48 ASA A. SCHAEFFER. 



comparison with other sources of stimuli it was thought essential, 

 in my own work, to reduce the areas of light to a size comparable 

 with the size of food and other objects which were placed at the 

 disposal of the ameba. By throwing the beams of light vertically 

 through the slide on which the amebas were placed, the area of 

 the cross section of the beam of light used for stimulating the 

 ameba could be varied within the desired limits. 



As sources of light the Welsbach gas mantle and the Leitz 

 Lilliput arc were used. The gas light was passed through five 

 cm. of distilled water containing a very small quantity of an 

 ammoniacal solution of copper sulphate to absorb the excess of 

 green and yellow rays and the distinctive heat rays. Between 

 the water and the microscope was interposed an opaque screen 

 with several small, clear-cut pinholes in it. By moving the 

 screen away from or toward the microscope, and focusing with 

 the substage condenser, the size of the projected beams of light 

 through the microscope could be easily controlled. The ordinary 

 mirror of the microscope was discarded and a front surface 

 mirror placed in its stead, in order to avoid reflection from the 

 front surface of the glass and so producing a subsidiary image on 

 the slide. The great sensitiveness of ameba to light makes this 

 precaution absolutely necessary. 



When spectral light was used, the arc was employed as a source, 

 and either prism or grating interposed between clear distilled 

 water and the screen. 



The amebas were placed on large, clear thin cover glasses in 

 clear culture fluid, without a cover glass over them. Usually in 

 these light experiments the beams of light were left stationary 

 while the coverglass containing the ameba was shifted, whenever 

 shifting was necessary for experimental purposes. Both inter- 

 mittent and continuous beams of light were employed. Inter- 

 mittent light was produced by moving an opaque object up and 

 down between the screen and the microscope. In a general way, 

 continuous and intermittent light had about the same effect on 

 the ameba. After orientation, it is worth noticing, the ameba 

 advanced as definitely and as uniformly toward intermittent as 

 toward continuous light. 



The work was done in a dark room in which there was very 



