376 WAVE-LENGTH AND PHOTOSENSORY PROCESS 



in greater detail, their identification becomes increasingly possible. 

 Heretofore attention has been centered mainly on the kinetic and 

 dynamic relations of the sensitive system. It is the purpose of the 

 present observations to furnish the basis for the description of a 

 physical property of the photosensitive substance S. 



2. It is axiomatic to say that in a photochemical transformation 

 only the light which is absorbed is active in producing chemical 

 change. Although the converse, — that all the light which is absorbed 

 gives rise to chemical action, — ^is not proved, it is well known that the 

 absorption spectrum of a chemical system is intimately connected 

 with its photochemical behavior (Sheppard, 1911, p. 140). Thus, if 

 in a given photochemical system we know the relative effectiveness 

 of a series of lights of different wave-length, it is possible to draw cer- 

 tain conclusions with regard to the absorption spectrum of the sensi- 

 tive substance in the range of wave-lengths which have been investi- 

 gated (Lasareff, 1907). Experiments were therefore undertaken to 

 determine the effectiveness of light of different wave-length on the 

 photosensory responses of Mya arenaria. 



3. The method used is in its essentials as follows. The reaction 

 time of Mya to light varies inversely as the logarithm of the intensity 

 (Hecht, 1920-21, e). The relation between these two variables may 

 be found for light of any quality. When using monochromatic illu- 

 mination, the curve expressing the relation between the intensity (/) 

 and the reaction time (/) may be called an It-isochrome, in analogy to 

 the curve at constant temperature called an It-isotherm. The experi- 

 ments consist in mapping out the It-isochromes for the photic response 

 of Mya to lights of different composition. The relations among the 

 isochromes will give the relative effectiveness of the different lights. 



The experiments were performed in the Marine Biological Labora- 

 tory at Woods Hole, Massachusetts, during the summer of 1920. 



II. 



1. Monochromatic light is secured by means of Wratten Light Fil- 

 ters Nos. 70 to 76 inclusive. These are made by the Eastman Kodak 

 Company, and consist of specially stained gelatin films cemented 

 between glass plates. The absorption spectra of these light filters 



