354 THE SPECIAL SENSES. 



With test III, red, the red bhnd 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. 



In the practical tests made upon railway employees and others 

 who must recognize red and green signals at night the Worsted 

 test should be supplemented by the use of lanterns with colored 

 lights (e. g., the Williams lantern test) arranged to enable the 

 observer to detect quickly any confusion of colors on the part of 

 the individual examined. 



Achromatic Vision. — A number of cases of total color bhnd- 

 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 hght and nystagmus (rolling movement of the 

 eyeballs) are also characteristic. Since the peripheral field of 

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

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

 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. It is usually stated that the extreme per- 

 iphery of the retina has no color sense, giving only the white-black 

 series (achromatic) of sensations. As we pass in toward the center 

 the color sense develops gradually, the blue colors being perceived 

 fii-st and the greens last — that is, nearest to the center — so that in 

 a certain zone the normal eye is red-green blind. The distribution 

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

 perimeter (see p. 337). It will be found to vary with each indi- 

 vidual, 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. Usually for the colors red, green, and blue, the blue has the 

 most extensive 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 fx/j.) — that is, the complementary of the red — it is stated that 

 their fields are co-extensive, f From this standpoint the retina 

 presents three concentric zones : an extreme peripheral zone devoid 

 of color vision, an intermediate zone in which yellow and blue are 



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

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

 lication, No. 29, 1905. 



