QUANTITATIVE ANALYSIS BY MEANS OF THE MICROSCOPE 207 
follows: By means of the plane mirror and Abbe condenser 
project the image of a coordinately ruled screen (photographic 
positive on glass) into the plane of the objectd The most con¬ 
venient magnitude of the rulings may be selected by varying 
the distance of the screen from the microscope; a great advantage 
at times. When dealing with thick particles rulings on the cell 
itself may almost disappear if the microscope is focused upon 
the upper surfaces of objects, but in the projected image method 
it is merely necessary to shift the substage condenser slightly 
Fig. 131. Counting Cell. (After Whipple.)* 
viates the purchase of a number of expensive, specially ruled cells 
for special purposes. 
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 
in the quantitative determination of the microscopic organisms 
in water may be profitably employed. This cell, Fig. 131, 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 
* See Method 4, Micrometry, Chapter VII. 
* From The Microscopy of Drinking Water, by G. C. Whipple, p. 35, Third Ed. 
John Wiley & Sons, Inc. Reproduced here through the courtesy of the author. 
