A RETINAL MECHANISM OF EFFICIENT VISION 345 



pushes outward to mask the visual cells, while these latter occupy 

 mutually reciprocal positions — rods elongated, cones shortened. 



Such positional changes have been interpreted as of use in 

 furthering efficient vision. It seems logical that the masking 

 pigment of the light phase would serve to protect the delicate 

 visual elements from the overstrong influence of light (Chiarini, 

 '06), while the insinuation of such pigmented processes between 

 the individual visual elements effects for the latter a certain 

 degree of optical isolation by acting as an absorbent of dis- 

 persed rays refracted from neighboring cells (Garten, '07). 

 From the standpoint of sensory reception, the sharpness of the 

 retinal image is in this way enhanced. The withdrawal of the 

 pigment in dim light might be thought to involve a response 

 which allows the. visual cells to utilize all the weak light available. 

 . Furthermore, there is good reason to believe that the cones are 

 concerned with bright-light vision, the rods with dim-light or 

 twilight vision. Hence a shortening of the cone in bright light, 

 drawing it down nearer the source of illumination, while the rod 

 at the same time elongates and is thus moved out of the way, 

 would appear to be a useful maneuver (Herzog, '05; Exner and 

 Januschke, '06) . The converse procedure in dim light — by which 

 the rods are shortened and the highly refractile cones, with their 

 lens-like ellipsoids no longer masked by pigment, are length- 

 ened — would be equally advantageous (Garten, '07; Arey, '15 a). 



For the detailed applications of these apparently adaptive 

 responses the reader is referred to Garten ('07), who gives an 

 extended consideration of the correlations within the vertebrate 

 classes between the morphology, optical qualities, and distribu- 

 tional ratios of the rods and cones on the one hand, and, on the 

 other, their movements together with the migrations of the 

 retinal pigment. 



Interesting and logical as these speculations may be, they nev- 

 ertheless lack a sound experimental basis. It is certain that the 

 movements as summarized in a preceding paragraph occur re- 

 spectively in daylight and in darkness; but since responses in 

 total darkness cannot be useful in the manner suggested, it is 

 obvious that in order to derive the reputed adaptive benefits 



