192 
ELEMENTARY CHEMICAL MICROSCOPY 
standard load is 3000 kilograms, for soft materials 500 kilo¬ 
grams. The number used to express the “ Brinell Hardness ” 
is the ratio of the applied load to the area of the indentation 
produced. To calculate the area of indentation we may measure 
either the depth of the indentation or its diameter. 
Micrometric microscopes of short range were formerly 
employed for these measurements, but in America the practice is 
generally to measure the depth of the indentation with some type 
of depth gauge. All measurements are expressed in millimeters. 
Since in many cases the forcing of the steel ball into the test 
piece causes the edges of the impression to become slightly 
raised above the surface of the surrounding metal, great care 
must always be taken in focusing the microscope. If the depth 
is to be determined by means of the fine adjustment, focus the 
instrument upon the very center of the spherical depression, 
then move the test piece until an area of the true surface is 
brought into the field of view. Read the graduated circle of 
the fine adjustment and carefully focus up with the final adjust¬ 
ment until the surface is sharply defined, read the graduated 
circle again. The difference in the readings will give the depth 
of the indentations in terms of the fine adjustment graduations. 
Knowing the value of one division of this scale, the depth 
expressed in millimeters may be calculated. Make several 
determinations, always focusing up, as directed above for the 
calibration of the fine adjustment. Make all measurements 
as near as possible to the circumference of the indentation yet 
scrupulously avoiding the ridge of metal right at the brim. 
The diameter can rarely be measured with an eyepiece microm¬ 
eter in an ordinary compound microscope since it is of too great 
a magnitude for even very low powers. Recourse should be 
had to graduated mechanical stages. In such cases the accuracy 
of the measurements will not be greater than one-tenth of a 
millimeter. Always measure several diameters. 
If h be the depth of the indentation: The Brinell Hardness 
may be calculated from the formula: 
3000 
95-5 
