NEW EXPERIMENTS ON LIGHT REACTIONS 9 



distance found by preliminary trials, after a very few hours we 

 find the plants in red, yellow and green bent far over towards 

 the measuring light, those in green, blue and a part of violet, 

 towards the spectrum, those in the outer edge of violet and ultra- 

 violet again bent toward the measuring light. Through this 

 experiment we have found two lights in the spectrum whose 

 heliotropic strength is equal to that of the composite light. 

 The circumstance that the plants bend over on each side of these 

 two colors in opposite directions make a comparatively exact 

 spectroscopic determination of their respective wave lengths 

 possible. By repeating such experiments, taking different dis- 

 tances of the lamp from the plants, I obtained each time two new 

 points for the construction of curves. You see here the curve 

 of the motor irritative values of the different lights of the spec- 

 trum for the invertebrates, next to it the curve of some among 

 the plants (Brassica napus) which I have observed and you can 

 see from these that there can be no question of identity between 

 the two results ; the curve for animals has its maximum in yellow- 

 green, with a wave length of about 526/*/*, that for Brassica napus 

 has its maximum in blue or in the beginning of violet, with a 

 wave length of about 475/*/*! In yellowish-green, where we find 

 the maximum for animals, the heliotropic effect on the plants 

 has already reached nearly its minimum. 



A second method for the investigation of certain questions 

 occupying my attention, I worked out in this way: I have 

 already shown that one can obtain beautiful and convincing 

 results if a reservoir is lighted by rays reflected from colored 

 paper at both ends, and direct light from the window is shut 

 off by placing shades as required. Animals seeking the light, 

 without exception hasten to that end which is lightest in the 

 opinion of a color-blind individual quite irrespective of the way 

 in which normal sight interprets the values. The heliotropic 

 movements of plants have hitherto been observed only when 

 caused by light from the spectrum or through colored glasses; 

 it had never been attempted to find out whether heliotropic 

 movements appear also when light from such reflecting surfaces 

 alone is used. After I had found in a few introductory experi- 

 ments that such is in fact the case to a quite surprising degree, 

 I used this method for the solution of the problem before us. It 

 is one easily adapted to the use of the interested layman. 



