Appendix II.

395

The Fault of the common Spectaplesconsists in two Particulars, viz, ist, TheLargeness of the Aperture or Diameter ofthe Glasses; and, 2dly, their oblique Posi-tion to the Axis. of the Eye; both whichmust in Time have a very sensible EffectUpon the Texture of that curious and de-licate Organ of Sight, the Eye. But theHarm we receive being not immediately sen-sible, is not considered by common People,and therefore not regarded ; and when atLength they find their Eyes weakened andimpaired, they reckon it as the natural Con-sequence of Age, without being apprizedhow far their Glasses have contributed to pro-duce that Effect.

That the Area of the Glasses in commonSpectacles is visibly larger than is necessary,no Body will dispute, when they considerhow small the Pupil of the Eye is whencompared therewith ; and that no moreLight can be useful than that which entersthe Pupil. Suppose, for Instance, the Pu-pil of the Eye were 4 Part of an Inchin Diameter (which is larger than any Ihave yet observed) and that the Glass be oneInch and a half in Diameter, then will theArea of the Glass be to that of the Pupilas 225 to 4, which is more than 50 to 1;

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