The glass is ground with emery to the correct spherical figure as given by a spherometer.
It is well to control this process by means of a spherometer, so that the desired radius may be approximately reached.
The spherometer is accepted as the sole guide in obtaining the proper curvature.
Before the grade of emery is changed all three surfaces must agree, as nearly, at least, as the spherometer will show.
The spherometer is accepted at all stages of the process as the final arbiter as to curvature.
The squares of glass are cemented to the roughing tool, and this is ground to the spherometer by means of the counterpart tool.