328 



THE SPECIAL SENSES. 



only achromatic vision. The external world appears to them only 

 in shades of gray. In the majority of these cases (f) there 

 is a region of blindness in the fovea (central scotoma), and an 

 unusual sensitiveness to light and nystagmus (rolling movement of 

 the eyeballs) are also characteristic. Since the peripheral field of 

 vision is nearly normal as regards sensitiveness to light, while the 

 central field is frequently blind or amblyopic, it has been assumed 

 that this condition represents one of loss of function in the cones. 



Distribution of the Color Sense in the Retina. What has 

 been said above in regard to color blindness refers especially to the 



Sl 081 oU 



Fig. 141. Perimeter chart indicating the average fields of vision for blue, red, and 

 green compared with white (gray). Right eye: The outlines of the color fields are repre- 

 sented as smooth since the chart is an average from many determinations. As a matter of 

 fact, in each individual the outline is highly irregular. Normally green (bright green) is the 

 smallest field, green objects outside the limit appearing yellow and farther out colorless 

 (gray). 



central field of vision. When we examine the peripheral field in 

 the normal eye it is found that on the extreme periphery the retina 

 is totally color blind, perceiving only light and darkness, that is, 

 the shades of gray. As we pass in toward the center the color 

 sense develops gradually, the blue colors being perceived first and 

 the greens last, that is, nearest to the center, so that in a cer- 

 tain zone the normal eye is red-green blind. The distribution of 

 the color sense may be studied conveniently by means of the pe- 

 rimeter (see p. 313). It will be found to vary with each individual, 

 so much so that it is possible that a test of this character might be 



