in the irritability of the elements 

 of color sensations. 



The negative after-image of a color sensation has the 

 color complementary to the original color. A colored light 

 upon a colorless background calls forth a colored simul- 

 taneous contrast, in which the background has the color 

 complementary to that of the colored light. 



3. VISUAL PERCEPTION 



I. Monocular vision. 



Space sensation of the retina. We are able to see 

 objects of the outer world clearly in their position because 

 we can distinguish different luminous object points. The 

 distinguishing of several object points is rendered possible 

 by the mosaic construction of the retina from elements each 

 of which produces a separate sensation when it is stimulated 

 by light from a luminous point. In the fovea centralis each 

 cone may be regarded as such an element, but at the 

 periphery of the retina several of the rods and cones collect- 

 ively form one element. 



From experience we seek the source of the light which 

 has stimulated a sensational element, in the line drawn from 

 this element through the nodal point outward (see pages 

 258 and 261). Hence by vision with one eye we are able to 

 tell the direction in which the object lies. 



