The measurements are made
with the help of a scale engraved (or, better, photographed) on a
circular piece of glass which rests on the diaphragm of the eyepiece.
This scale and the object upon the stage can be easily brought into
focus at the same time. The button of gold obtained by cupelling is
loosened from the cupel by gently touching with the moistened point of a
knife; it generally adheres to the knife, and is then transferred to a
glass slide. The slide is placed on the stage of the microscope,
illuminated from below; and the button is brought into focus, and so
placed that it apparently coincides with the scale. The diameters in two
or three directions (avoiding the flattened surface) are then read off:
the different directions being got by rotating the eyepiece. The mean
diameter is taken. The weight of the button is arrived at by comparing
with the mean diameter of a _standard prill_ of gold of known weight.
The weights are in the proportion of the cubes of the diameters. For
example, suppose a prill has been obtained which measures 12.5 divisions
of the scale, and that a standard prill weighing 0.1 milligram measures
11.1 divisions. The weight will be calculated as follows:
11.1^{3} : 12.5^{3} :: 0.1 : _x_
0.1?12.5?12.5?12.5
_x_ = -------------------- = 0.143 milligram.
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