PROPERTIES OF THE RETINA. 



329 



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. 141. If the green chosen is blue green (490/*/*) 

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

 are co-extensive.* 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 perceived, 

 and a central zone sensitive to red and green. The outlines of 



*' ON 'W OH u )V 



Fig. 142. Perimeter chart showing the highly restricted color fields in the left eye 

 of a typical case of so-called red-green color blindness. The ability to distinguish red and 

 green, by whatever characteristics of intensity or color they possessed extended for a very 

 short distance outside the fovea. It is interesting that the ability to distinguish blue was 

 in this case limited as compared with a normal eye. 



the different fields usually show many irregularities, and in some 

 cases it will be found that bright green is perceived over a larger area 

 than the red. The fields are not identical in the two eyes, and in 

 each eye it is, as a rule, more extensive upon the nasal than 

 upon the temporal side of the retina. In the red-green blind the 

 peripheral fields of color vision, judged by the individual's own 

 standards, may be markedly constricted as compared with the nor- 

 mal retina (see Fig. 142). 



* Baird, "The Color Sensitivity of the Peripheral Retina," Carnegie 

 Publication No. 29, 1905. 



