DIATOM MOTION. 1 29 



In the second series (Figures IV to VII) the illuminated 

 area was much reduced by the interposition of a card, close 

 up tt) the mantle, pierced by a rectangular hole of such dimen- 

 sions that the sharply focussed image made a light area of 

 the apparent dimensions of one and a half by four centime- 

 ters. In both series, the top illumination of the stage was 

 negligible, being only such diffused rays as came from the 

 dull gray walls of a dimly lighted room. In the second series, 

 especially, the gloom of the room was marked, owing to the 

 screen placed in front of the mantle. Under these conditions 

 a small diatom, unless in or near the lighted area in the 

 centre of the field, was only barely visible, and small, inert 

 and motionless objects could V)e seen only with much diffi- 

 culty. Conditions being as stated, a drop of the gathering- 

 was put upon a glass slip having a shallow cell open at the 

 ends, a cover glass was superposed, and a particular diatom, 

 observed as in active motion, was brought nearly into the 

 centre of the field, and often into the illuminated area. A 

 diagram of the field being at hand, the first position of the 

 centre of the diatom was marked, and simultaneously the 

 hour and the minute. Successive positions were marked at 

 intervals of a minute or less, until the diatom either came to 

 rest or passed from view. These positions were connected 

 in the diagram, at the time of observation, with a line repre- 

 senting as nearly as possible the veritable track of the diatom. 

 The observations were all made at night, but not all during 

 the same night. Times are given in hours, minutes and frac- 

 tions of a minute. 



