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June, 1930] LENS WHEELS FOR PROJECTORS 631
condition of the surface being produced. When polishing very little attention is required. No particular skill is necessary for either grinding or polishing as the accuracy of the work is automatically controlled by the machine, and one attendant can look after several grinding machines or many polishers.
The device is suitable for grinding and polishing concave and convex surfaces and the radius of curvature, which is determined by the radius of oscillation, is easily adjustable over a wide range to any required exact value. When the machine is changed over to grind to a new radius of curvature a lap of corresponding curvature must be substituted for the former one. The machine is provided with stops which may be set to limit the extent of grinding, thus making it possible to accurately control the thickness of the lenses produced. For grinding spherical surfaces the centers of the universal joints must coincide. Aspherical surfaces may also be produced by making one universal joint eccentrically adjustable or by displacing one center of oscillation along the vertical axis.
This machine is simple, compact, self-contained and of rugged mechanical construction; is capable of producing the most accurate surfaces, and is quickly adjustable to a wide range of work. It is to be noted that the accuracy of the lenses is determined entirely by the accuracy with which the centers of oscillation are maintained. The universal joints are provided with ball bearings throughout. The inner and outer sleeves rotate, the one upon the other, on ball bearings, and the construction of these parts is such that they are effectively sealed to prevent entrance of abrasives, hence the device will operate with little attention over long periods with extreme accuracy. It is evident, therefore, that this invention makes entirely practical the production of large quantities of optically accurate and truly matched lenses at a low cost.
LENS SETTER FOR POSITIONING LENS SECTORS ON LENS WHEEL
Having secured exactly matched lenses properly contoured to form sectors of the lens wheel, we now face the third and last problem in lens wheel construction, namely, that of exactly positioning the optical centers of these lens sectors in the final lens wheel assembly. It will be obvious that the correct arrangement of these optical centers will represent a symmetrical pattern about the axis of the lens wheel shaft. That is, all the optical centers should lie on a common circle and the angular spacing between them should be equal. The nearer