102 TBOPISMS 



tions caused by light in lower animals are the same as 

 those caused in a human being. But even if the relative 

 efficiency of the various parts of the spectrum were the 

 same for the sensations of brightness in human beings and 

 for the heliotropic reactions in lower animals, it would not 

 prove that the latter also have sensations of brightness. 

 For w T e have no guarantee that the heliotropic reactions 

 of lower animals are due to or accompanied by sensations 

 of brightness. If the yellow-green rays are the most 

 efficient in causing heliotropic reactions in an organism, 

 it suggests only that in such an organism the photosensi- 

 tive substance responsible for the heliotropic response is 

 most easily decomposed by the yellow-green part of 

 the spectrum. 



A similar error of reasoning as that by Bert has 

 recently been made by Hess. Hess corroborated what the 

 writer had already pointed out, that the red rays of the 

 visible solar spectrum are the least efficient, and he found, 

 moreover, as Bert had found for Daphnia, that for the 

 heliotropic reactions of a number of animals, from the 

 fishes down, the yellow-green region of the solar spectrum 

 is the most efficient. Now it happens that to a totally 

 color blind human being, to whom the different parts of 

 the solar spectrum appear only as shades of gray, the 

 region A = 540 /*/* in the yellow-green appears to be the 

 brightest; while the red part of the spectrum gives a 

 very faint sensation of brightness. From this similarity 

 or apparent identity between the relative effects of dif- 

 ferent wave lengths upon the heliotropic effects of certain 

 lower animals and upon the sensations of brightness of 

 a totally color blind human being, Hess draws the con- 

 clusion that these animals are totally color blind. In our 

 opinion the only conclusion which Hess has a right to 



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