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 object. 1 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 

 in order to bring the scale sharply in focus in 

 the same plane as the image of the powdered 

 material. This method of projecting the image 

 of a scale permits the use of ordinary slides and 

 of rulings of all sorts and magnitudes. It ob- 



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 



1 See Method 4, Micrometry, Chapter VII. 



2 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. 



