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In the new or metric system of numbering spectacles and test lenses, the unit is the dioptry (written also dioptre and dioptric, and abbreviated D) which is a lens having a focal distance of 1 meter=39.37 English or American inches, =36.96 Paris or French inches. In the following table the first column gives the number of the lens, showing refractive power in dioptries; the second column gives the focal distance of each lens in English inches; the third gives the focal distance in French inches; and the fourth in millimetres. Fractions of less than one millimeter or less than one-tenth of an inch, are disregarded The fifth column shows the nearest equivalent number according to the old system in which lenses were designated by the radius of curvature of the equal convex surface in French inches, this being about equal to the focal distance in English inches.

This table shows for the different ages from ten to seventy-five years the normal average power of accommodation in dioptries, and the distance, in inches, of the near point for the emmetropic eye; the strengh of the convex lens required to correct the presbyopia of such an eye having full vision, in dioptries, and the distance to which it should bring the near point in inches:

Presbyopia And Asthenopia. (A Remedy making Spectacles unnecesary.)

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℞ Eserinae sulphatis......gr. xv

Aquae...........℥ j

M. S. Put one drop in each eve at bed time.

- Dr. W. Cheatham.