WAVE LENGTH 



writer showed that the animals are neither fond of blue 

 nor of red but are oriented by the light in the same way 

 as are plants, and the statement that animals which were 

 "fond" of light also were "fond" of blue and those 

 which were "fond" of "dark" were "fond" of red 

 the writer explained in a simpler way, namely that the 

 light filtered through red glass had a smaller orienting 

 effect than the light filtered through blue glass. 287 Hence 

 red glass acted like an opaque, blue glass like a trans- 

 parent screen. This had already been known to be true 

 for the heliotropic reactions of plants for which Sachs 

 had shown that they occur behind a blue glass in the same 

 way as behind common window glass, while behind red 

 glass heliotropic reactions do not occur at all or occur 

 very slowly as if the light were weak. The writer was 

 able to show that the same is true for animals. 267 When 

 positively heliotropic animals are put into a box covered 

 with blue glass they go as rapidly to the window side 

 as when the box is uncovered; while when it is covered 

 with red glass the animals will go to the window but more 

 slowly and irregularly. Behind a red screen they behave 

 therefore as if they were exposed to weak light. 



Blue glass is permeable not only for blue but also for 

 rays which produce the sensation of green. Paul Bert 38 

 had already made experiments with positively heliotropic 

 Daphnia in a solar spectrum and found that the animals 

 "accouraient beaucoup plus rapidement au jaune ou au 

 vert qu' a toute autre couleur." a Bert concluded from 

 this that to the eye of a Daphnia those parts of the spec- 

 trum appear brightest which also appear brightest to the 

 human eye. Bert's aim was to find out whether the sensa- 



a This method of ascertaining the most efficient part of the spectrum is 

 not reliable and has been replaced by other methods. 



