DIATOM MOTION. 7 I 



it was permitted to enter through the substage diaphragm. 

 The objective of one inch focal distance was used, with short 

 eye-pieces, and the apparent diameter of the field was i6 cen- 

 timeters. The amplication was found to be approximately 80 

 diameters. The source of light, as in previous experiments, 

 was a Welsbach mantle, the volume of light being minimized 

 by the interposition of a screen pierced with a rectangular 

 hole. This screen left only a narrow beam of light to fall 

 upon the mirror of the microscope, and at the same time 

 greatly lessened the general illumination of the room. 



A fresh gathering of naviculoid forms was made in the 

 afternoon and during the evening these were examined and 

 found to be in active motion. The water in which they were 

 seemed highly impregnated with oxygen, and a nearly color- 

 less, freshly made, aqueous solution of haematoxylin, when 

 mixed with it, turned a deep blood red in three minutes. 

 There was, therefore, no fear of the diatoms coming to rest 

 for want of oxygen. 



Experiment I. — A few of the diatoms were put into a 

 shallow, rectangular cell, open at the ends, and a thin cover 

 superposed. The slip was then put upon the stage of the 

 microscope, which had been arranged as described for exclusion 

 of light. The diaphragm having been opened slightly, it was 

 seen that the Naviculce were moving normally in every direction. 



INTERVALS OF OBSCURITY EXPOSURES TO LIGHT 



9.05 — 9. 10 o'clock 15 seconds 



9.io}( — 9.20 " 15 " 



9.2oi<— 9.50 



At all three exposures the activity' of the moving diatoms 

 was seen to be undiminished, and new groupings were visible 

 each time. From 9.05 to 9.50 o'clock, therefore, they con- 

 tinued their motions in darkness, with only 30 seconds of 

 exposure to a feeble light. 



