Turned-up Edge
This is a narrow edge zone of shortened radius of curvature. It is conspicuous by a thin strip of shadow just inside the diffraction ring around the right-hand edge.
The corresponding zone on the left edge is very bright, and if the zone is very narrow, this may be mistaken for the normal diffraction ring of a perfect edge. Turned-up edge seldom occurs in more than mild form, and usually works out under the long and overhang strokes used to correct the oblate spheroid, or to parabolize.
It may result from the short strokes used to correct a turned-down edge, or from a lap that has too small a diameter. If it is stubborn, polish upside down for a few minutes, with short strokes. This will probably cause the edge to turn down, and some correction will be necessary to restore the diffraction ring. When satisfied with the edge, go after the rest of the figure.
Next- Correcting the oblate spheroid |
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