REACTION IN AMOEBA TO LIGHT 
269 
response is dependent primarily upon the time rate in change of 
intensity. What bearing has this fact on the analysis of orienta- 
tion? 
ORIENTATION 
The observations on orientation were made' under a compound 
microscope situated in diffuse dayhght without any screen around 
it. Mirrors were so arranged that two horizontal beams of direct 
sunlight were reflected on the stage at right angles to each other 
after passing through 8 cm. of water to eliminate the heat. Speci- 
mens exposed in one of these beams without any light from the 
sub-stage were found to direct their course in a general way from 
the source of light. In one instance, after a sHde had been exposed 
for fifteen minutes, there were eleven specimens in one field of 
the low power, all but two of which were moving from the source 
of light. In another field there were twelve specimens; all but 
four of these were directed from the source of light. Of these 
four, two were proceeding at right angles to the rays and two 
were going toward the light. In still another field containing 
nine specimens, seven were negatively oriented, one positively 
and one at right angles to the rays. Orientation was however, 
not very precise in any of the specimens. Amoebae usually take 
a sort of zigzag course. Pseudopods were frequently seen to 
extend toward one side for some distance then stop as though they 
had been checked, after which new ones were ordinarily seen to 
extend on the opposite side for some distance and stop, etc. 
The details in the process of orientation were observed as fol- 
lows : a specimen which had oriented in one of the beams of light 
was selected, after which the light in this beam was intercepted and 
that in the other simultaneously turned on. The reaction of 
numerous specimens was observed in this way and the movements 
in several were recorded by means of camera sketches made at 
short intervals. A typical record is presented in figure 2. A 
majority of the specimens did not, however, orient as precisely 
and definitely as this one did. By referring to the figure it will 
be seen that the amoeba under observation gradually turned 
from the side most highly illuminated, sending out pseudopods 
only on the shaded side. What is the cause of this? 
