PART IV 



REACTIONS IN LIGHT OF DIFFERENT WAVE- 

 LENGTHS OR COLORS 



CHAPTER XV 



ENERGY, PHOTOCHEMICAL REACTIONS AND BRIGHTNESS 



It is assumed by some investigators that the reactions 

 to light in lower forms, plants as well as animals, are all 

 induced by waves of approximately the same length, and 

 that these waves are the more refrangible in the spectrum, 

 the so-called actinic rays, the rays which are generally sup- 

 posed to have the greatest effect on chemical reactions. 



Davenport (1897, p. 202) closes a brief review of the 

 literature on this subject with the following words: '* Thus, 

 without multiplying cases, the results of experiments may 

 be summed up as follows: positively phototactic or posi- 

 tively photopathic organisms are such only in the presence 

 of the blue rays." Referring to experiments with numer- 

 ous different animals, Loeb says (1905, p. 294),^ " [I] found 

 a universal confirmation of the fact . . . that the more 

 strongly refrangible rays of the visible spectrum are the 

 most active heliotropically, as in the case of plants." 



Other investigators have, however, arrived at different 

 conclusions. They claim that all the rays in the visible 

 spectrum and some in the ultra-violet may be active in 

 stimulating organisms and that not all organisms are 

 equally stimulated by the different rays. After disagree- 

 ing with Loeb's statement that the shorter waves are the 

 more efficient in all plants and animals, Nagel adds (1901, 

 p. 294), " Es ist sehr wahrscheinlich dass fiir sehr viele 



^ Original in Pfiiiger's Arch., Vol. 54, 1893. 



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