REACTION IN AMOEBA TO LIGHT 
267 
the question of local reaction due to increase in illumination of a 
limited part of the organism and consequently throw no light on 
the factors involved in orientation. It was therefore necessar>^ to 
study the effect of local increase in light intensity. This was done 
as follows : In a dark room a limited area of a luminous Welsbach 
mantle was focused on a slide containing amoebae by means of 
the mirror and the Abbe condenser attached to a microscope, so 
as to form an intensely illuminated and well-defined area about 
0.5 mm. square. By carefully manipulating the mirror the intense 
Kght in the illmninated area could be suddenly flashed on any 
portion of an amoeba. It was found that an increase of intensity 
produced thus, usually caused a cessation in movement in the part 
stimulated, especially in case of newly formed pseudopods. But 
more convincing results were obtained by studying the reactions 
of specimens as they came in contact with the illuminated area 
in their random movements. Many observations were thus made 
on different individuals and it was found in nearly all instances 
that they stopped when they reached the light and proceeded 
in a different direction. 
The details in the responses which resulted in a change in the 
direction of motion and thus kept the organism out of the intense 
light were essentially the same in all of the specunens observed. 
They are graphically recorded for a single individual in figure 1. 
By referring to this figure it will be seen that after one pseudopod 
came in contact with the illumination and was stopped, the amoeba 
did not at once proceed in the opposite direction so as to avoid 
the light but sent out other pseudopods at only a slight angle with 
the first apparently trying to get around the obstacle in this way. 
The character of the response did not change after the first pseudo- 
pod came in contact with the light nor did it change after the 
second and the third came in contact with it. But after the fourth 
became exposed the direction of motion was nearly reversed. This 
indicates that the reaction was modified, that the response to 
a given stimulus depends upon the preceding experience. 
It is evident then judging from the reactions described that a 
sudden increase of intensity tends to inhibit the movement 
momentarily in Amoeba either locally or entirely and that this 
THE JOURNAL OF EXPERIMENTAL ZOOLOQY, VOL. 9, XO. 2. 
