SELIG HECHT 387 



500 mm) • At any other wave-length X„, then, the per cent of absorp- 

 tion will be given by the ratio of the energy at \max divided by the inci- 

 dent energy at X„. 



The absorption at \max will then be 100 per cent, and at any other 

 value of X„ it will be equal to the reciprocal of the incident energy. 

 Therefore Fig. 5 represents the absorption spectrum of the photosen- 

 sitive substance S, the ordinates now being per cent of absorption. 

 Absorption spectra are not infrequently given in this way (Henri, 

 1919, p. 42, 65), I shall therefore not give the absorption spec- 

 • trum in terms of the absorption index. The latter can be calculated 

 according to certain assumptions as to the thickness of the absorbing 

 layer etc. (C/. Bovie, 1918-19), in themselves, however, only of 

 speculative interest. 



3. Because of its interpretation as the absorption spectrum of S, 

 the shape of the curve in Fig. 5 becomes of significance. Many 

 known substances possess absorption bands in position and extent 

 similar to that shown here (C/. Uhler and Wood, 1907, Fig. 17 and those 

 following). As a rule such curves are symmetrical with regard to 

 the point of maximum absorption. This, however, is by no means 

 universal, because many substances show skew absorption spectra 

 similar to that in Fig. 5; for example, uranine, as studied by Uhler 

 and Wood (1907, Fig. 15). In such cases it is usual to assume that 

 there are really two, or more, vibrators in the molecule, their com- 

 bined effect being given by the total curve as found. If we suppose 

 that in the photosensitive substance 6* there are present two vibrators, 

 one whose period corresponds to 500 mm and the other to about 570 

 MM, each giving a symmetrical resonance curve, the compound curve 

 of Fig. 5 would be their resultant. 



Leaving aside these speculative matters, it may be noted that the 

 appearance of the absorption curve, though not particularly distinc- 

 tive, is sufficiently so to serve as a corroboration of the identity of the 

 photosensitive substance S in future experimentation. 



In recent years there has appeared a number of careful measure- 

 ments of the most effective portion of the spectrum for the stimula- 

 tion of different organisms (Laurens and Hooker, 1920; Loeb and 



