486 S. O. MAST 



The two slits in the spectrometer were both, unless otherwise 

 stated, 0.75 mm. wide, insuring a fair degree of purity in the 

 spectral colors used. All of the observations on the microscopic 

 forms were made in a small rectangular aquarium under a 

 binocular supported by an extension arm. The aquarium, which 

 was 26 mm. wide, 26 mm. long and 9 mm. deep, was made of 

 the best qualitj'- of colorless glass slides accurately cut and 

 ground and glued with Khotinsky cement. An opaque screen 

 containing an opening 3 x 10 mm. was fastened to each of the two 

 adjoining sides of the aquarium in such a position that the long 

 axis of the opening was parallel with the bottom, the lower edge 

 of it just a trifle lower than the upper surface of the bottom 

 and the ends of it equal distance from the ends of the aquarium. 

 Thus each of the two horizontal beams of light crossing each other 

 at right angles in the aquarium was 10 mm. wide. The aqua- 

 rium was supported by a glass plate containing ledges so arranged 

 that the aquarium could be easily removed and returned to pre- 

 cisely the same position. The glass plate contained a line which 

 passed under the center of the aquarium and bisected the angle 

 between the two beams of light. Some distance under the 

 center of the glass plate there was a 2 c. p. iiiby electric bulb and 

 above this a plate of ground glass and a petri dish containing 

 water for the purpose of diffusing the hght and reducing the heat 

 (fig. 2). In some of the woz'k the dish was replaced by a few ad- 

 ditional glass plates. The hght from below made it possible to 

 see the organisms and the line on the glass plate clearly, but 

 being red it did not appreciably influence their direction of 

 movement. For further mformation regarding apparatus used 

 see page 517. 



In making the observations, a considerable number of the or- 

 ganisms was put into the aquarium in clear pond or tap water 

 about 4 mm. deep. The aquarium was then put in place and the 

 illumination in the beam of white light adjusted by altering the 

 size of the opening in the Lummer-Brodhun sector until the 

 organisms travelled parallel with the hne under the aquarium 

 on the glass plate. This adjustment was made in darkness, so 

 as to eUminate the personal factor as a possible error. After the 



