PROPERTIES OF THE RETINA. 357 



to have the same color. Those who are red or green blind will see 

 the test skein as a gray with some yellow or blue shade and will 

 select, therefore, not only the green skeins, but the grays or grayish 

 yellow and blue skeins. To ascertain whether the individual is red 

 or green blind tests II and III may then be employed. 



With test II, medium purple, the red blind will select, in addition 

 to other purples, only blues or violets ; the green blind will select as 

 "confusion colors" only greens and grays. 



With test III, red, the red blind will select as confusion colors 

 greens, grays, or browns less luminous than the test color, while the 

 green blind will select greens, grays, or browns of a greater brightness 

 than the test. 



Achromatic Vision. A number of cases of total color blind- 

 ness have been carefully examined.* It would seem that in 

 such individuals there is an entire loss of color sense, they possess 

 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 is 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 

 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. 341). It will be found to vary with each individual, 

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

 used for the identification of individuals. Exceptionally it is found 

 that the entire retina possesses a nearly normal color sense. Usu- 

 ally for the colors red, green, and blue, the blue has the most exten- 

 sive field and the green the least, as is indicated in the perimeter 

 chart given in Fig. 150. If the green chosen is blue green (490 wO 

 that is, the complementary of the red it is stated that their fields 

 are co-extensive, f From this standpoint the retina presents three 



* Grunert, "Archiv fur Ophthalmologie," 56, 132, 1903. 

 t Baird, "The Color Sensitivity of the Peripheral Retina," Carnegie Pub- 

 lication, No. 29, 1905. 



