THEORIES OF PHOTOMECHANICAL CHANGES 87 



dog and the rabbit. A similar view was held by Kolmer 

 (1909), who described the presence of 'droplets' on the outer 

 segments of the rods — supposing them to be concerned with 

 the visual purple functions and having their origin in the 

 pigment epithehal cells. His observations will be discussed 

 more fully later. Comparable results were obtained on the 

 frog by Dreser (1886). 



Klihne (1879) stressed the parallelism between the time 

 needed for the regeneration of visual purple in the frog and 

 the length of time for dark adaptation, as indicating a sig- 

 nificant relationship between the two. Chiarini (1906) 

 expressed the view that the function of the migrated pig- 

 ment is nutritive to compensate the rods and cones for losses 

 which they sustain during functional activity. Herzog (1905) 

 considered the migration of pigment as useful in storing up 

 radiant energy so that the ''dark heat waves" thus made 

 available would accelerate the chemical processes taking 

 place in the visual cells. 



Herzog (1905) and Exner and Januschke (1906) main- 

 tained that these retinal changes represent a mechanism 

 for adapting the eye to daylight and to twilight vision. In 

 dim light or in darkness when the rods are capable of being, 

 stimulated, the pigment moves back (contracts towards the 

 perikaryon of the epithelial cell) and leaves free the spaces 

 between the rods, resulting thereby in a less complete insula- 

 tion of these cells. Under these conditions, with the entrance 

 of a small amount of light, the part played by the individual 

 rods in the perception of light, owing to refraction and diffu- 

 sion, is greater than if they were covered by a thick mantle 

 of pigment, in which case only the light which passes through 

 the retina in the direction of the long axis of the rods could 

 enter them. The presence of a light reflecting tapetum 

 further enhances favorable conditions. The cones, under 

 these conditions, are not functional, by reason of their high 

 thresholds. Consequently they elongate and move out of 

 the way. The rods contract so that optimum conditions are 

 presented for their stimulation. 



