276 
S. O. MAST 
SUMMARY 
1. A sudden and sjiarp increase of light intensity causes 
retardation or cessation of movement in Amoeba proteus. This 
effect may be local if the increase of intensity is local. 
2. If the intensity remain constant for a few moments after 
a response, then the movement usually begins again. If the in- 
tensity is very gradually increased it produces no response, show- 
ing that the reaction to light in Amoeba is dependent primarily 
upon the rate of change of intensity. 
3. Amoeba proteus is negative in strong light and orients 
fairly accurately. Orientation is brought about by the inhibition 
of the formation of pseudopods on the more highly illuminated 
side. This is probably due to local changes of intensity owing 
to the movement of the protoplasm and the resulting shadows of 
one part passing over others. 
4. There is no evidence indicating that the direction of the 
rays through the tissue or absolute difference of intensity on 
different parts of the organism is functional excepting in so far 
as it may result in changes of intensity on the organism. 
5. The blue (430 to 490AtAt) in the solar prismatic spectrum is 
nearly as efficient in causing reactions in Amoeba proteus as 
white light. Violet, green, yellow and red are only very slightly 
active. Not all organisms, however, are most strongly stimulated 
in the blue of the solar spectrum. Some respond most definitely 
in the violet, others in the green and yellow, and still others in the 
red. 
6. It is highly probable that different photochemical changes 
are associated with the reactions to light in different organisms. 
