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

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

The process of fine grinding is continually checked by the spherometer, and the art consists in knowing how to move the grinding tool so as to make the lens surface more or less curved.

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.