Abstract
By lens-grinders and spectacle salesmen, the powers of spectacle lenses are computed by simple additions and subtractions; a lens which has a + 5 D. surface on one side and + 5 D. surface on the other, is called a + 10 D. lens. Also a lens which has a + 20 D. surface on one side and a - 10 D. surface on the other, is called a + 10 D. lens. But it is found in practice that if this double convex +10 D. lens accurately measures a defect in an eye, and a meniscus lens of this form is ordered with the - 10 D. surface next the eye, an over correction of more than 2 D. will exist. In other words this meniscus lens with a conventional value of + 10 D. has a value equivalent to that of a double convex lens of more than + 12 D. Moreover the ordinary neutralization test fails in
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