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626 ARTHUR J. HOLMAN [j. s. M. p. E.
curvature and exact gauging to a master curve. Then the fine ground glass surface requires polishing and, since this operation is carried out on a pitch polisher which is more or less plastic, and therefore subject to continual change in shape, considerable skill is required on the part of the operator to produce a uniform highly polished surface without departing from the curvature to which the surface was ground.
The most accurate curved optical surfaces are at present produced by expert lens makers with hand controlled movements of the lens blank in conjunction with a power operated spindle on which the lap of the polisher rotates. Surfaces of lesser accuracy are produced in quantities on machines requiring no manual operation but the inherent characteristics of the hand and machine methods are essentially the same. The expert lens maker can "feel" the effects of grinding or polishing and control the hand movements accordingly, whereas the machine goes through the same cycle regardless of the progress of the grinding or polishing. The lens maker's skill resides in his ability to "feel" the effects of the grinding and to modify his hand stroke in such a manner as to correct uneven surfacing.
The cause of uneven wearing of the lap in grinding and uneven polishing of the fine ground glass surface is discovered when the relative movements of lens blank and lap or polisher are analyzed. When it is remembered that the lap or polisher usually spins on the axis normal to the curved surface at its center and the lens blank tends to spin on an axis passing through its center, it is not difficult to see that relative movements of lens blank arid tool are largely the resultant of each spinning about its own center, and therefore the rate of relative movement of elements on the surface of the lens blank across elements on the tool is dependent largely on the distance of these elements from the axis of rotation of the lens blank and the tool, respectively. As the local abrading action is dependent on the speed of relative movement, it is easily seen that it is most difficult to prevent uneven grinding or polishing in zones about the centers of rotation. This is the inherent and unavoidable cause of error in all present systems of lens manufacture.
The one way to avoid this error is to produce relative movements of lens blank and tool which are not primarily the resultant of rotation of these parts on their respective axes and my improved grinder and polisher accomplishes this desired result in the following manner. All movements of the lens blank and lap are constrained to take place about the center of curvature of the lap surface; a condition