268 HOVEY JORDAN 



watts) increased the reaction time about 120 per cent (24 to 53 sec- 

 onds). This rough inverse ratio between time and intensity, though 

 based on methods not rigidly exact, conforms fairly well to the Bunsen- 

 Roscoe law of photochemistry, viz., that the products of reaction time 

 and intensity of stimulation are constant for any effective illumination. 

 In my experiments upon the whole of one side the products for 100-, 

 60-, and 40-watt lights were 2400, 2100 and 2120, respectively. This 

 conformity to the Bunsen-Roscoe law may, perhaps, be taken to indi- 

 cate that photic stimulation of the integument of Epinephelus is photo- 

 chemical in nature, as Froschel ('08), Blaauw ('09), Loeb ('11), and 

 Parker and Patten ('12) have suggested for varous biological phenomena. 

 The phototropic reactions of the hamlet, then, are dynamic in char- 

 acter and the stimulus is probably produced by a photochemical process. 



C. Exhaustion of the photoreceptors 



1 . General evidence. A study of the responses both of normal and of 

 blinded fishes to successive stimulations by the same intensity of light 

 often shows a progressive increase in the time of reaction from the first 

 to the last response. A state of complete exhaustion is, of course, 

 reached when no response can be induced. There seems to be, how- 

 ever, no definite ratio between the number of stimulations and the in- 

 crease in reaction time. Indeed, in two or three individuals the first 

 response was slower than the second, or all responses occurred after 

 about the same interval of time. Even in the latter cases, however, a 

 condition of complete exhaustion was finally effected after a sufficient 

 number of exposures of adequate intensity, but its appearance was 

 abrupt rather than gradual. 



2. Photodynamic nature of exhaustion. The process of exhaustion, 

 moreover, appears in general to be photodynamic in character, for 

 the period of resistance of the receptors is lessened by an increase in 

 the intensity of the light and vice versa. This inverse ratio is, appar- 

 ently, similar to that which has just been described in connection with 

 the negative responses. 



In experiments on this point it was found necessary in order to pro- 

 duce a sta^te of exhaustion to stimulate the fish for 275 seconds with 

 the light from a 100-watt bulb; but when a 60-watt bulb was substi- 

 tuted, this period was more than doubled, being 562 seconds. Thus a 

 decrease of 40 per cent in the intensity of illumination (from 100 to 60 

 watts) increased the time of resistance of the photoreceptors more than 



