GB191210129A - Improvements in Bifocal Spectacle Lenses. - Google Patents

Abstract

10,129. Jameson, A. April 29. Spectacle lenses.-In a bifocal, cylindrical, or toric spectacle lens for the correction of astigmatisms, an insertion D, Fig. 1, of dense glass is made at the astigmatic surface B, and a spherical depression C of the same size and shape is made at the spherical surface of the lens to reduce excess spherical power. This insertion or wafer has the same curvature as the external astigmatic curvature of the lens. The spherical depression C may be filled by glass having a lower index of refraction than that of the spectacle lens. The excess spherical power occasioned by the convex spherical curve A given to the wafer can be reduced or avoided by fusing or cementing it into a cylindrically curved depression, and by so choosing the index of refraction of the glass wafer that the added cylindrical effect may be divided between the outer and inner surface of the wafer. If the seating of the wafer is made cylindrical or toric, the lens is applicable to the correction of astigmatic presbyopes, provided that the cylindrical curvature of the seating be made at right-angles to the convex surface of the lens so that the two cylindrical powers may act together to produce a spherical power. A stopper or plug A, Fig. 2, of high refractive index may be inserted instead of the wafer and serves to increase the cylindricity of the reading portion of a deep periscopic toric lens, the concave surface A compensating for increase of spherical power at the toric surface B. In manufacture, the lens blank A, F, Fig. 3, may be made large enough to provide for making both lenses at one operation, the blank being cut across at the line D-D after the insertion E E has been made. A larger number of bifocal blanks may be made in the form of an annulas which may be cut up radially. For the modification of prismatic power and to correct errors in centration without introducing a spherical effect, a supplementary lens may be inserted in a manner similar to that already described. A compound lens may he inserted in stopper form, the lenses forming the compound stopper A B, Fig. 6, being decentred before being fused together.