WAVE LENGTH 



107 



The method used for these experiments by Loeb and 

 Wasteneys is as follows : 



A carbon arc spectrum, about from 18 to 23 cm. wide, 

 was thrown on a black screen 88 (see Fig. 34) with two 

 slits a and b in the two different parts of the spectrum 

 which were to be compared in regard to their heliotropic 



FIG. 34. Method of determining the relative heliotropic efficiency of two different parts 

 of the spectrum. (After Loeb and Wasteneys.) 



efficiency. The two beams of light passing through the 

 slits are reflected by the two mirrors M and M^ into the 

 square glass trough cdfe in such a way as to strike the 

 same region g of the back wall of the trough. The glass 

 trough is surrounded by black paper except at R and R 19 

 where the two beams of light enter from the mirrors. Be- 

 fore the experiment begins, all the organisms are collected 

 in the spot g with the aid of an incandescent lamp. As 



