Publisher's Synopsis
This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1907 edition. Excerpt: ...at 1 meter the eye would be just 1 D. myopic. 'The concave. lens, therefore, which delays the focussing so that they meet at just 1 meter is the measure of the convergence of the emerging rays in excess of 1 D. Hence the strength of this concave' lens, added to 1 D., is the total measure of the myopia of the eye, and will be its proper correction. Examp1es.--Shadow is seen to move against the mirror. A 0.25 lens just reverses the shadow movement. Then the eye is myopic to the degree that 0.25 D. is less than 1 D., i. e., it is myopic 0.75 D., and a--0.75 D. lens will correct the myopia. ' Shadow is.seen at onceto move with the mirror. A--1.00 D. lens just reverses the movemcnt. Then the eye is myopic to the degree that 1 D. is in excess of 1 D., i. e., it is myopic 2 D., and a--2 D. lens will correct it. Direct Ophthalmoscopy in Myopia.--Under the direct method of ophthalmoscopy the rays emerging from the patient's eye are convergent, and the normal unaccommodated eye of the observer cannot focus them upon his retina, hence cannot get a clear view of the patient's fundus. A convex lens wheeled before the sight hole makes matters worse, as it converges the rays still more. We therefore try concave lenses, and the weakest ' con'cave lens which enables the observer to get a clear view of the patient's fundus, i. e., which renders the convergent emerging rays equivalent to parallel rays, is of 'course the measure of their convergence, i. e., of the myopia of the eye, and its correction. Ilxamp1e'.--Under direct ophthalmoscopy, with no lens at all, the observer cannot see the details of the patient's fundus. A weak convex lens makes it still more indistinct, but a weak concave lens improves it, ..
