4 CARL VON HESS 



the following interesting phenomenon appertaining to Centro- 

 stephanus longispinus. The animals have surrounding their abo- 

 ral pole, 20 or 30 beautiful lilac colored, clublike processes about 3 

 mm. long, concerning which we knew hitherto only that they some- 

 times move in rotation, at other times are quiescent. I noticed 

 that if a specimen at rest was slightly shaded, for example, if 

 one's hand were passed quickly between window and reservoir, 

 the little clubs began to rotate in a most lively manner. Further 

 experiment showed that in order to bring about such agitation 

 an exceedingly slight lessening of the lighting suffices. If, for 

 instance, the greater part of the light reaches the animal from 

 a gray pasteboard held at the proper angle, and I replace this 

 board with one which is only a little darker in shade, the clubs 

 begin to rotate quickly. Even with this method it was possible 

 to a certain degree to make determining measurements, and I 

 was able by the further use of differently colored boards for the 

 lighting again to show convincingly that these animals also 

 behave like totally color-blind human beings brought under cor- 

 responding conditions. Still more delicate, surprisingly exact 

 measurements were made by using the method which I shall 

 now describe. 



Ill 



Several writers have thought to deduce an argument against 

 the experiments I have so far made with the qualities of sight 

 in animals from the idea that I bring the " objective light-reac- 

 tion " of animals into relation with the " subjective light sensa- 

 tion " of man. For anyone to whom the science of color is fami- 

 liar, this argument is easily controverted. Still it is evident 

 that there is a great advantage in showing that the problem may 

 be attacked from quite a new direction. Therefore in a new 

 series of experiments on a large scale, I brought the light sensi- 

 tiveness of animals into relation, not to the ' ' subjective light 

 sensation " of human beings, but to the " objective light reac- 

 tion " in the human eye, to the changes in the size of the pupil 

 caused by light. This correlation was successful after I had 

 made extended and rather difficult preparations, as follows: 



We know from former experiments of M. Sachs (1893) that 

 the degree of contraction of the pupil caused by a colored light, 

 the "motor irritative value" of a colored light, depends on the 

 strength of luminosity in which the colored light is seen. Until 

 now we lacked a practical method of comparing the changing 



