PROPERTIES OF THE RETINA. 



337 



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. 143. 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 



Fig. 143. 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). 



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. 144). 



Functions of the Rods and Cones. Many facts unite in mak- 

 ing it probable that the rods and cones are different in function. 



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

 lication, No. 29, 1905. 

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