MOVEMENTS IN THE VISUAL CELLS 127 



of the time which they require in assuming the positions charac- 

 teristic of Hght and of darkness. In most cases it is not easy 

 to state definitely when adaption is completed, for the response 

 becomes less vigorous as it nears the end and consequently 

 the factor of personal equation is unavoidable. 



Light intensity and temperature "(Dittler, '07) represent 

 variables which undoubtedly play a part in the determination 

 of adaption time. No attempt was made to discover the exact 

 role of either of these factors, although the general experimental 

 conditions were kept approximately uniform during successive 

 trials. 



The effect upon adaption time of a long or short prehminary 

 subjection to light or darkness, has never been taken into ac- 

 count, although such influences were asserted by Gaglio ('94). 



a. Retinal pigment 



Pergens ('96) found that after 2 minutes' illumination the 

 retinal pigment of I.euciscus began to expand. After 1 minute 

 of darkness a noticeable contraction occurred, which was com- 

 pleted in 20 minutes. Chiarini ('04b), working on the same 

 fish, came to somewhat different conclusions. He observed 

 a sensible pigment expansion after direct exposure to sunlight 

 for 1 minute, although complete light adaption necessitated 

 a period of 1 hour. The reverse process of dark adaption was 

 not initiated until the animal had been subjected to darkness 

 for from 4 to 5 minutes, and a minimum of 1 hour was required 

 to complete the contraction. 



When the retinal pigment of fishes has undergone a maximum 

 expansion, it accumulates distally' near the external limiting 

 membrane, whereas the proximal portions of the cells are to a 

 greater or less extent devoid of pigment (figs. 9, 10). Such a 



^ The term 'proxiinal' as iisod in this jiaper will refer to movements toward 

 the nuclei of a given cell, either pigment or visual. In like manner 'distal' will 

 have reference to movements away from the nucleus. Since the distal movement 

 in the pigment cells is the reverse of that in the rods and cones, an arbitrary 

 nomenclature with reference to the eye-ball becomes confusing, while the ter- 

 minology here suggested lends itself readily to descriptions of the moving parts. 



