OBSERVATIONS ON UNICELLULAR FORMS 93 



dark room were used in all quantitative work, and in all 

 work in which it was desirable to regulate the direction 

 of the rays. I have elsewhere pointed out the advan- 

 tageous features of these glowers for such work (1906, 



P- 363)- 



The general movements of Euglenae could readily be 

 followed under a Braus-Driiner binocular, but it was found 

 necessary to use a compound microscope in working out 

 the details in the reactions owing to the small size of the 

 organisms. They progress so slowly however that every 

 movement can easily be followed even under the highest 

 magnification. They are consequently very favorable for 

 the work in hand notwithstanding their minute size. 



In studying the process of orientation the microscope was 

 placed either in front of two windows in the laboratory 

 so situated that the general direction of the light entering 

 them was at right angles on the stage, or in the dark room 

 in a similar relative position in front of two Nernst glowers 

 (see Fig. 13). The two glowers were mounted so that the 

 rays were practically parallel with the plane of the stage. 

 One was stationary; the other was mounted on a track so 

 that the distance between it and the aquarium could readily 

 be varied, and thus the intensity of the light from it on the 

 stage changed without any change in the direction of the 

 rays. Both glowers were of the same kind and both were 

 in the same circuit, so that any fluctuation in the current 

 affected both alike. The relation in light intensity from 

 the two sources could thus be regulated as desired. The 

 glowers were so screened that only a small beam from each 

 reached the stage, and this could readily be cut off from 

 either or both. 



The Euglenae were either mounted on a slide under a 

 cover-glass or exposed in a rectangular glass aquarium made 

 for the purpose by cementing slides together with balsam 

 and linseed oil. After they had oriented in light from one 

 of the two sources, the light from that source was cut off 

 and that from the other turned on simultaneously. In 



