COLOR VISION 363 



Daphnia reacts more strongly to green or yellow and Hydra 

 to blue than to white light. These reactions have much 

 in common with the reversible photochemical reactions of 

 certain chemical compounds, particularly the fulgides, in 

 which the reaction in one direction proceeds most rapidly 

 in the shorter wave lengths and in the opposite direction 

 in the longer, while in a mixture of rays the reaction pro- 

 ceeds more slowly in one or the other direction, depending 

 upon the relative amount of the different rays. 



(6) Considering the results set forth above it is evident 

 (a) that, contrary to the hypothesis of Sachs, Loeb and 

 Davenport, the shorter waves are not the more active in 

 all plants and animals; the different rays do not have 

 the same relative stimulating efficiency in all organisms; 

 and (b) that the stimulating efficiency of the different 

 rays, probably in all organisms, is not proportional to the 

 energy they contain, but that for a given ray or color there 

 is a definite relation between the energy and the stimula- 

 tion which is probably in accord with Weber's law. 



(7) Light, as we have seen, causes reactions between 

 many different chemical compounds. In these reactions 

 the different rays have a specific effect. That is, certain 

 reactions are produced only by waves of a given length and 

 others only by waves of a different length. If reactions 

 in a given chemical solution take place in light waves of 

 a given length, and those in another solution in waves of a 

 different length, we may be fairly certain that the reacting 

 compounds in the two solutions differ. The reactions of 

 organisms are caused by, or at least associated with, chem- 

 ical changes in the organisms. The organisms probably do 

 not react to the external agents directly, but to the chem- 

 ical changes within produced by these agents. Since the 

 reaction to the different rays is not the same in different 

 organisms, it is clear that the chemical changes associated 

 with the reactions in the different organisms are not the 

 same. For example, in Amoeba the maximum power of 

 stimulation is in the blue; this must be associated with 



