HELIOTHOPIC KEACTIONS ANIMALS AND PLANTS 227 



periment begins, all the organisms are collected in the spot g 

 with the aid of an incandescent lamp. As soon as the spectrum 

 is turned on, these organisms are simultaneously exposed to two 

 different beams of light which come from the two mirrors M and 

 Ml. When one type of light, e.g., that from M, is much more 

 efficient than the other coming from Mi, practically all the or- 

 ganisms are oriented by the light from M and move toward this 

 mirror, collecting in the region R. When the relative efficiency 

 of the two types of light is almost equal the organisms move in 

 almost equal numbers to R and Ri. By using as a standard of 

 comparison the same region of the spectrum and successively 

 altering the position of the other slit in the spectrum we were 

 able to ascertain with accuracy the relative efficiency of the dif- 

 ferent parts of the spectrum for the two forms of organisms. 

 When the two parts of the spectrum which are to be compared 

 are very close to each other it is necessary to deflect the beams 

 with the aid of deflecting prisms, before they reach the two 

 mirrors. 



III. THE DISTRIBUTION OF ORGANISMS IN THE CARBON ARC 



SPECTRUM 



The spectrum used was a carbon arc spectrum and its visible 

 part had a width varying from 18 to 23 cm. in different experi- 

 ments. We used very dense cultures of Euglena viridis and 

 filled the trough with this greenish suspension of Euglena. After 

 an exposure varying in length between 30 and 180 minutes the 

 results were ascertained, and in some cases the trough was 

 photographed. Figure 2 gives the photograph of the trough after 

 30 minutes' exposure. A very dense mass of Euglena was 

 gathered at the bottom of the trough in front between violet 

 (410 mm) and green (515 mm). In the photograph this mass is 

 visible as a thick, dark, horizontal streak at the bottom. In 

 addition some vertical streaks of organisms are visible in the 

 blue and indigo. The reader will recognize how difficult it is to 

 ascertain the most efficient wave length by this method. We 

 can only say the blue is the most efficient light and the wave 

 lengths > 515 Mi" are practically without orienting effect. 



