In recent years, so numerous have been the varieties of lenses produced by zealous manufacturers that a sporadic survey of the subject is necessary to evaluate properly the progress made. Cowan1 and Jackson2 have repeatedly inveighed against "copyright glasses" and insist that the physician should have a precise knowledge of the glass he prescribes.
IMPROVEMENTS IN LENS FORM
As long as very small lenses were in vogue, the flat lenses then used were fairly satisfactory. But in large flat lenses a noticeable marginal blur occurs, owing to the astigmatism of oblique eccentric rays. This has been almost entirely eliminated within a solid angle of 60 degrees by making the lens of proper shape. "If the lens contains a cylinder, only one principal meridian can be corrected—usually the horizontal. Moreover, a lens corrected for infinity is not quite correct for reading distance" (Cowan).
Cowan suggested that convex lenses
LEBENSOHN JE. MODERN DEVELOPMENTS IN SPECTACLES. Arch Ophthalmol. 1932;7(1):113–119. doi:10.1001/archopht.1932.00820080125009
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