BENDING OF HIGHER PLANTS TOWARD THE LIGHT 63 



of the lens, it Is evident that the rate of diminution in 

 intensity depends upon the ratio of the altitude of the 

 triangular opening to the length of its base; i.e., decreasing 

 the altitude or increasing the base causes an increase in 

 the rate of diminution, and vice versa. Yerkes (1903) was 

 the first to make use of a cylindrical lens in studying re- 

 actions to light. 



c. Experiments. — In these experiments the light grader 

 was placed in a horizontal position in such a way that the 

 glower was vertical. The lens was covered with an opaque 

 screen containing two triangular openings with the apexes 

 facing each other and only a millimeter apart. In this 

 way two parallel horizontal beams of light were produced, 

 the intensity of which gradually diminished from side to 

 side (see Fig. 5). The object of having two beams was to 

 neutralize any possible effect from diffusion of light by the 

 lens. 



A single plumule at a time was exposed In one of these 

 beams of light. In some cases it was allowed to grow up 

 into it from a small pot of sphagnum in which It was ger- 

 minated; In others the seedlings were transferred to the 

 light grader after the plumules were about one centimeter 

 long. 



In former experiments with this apparatus aquatic or- 

 ganisms were used; it was therefore necessary to expose 

 them in an aquarium containing water. Under such con- 

 ditions it is Impossible to eliminate light reflected from the 

 glass walls of the aquarium and from particles in suspension 

 in the water. With the plumule growing in air, however, 

 and with only one exposed in the beam of light at a time, 

 it is evident that all such reflections are done away with. 

 Thus the objections to Oltmanns' experiments with the 

 hollow prism have been obviated, and likewise those brought 

 forward against Darwin's work. 



All the following experiments were performed in a large 

 dark room. During the first part of the work the apparatus 

 was situated several meters from a dead black wall upon 



