208 Wager: Movements of Aquatic Micro-Organisms. 
approximately equal numbers (Fig. XI.). If unequal they 
accumulate unequally (Fig. X.), and by varying the intensity 
of one of the sources of light it is found that, although all the 
Euglenae at the beginning of the experiment are within the 
range of the two different sources of light, they are attracted 
by, and accumulate at, these two sources in such numbers as 
can only be explained by the fact that they become oriented 
by the stronger of the two lights falling upon them. That this 
ao 
Fig-. XII. — When light at a is equal to b, the region of influence of each 
is equal, as indicated by i. If light at b remains the same, but that at a 
increases in intensity to 2,3, 6 or 12 times as much, the region of intensity 
due to a will be correspondingly increased. 
must be so can be shown by means of the diagram (Fig. XII.), 
representing a shallow circular cell in which Euglenae are, at the 
beginning of the experiment, evenly distributed and exposed to 
light from two different sources, a and b. If the light at a is 
equal to that at b (approximately equal), then the line 1 — i 1 
marks the neutral region where the intensity of the light 
from either source is equal. This line divides the circle into 
two equal halves and it is obvious that if the Euglenae move 
Naturalist, 
