8 CARL VON HESS 



values in terms of one and the same measurable variable light 

 with which each colored light is compared. Besides this, we 

 are now in a position to bring all these reactions of animals in 

 relation to the motor-sensitive values which the same colored 

 lights have for the pupil of the normal, the red-blind, and the 

 totally color-blind human eye. 



That it would be possible to carry out such exact measure- 

 ments by this new process, I myself could not foresee at the be- 

 ginning of these tests; as the results obtained coincide in every 

 detail with those of my former widely differing experiments, 

 they prove most satisfactorily the accuracy of the statements 

 I have previously made about the sight qualities of animals. 



IV 



The long well-known fact that, on plants, red lights have 

 comparatively slight, blue, on the contrary, strong heliotropic 

 effect, that therefore in this respect there exists a certain simi- 

 larity between the effect of colored lights on plants and on 

 animals, gave J. Loeb occasion to accept the " Identity of ani- 

 mal and plant heliotropism." Some time ago, referring to older 

 experiments made by Wiesner and to more recent ones by 

 Blaauw, I had expressed doubts of this theory; as in spite of this, 

 Loeb's followers have again energetically taken up the defence 

 of the identity of the two tropisms, it seemed to me advisable 

 to attack this interesting question with new methods. In order 

 to settle it so that every possible objection should be met, both 

 reactions must be studied under identical conditions, with the 

 same colored lights, and especially in quantitative experiments, 

 the same light for measuring must be used for both. 



These conditions were fulfilled by the following procedure. 

 Etiolated seedlings of various kinds, in long narrow boxes, were 

 exposed on one side to the rays of a suitable Nernst-light spec- 

 trum and simultaneously from the opposite side, to the light 

 used for measurement and comparison, the latter being variable, 

 an electric light placed in a tunnel adapted to the purpose. 

 Its strength I varied partly by changing its position as required, 

 to distances nearer to or farther from the plants, and partly 

 by means of an episkotister. This method proceeds in the same 

 lines as those developed in my experiments with Artemia and 

 other animals which shun the light. Starting from a medium 



