QUANTITATIVE ANALYSIS BY MEANS OF THE MICROSCOPE 211 



properly calibrated and inserted into the eyepiece or to cut a 

 square opening in a disk of dull black paper, thin card, metal or 

 blackened mica, and drop the disk into the proper eyepiece by 

 removing the eye-lens and allowing the disk to rest upon the 

 diaphragm of the eyepiece. The proper size of opening is ascer- 

 tained by eyepiece and stage micrometers, and a square hole of 

 this calculated size is cut in the paper and the perforated disk 

 is inserted in the eyepiece. The final adjustment is then made 

 with the draw- tube. 



When the particles of material are of a sufficiently low density 

 to remain suspended for a few seconds and one cubic centimeter 

 portions can be removed the Sedgwick-Rafter counting cell used 



FIG. 118. Counting Cell. (After Whipple.) ' 



in the quantitative determination of the microscopic organisms 

 in water may be profitably employed. This cell, Fig. 118, con- 

 sists of a glass object slide of standard size to which is cemented 

 a brass cell 5 centimeters long by 2 centimeters wide; its area is 

 therefore 1000 square millimeters and being made exactly i 

 millimeter deep, its capacity when closed with a cover glass is 

 i cubic centimeter. Counts of particles are made in as large 

 a number of fields as possible, using a net eyepiece micrometer 



1 From The Microscopy of Drinking Water, by G. C. Whipple, p. 35, Third Ed. 

 John Wiley and Sons, Inc. Reproduced here through the courtesy of the author. 



