50 ASA A. SCHAEFFER. 



-16. Finally a pseudopod was thrown out on the right partly 

 encircling the light 19. After breaking up into a number of 

 pseudopods the ameba moved away. The ameba was shifted 

 with the beam of light on the left 22. Movement became some- 

 what uncertain at first, then streaming was reversed 25. As 

 the ameba moved forward in the new direction, it gradually 

 turned to the right until it was flowing directly toward the beam 

 of light 29. After partly surrounding the beam 35 the 

 ameba moved away. 



Summary. From the foregoing experiments it is clear that 

 ameba responds positively to white light under the given con- 

 ditions. The ameba is attracted from a distance of sixty-five 

 microns or more. Since the beam is projected vertically, the 

 question at once arises: How does the ameba become aware of 

 the beam of light? Is some light reflected horizontally by par- 

 ticles of solid matter in the water; or is some of the light energy 

 transformed yito heat or other form of energy which, being radi- 

 ally propagated, stimulates the ameba? These are very im- 

 portant questions in sense perception, but they must remain 

 unanswered for the present. The light apparatus at my dis- 

 posal was too crude to attempt a solution of them. 



In most cases the ameba reacts positively until it comes into 

 contact with the beam of light, when negative behavior usually 

 sets in. This difference in behavior may be due to differences in 

 the intensity of the stimulus. Both proteus and dubia react 

 positively to white light. 



It is quite clear from these experiments that light of the in- 

 tensity used does not tend to inhibit directly the formation of 

 pseudopods, nor does it seem to have any other direct effect on 

 the movement or form of ameba. 



Red Light. The apparatus for producing monochromatic light 

 of the various wave-lengths was a Leitz Lilliput arc light, fifteen 

 cm. of distilled water, a piece of grating with a slit two milli- 

 meters wide, a screen of heavy drawing paper with a pinhole about 

 one half mm. in diameter with clear-cut blackened sides, front 

 surface mirror and condenser. The screen was blackened on the 

 far side and lighted on the near side with just enough light from 

 a Welsbach mantle to barely see the amebas. The spectrum on 



