194 
ELEMENTARY CHEMICAL MICROSCOPY 
crystals are checked as counted by means of a pencil made for 
writing on glass. 
The magnifications recommended by the American Society 
for Testing Materials are as follows according to the character 
of the specimen: 
Non-ferrous alloys. .25, 75, 150 or 250 diameters. 
Steels.50, TOO, 250 or 500 diameters. 
To obtain the number of 
total number of grains found 
grains per square millimeter, the 
is multiplied by a factor depend- 
mg upon the magnification used. This factor = / = - , 
5000 
employed. If instead of express- 
square millimeter it is desired to 
of the crystals in millimeters or 
in square microns the following 
where M is the magnification 
ing the number of grains per 
obtain the average diameter 
to learn their average areas 
formulas may be employed: 
n = fx\ 
d = 
V^’ 
a = 
1,000,000 
! 
n 
X = total number of grains found; 
/ = factor for magnification, / = 
5000 
n = number of grains per square millimeter; 
d = diameter of average grain counted expressed in mm.; 
a = area of average grain in 
Calibration of Sieves. — Sieves to be used in accurate analysis 
in grading or classifying finely divided material must have the 
wire cloth carefully made and applied to the metal frames. 
Carelessness in weaving or in stretching the cloth too tightly 
upon the frames will give rise to irregular openings and the sieve 
becomes thereby unreliable and useless for the purposes to 
which it is to be put. The U. S. Bureau of Standards has issued 
