136 VERTEBRATE PHOTORECEPTORS 



previous work on simple dark- and light-adaptation at room 

 temperature that the droplets are typically numerous in 

 dark-adapted eyes and sparse in light-adapted ones. Accord- 

 ing to Wald's findings, retinae dark-adapted at room tem- 

 perature are rich in visual purple, but after treatment with 

 acids, the visual purple is converted to retinene. Under 

 these conditions droplets should be numerous. Further, 

 after prolonged exposure to light, visual purple is bleached to 

 vitamin A and protein, but with traces of retinene and visual 

 purple. Under these conditions the droplets should be few 

 or lacking. Experiments 1, 2, 6, and 8 (Table 4), consisting 

 of light- and dark-adaptations, at room temperature (Figures 

 91 and 94, 93 and 96), have supported earlier findings 

 (Kolmer, 1909, and Detwiler, 1923c). The experiments in- 

 volving dark- and light-adaptations at 0°C yielded results 

 which accord with the theory that the droplets exhibit a 

 behavior parallel to that of retinene under various conditions 

 of temperature and illumination (Experiments 3, 4, 5, 7, 

 and 9, Table 4). 



Animals were light-adapted by placing them for at least 

 one hour or longer under two 60-watt lamps, after which 

 the eyes were excised and fixed in modified Held's solution 

 (Kolmer's bichromate-acetic acid solution). Animals were 

 dark-adapted at room temperature by leaving them in total 

 darkness for four hours (some cases over night), after which 

 the eyes were excised under faint red light (Wratten filter, 

 series 2), and fixed in modified Held's solution in the dark. 



When animals were subjected to low temperatures, they 

 were first ^adapted' at room temperature to the desired 

 illumination, and were then placed in a dish of water, which 

 was kept at 0°-0.5°C by immersing it in another dish filled 

 with cracked ice and salt — the required conditions of 

 illumination being maintained. At this temperature of the 

 water, the oesophageal temperature of the frogs never 

 exceeded 1.5°C. 



The eyes under all conditions of illumination and tempera- 

 ture were fixed alike (modified Held's), cut at 8 m and stained 



