430 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



to utilize for the sterilization of water and milk; a difficulty is created, 

 however, by the deficient power of ultraviolet light to penetrate 

 liquids, which necessitates the exposure of only thin layers to the toxic 

 rays. 



The region of the spectrum which is most efficient in causing helio- 

 tropic movements or curvature varies in different species of animals 

 and plants. Among the plants the rays of the blue end of the spectrum 

 are usually the most efficient. Thus Blaauw has determined the dura- 

 tion of exposure to light derived from various parts of the carbon arc 

 which is necessary to induce heliotropic curvature in the seedlings of 

 Avena. The following are his results: 



Duration of illumination Position of the rays in 



in seconds. the spectrum. 



6300 '.. .... : . . . . . 534 JUM 



1200 510 " 



120 499 



15 491 " 



5 487 " 



4 478 " 



3 . . . . . . . . . . ... ... ... 



4 , ... 466 " 



6 448 " 



The maximum effect is therefore obtained between 478 MM an d 466 ///z, 

 that is, in the blue region of the spectrum. Among animal forms 

 Loeb and Wasteneys have found that Eudendrium and Arenicola are 

 similarly affected chiefly by the blue rays, while other animals, for 

 example the crustaceans and insects are primarily affected by the rays 

 lying en the border of the green and yellow. Even closely allied forms 

 may, however, differ very decidedly in the part of the spectrum which 

 is most efficient in eliciting heliotropic curvature. Thus Loeb and 

 Wasteneys have observed that the green flagellate Euglena viridis 

 is most affected by the blue rays lying between X = 470 and 480 ///*, 

 while the closely allied flagellate Chalmydomonas pisiformis is especially 

 sensitive to the yellow-green rays in the neighborhood of X = 534 w 



That the foundation of these light effects resides in a Photochemical 

 Reaction which is induced within the organism is shown by the applica- 

 bility of the Bunsen-Roscoe Law, which is generally characteristic of 

 photochemical reactions and applies, for example, to the blackening of 

 a sensitized photographic plate by exposure to light. This may be 

 enunciated as follows: The chemical effect induced by light is pro- 

 portional to the product of the intensity multiplied by the duration 

 of illumination, or in symbols: 



E = Kit 



where E is the extent of photochemical transformation, i the intensity 

 of the light, and t the duration of the illumination and K a propor- 

 tionality-factor which is constant for the particular photochemical 

 transformation under consideration. 



