366 HENRY LAURENS AND HENRY D. HOOKER, JR. 



It does not seem possible at the present time to assign a satis- 

 factory explanation to the maximal stimulating effect of light 

 of a particular wave-length. The following considerations are, 

 however, of interest. The effect of the light is a result of its 

 absorption by the photosensitive substances. Only the light 

 which is absorbed is chemically active, though all of the rays 

 absorbed are not necessarily active in producing chemical change. 

 When light is absorbed, the amplitude of the vibrations is a 

 maximum when the free periods of the vibrating particles co- 

 incide with the period of the incident light. 



The wave-lengths of maximum stimulating efficiency owe their 

 action to maximum absorption and to the hypersensitivity of 

 the photosensitive substance to the influence of these particular 

 rays. 



But absorption alone cannot be used as a measure of physio- 

 logical action, because it depends upon the kind of processes 

 initiated by the transformation of the absorbed energy (Bovie, 

 '18, p. 253). The kind of processes initiated are dependent upon 

 the nature of the original photochemical substance. 



SUMMARY 



1. The sensibility of Volvox to radiation of different wave- 

 lengths but of equal energy (sensibility to radiation at equal 

 energy) has been investigated by two methods: (a) the relative 

 duration of the presentation time, and (5) the relative rate of 

 locomotion (and precision of orientation). 



2. Wave-length X494mM is found by both methods to have the 

 highest stimulating value. The efficiency of the other wave- 

 lengths show a gradual decrease as shorter and longer wave- 

 lengths are tested. 



3. The necessity of using an equal-energy spectrum for such 

 work is emphasized. 



4. Attention is directed to the present inability of making 

 comparisons with human vision (either normal or color-blind), 

 that is, with photopic and sco topic (or achromatic) luminosity 

 curves : (a) because of the lack of sufficiently exact work on lower 

 organisms; (b) because of our lack of knowledge regarding photo- 



