WAVE LENGTH 103 



draw is that the photosensitive substance which causes 

 sensations of brightness in the eye of the color blind 

 human being is either identical with or is affected in a 

 similar way by light waves as is the substance giving 

 rise to heliotropic reactions in certain animals. This 

 assumption is entirely adequate and harmonizes better 

 with the facts than the assumption made by Hess. The 

 substance responsible for the sensations of brightness in 

 the eyes of the totally color blind human being is visual 

 purple which is bleached most rapidly by light of 

 A = 540 P-P-. That our objection is justified is proved by 

 the experiments of v. Frisch on bees. 



v. Frisch 168 has shown by very ingenious and care- 

 ful experiments that bees can be trained to discriminate 

 between blue and yellow but not between different shades 

 of gray. On a table were put square cardboards of dif- 

 ferent shades of gray and among them one blue piece of 

 cardboard. On each gray square was put a watch crystal 

 containing water, while the watch crystal on the blue con- 

 tained sugar water. The bees of an observation hive 

 visiting the sugar crystal were marked with a fine paint 

 brush. After a sufficient period of training it was found 

 that the marked bees always went directly to those crys- 

 tals which were on a blue piece of cardboard, whether they 

 contained sugar water or pure water; and when there 

 was no sugar water on the blue cardboard they alighted 

 on any blue object, e.g., a blue pencil. The crystals and 

 cardboard pieces were always renewed in different tests, 

 to avoid any influence of odor. It was never possible to 

 train the bees to select a piece of cardboard with a definite 

 shade of gray among cardboards of different shades of 

 gray. 



Hess had shown that for the heliotropic reaction of 



