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It is well to control this process by means of a spherometer, so that the desired radius may be approximately reached.

Here is a very fine spherometer that Dr. Hastings works with from time to time, and which he calls his standard spherometer.

A check by spherometer will be applied at intervals according to the judgment of the operator, but, in any case, the fellow tool and lens should be kept at very nearly the same figure.

The rough grinding is checked by the spherometer, and is interrupted when that instrument gives accordant and correct measurements all over the surface.

The squares of glass are cemented to the roughing tool, and this is ground to the spherometer by means of the counterpart tool.