246 NATURE OF PHOTOSENSORY PROCESS 



sity is decidedly not the effective stimulus. Therefore the contro- 

 versial division of organisms into the two groups previously mentioned, 

 though traditionally sacred, is fundamentally without significance. 

 The effective stimulus in photic irritability is of the same nature 

 whether the response is to a Hght which is obviously continuous, or 

 to a hght whose intensity is suddenly augmented. 



SUMMARY. 



1. In order to produce a response in Mya, the minimum amount of 

 light energy required is 5.62 meter candle seconds. This energy fol- 

 lows the Bunsen-Roscoe law for the relation between intensity and 

 time of exposure. 



2. The necessary minimum amount of energy varies but little with 

 the temperature; the temperature coefficient for 10°C. is 1.06. 



3. In view of these facts it is concluded that the initial action of the 

 light is photochemical in nature. This substantiates the hypothesis 

 previously suggested to account for the mechanism of photoreception. 



4. The constant energy requirement for stimulation of Mya shows 

 that the traditional division of animals into those which respond to a 

 constant source of hght and those which respond to a rapidly aug- 

 mented light is without any fundamental significance for sensory 

 physiology. 



BIBLIOGRAPHY. 



Blaauw, A. H., Die Perception des Lichtes, Rec. irav. hot. Neerlandais, 1909, 



V, 209. 

 Bunsen, R., and Roscoe, H., Photochemische Untersuchungen. IV, Ann. Physik., 



1862, cxvii, 529. 

 Froschel, P., Untersuchung iiber die heliotropische Prasentationszeit. II, Sit- 



zungsh. Wien. Akad., 1909, cxviii, 1247. 

 Hecht, S., (o) The photic sensitivity of Ciona intestinalis , J. Gen. Physiol., 1918- 



19, i, 147; {b) Sensory equilibrium and dark adaptation in Mya arejiaria, 



545; (c) The nature of the latent period in the photic sensitivity of Mya 



arenaria, 657; {d) The effect of temperature on the latent period in the 



photic response of Mya arenaria, 667. 

 Loeb, J., Monographs on experimental biology, Forced movements, tropisms, 



and animal conduct, Philadelphia and London, 1918. 

 Sheppard, S. E., Photochemistry, New York, 1914. 

 Schwarzschild, K., Ueber abweichungen von Recprocitatsgesetz fur Bromsil- 



bergelatine, Photograph. Correspondenz, 1899, xxxvi, 109, 



