226 



JACQUES LOEB AND HARDOLPH WASTENEYS 



animals, some blue light will also reach the animals in the green 

 and vice versa. In this way a comparatively denser gathering 

 of animals may occur in that part of the spectrum which is more 

 effective. It is obvious, however, that this method is not very 

 exact since in an aquarium with animals the scattered light from 

 one part of the spectrum can chiefly reach only those individuals 

 which are not too far from this spot. 



The second method consisted in the comparison of the relative 

 efficiency of two narrow parts of the spectrum. It was described 

 briefly in a former note. 



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

 thrown on a black screen SS (see fig. 1) with two slits a and b 

 in the two different parts of the spectrum which were to be com- 

 pared in regard to their heliotropic efficiency. The two beams 

 of light passing through the slits are reflected by the two mirrors 

 M and Mi into the square glass trough 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 Ri, where 

 the two beams of light enter from the mirrors. Before the ex- 



