386 WAVE-LENGTH AND PHOTOSENSORY PROCESS 



of the spectrum for the sensory stimulation of Mya lies between 490 

 and 510 ixji. The effectiveness of the spectrum drops rapidly on both 

 sides of this maximum, the parts above 610 /x/x and below 420 /x/x being 

 practically inefifective as stimulating agents. 



rv. 



1. On the basis of certain considerations it is possible to place a 

 physical interpretation on the results as presented in Fig. 5 and Table 

 II. Lasarefif (1907) has shown that for dyes that are bleached rapidly 

 by light, the photochemical effect is directly proportional to the energy 

 absorbed per interval AX, independent of the value of X and of the 

 position of the absorption maximum. These results have been con- 

 firmed on other photochemical reactions (Weigert, 1911, p. 90) ; and a 

 similar state of affairs has been shown to be true for the bleaching of 

 visual purple (Trendelenburg, 1911). 



This means that in order to produce a given photochemical effect, 

 sufficient light of any wave-length X„ must be delivered, such that 

 the amount absorbed will be the same at all values of X„. The relative 

 effectiveness of different parts of the spectrum therefore depends 

 solely on the absorption spectrum of the sensitive substance. The 

 greater the proportion of light of a given wave-length that is 

 absorbed, the less of that light is necessary to produce a given effect. 



2. If we apply these findings to the present data, it will become 

 apparent that Fig. 5 represents the absorption spectrum of the pho- 

 tosensitive substance 5 in the reaction 



which is responsible for the photic sensitivity of Mya. 



Let us assume that in the position of maximum effectiveness (Xma» 

 = 500 ixy) there is complete absorption of the incident energy. At 

 any other value of X„ the same amount of energy will be required to 

 be absorbed in order to produce the same effect. But the amount of 

 incident energy is greater than this, in inverse proportion to the 

 magnitude of absorption at that wave-length. We know the incident 

 energy from Figs. 4 and 5 and Table II. The absorbed energy is 

 equal to the amount required at the maximum effectiveness (X = 



