COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 9 



crease of distance from the fovea, until at the extreme periphery there 

 is found a zone " not of objective blackness and darkness, but of insen- 

 sibility and unconsciousness." Purkinje devised an apparatus for the 

 purpose of determining the spatial extension of retinal sensitivity, and 

 of investigating the phenomena of indirect vision in general. His 

 apparatus consisted of a sheet of heavy cardboard cut in the form of a 

 quadrant. The subject was seated in a dark room and the apparatus 

 was brought into such a position that the pointed apex of the quadrant 

 rested upon the bridge of his nose, while the side of the apparatus lay 

 against his cheek. A lighted taper was then moved along the graduated 

 arc of the quadrant and the point was noted at which the taper first 

 appeared or disappeared from view. Purkinje's measurements of the 

 absolute extension of the normal field of vision gave somewhat larger 

 values than had been obtained by Young (see p. 7), namely, 60 up- 

 wards, 60 inwards, 80 downwards, and 100 outwards. In a series of 

 experiments with color-stimuli,* he discovered that colored objects 

 appear gray when their images fall upon peripheral parts of the retina 

 and that they pass through regular transitions of color tone as their 

 images approach the fovea. Neue Beitrage, S. 15-16.) Thus, purple 

 appeared successively black at 90, blue at 80, violet at 70, and pur- 

 plish at 50 ; red appeared pale grayish-yellow at 90 to 70, then orange, 

 and finally its true color; "bright blue" appeared white at 90, bluish 

 at 80; "saturated blue" appeared white at 90, bluish at 80; violet 

 appeared black at 90, blue at 80 to 70, violet in different tones from 

 60 onward ; " saturated green " appeared black at 90 and 80, greenish 

 at 70 ; rose-red appeared white at 90 and 80, reddish at 70 ; bright 

 yellow and orange appeared in their own tones at 90. 



Purkinje confesses that he is unable to find a satisfactory explana- 

 tion of these phenomena. He points out that peripheral images must, 

 in the nature of the case, receive less light than central images, and 

 discusses the influence of the yellow spot and of different conditions of 

 optic refraction, upon color vision, but he regards these factors as in- 

 adequate to explain the transitions of color tone. He also discusses the 

 significance for practical life of the indistinctness of indirect vision, and 

 finally concludes (Neue Beitrage, S. 19) : "It is difficult to determine 

 whether the clearer vision of the central part of the retina is due to the 

 presence, at the fovea, of a greater light intensity, or to a peculiar struc- 

 ture and a closer relation to the Seelenkraft, at that point." 



*Just how these experiments were arranged is not clear from Purkinje's 

 description. It is probable, however, that he employed the same apparatus in 

 diffuse daylight, and that colored papers served as stimuli. 



