On diseases and injuries of the eye, a course of systematic and clinical lectures to students and medical practitioners - Tapa blanda
Sinopsis
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1882 Excerpt: ...Different glasses can only be allowed in cases in which the difference of refraction is slight. Even then it must only be done after minute examination, so as to make sure that it suits. In this, just as in correcting myopia, very great care must be taken; the eye must on no account be strained, as any straining may give rise to visual disturbances. Every case must be judged per se, and not in accordance with any fixed formula. When an eye takes part in binocular vision its function must bo maintained, even when it constantly receives very imperfect diffused images. Metrical System Op Numbering Glasses Within recent years a metrical system of numbering glasses has been introduced. The present generation, however, have been so long accustomed to the numbering by Paris inches, that they are by no means familiar with the new system, the more so as in the standard work on refraction (that of Donders) Paris inches are used; and, besides, provincial opticians, so far from having adopted, do not yet understand the metrical system, so that in ordering glasses the new has to be converted for them into the old system. The usage of ophthalmologists being on this point divided, it is, therefore, necessary to give both systems, and to show their relations to each other. According to the old, then, the number gave in inches the radius of curvature of the lens. It will be seen by referring to the formula for calculating the focal distance of a lens from its radius of curvature and index of refraction, that, when the index of refraction is 1, the focal distance is equal to the radius of curvature. Now, this is approximately the index of refraction of spectacles in general use, and therefore the number of a glass gives also its focal distance. In calculations for combinatio...
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Reseña del editor
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1882 Excerpt: ...Different glasses can only be allowed in cases in which the difference of refraction is slight. Even then it must only be done after minute examination, so as to make sure that it suits. In this, just as in correcting myopia, very great care must be taken; the eye must on no account be strained, as any straining may give rise to visual disturbances. Every case must be judged per se, and not in accordance with any fixed formula. When an eye takes part in binocular vision its function must bo maintained, even when it constantly receives very imperfect diffused images. Metrical System Op Numbering Glasses Within recent years a metrical system of numbering glasses has been introduced. The present generation, however, have been so long accustomed to the numbering by Paris inches, that they are by no means familiar with the new system, the more so as in the standard work on refraction (that of Donders) Paris inches are used; and, besides, provincial opticians, so far from having adopted, do not yet understand the metrical system, so that in ordering glasses the new has to be converted for them into the old system. The usage of ophthalmologists being on this point divided, it is, therefore, necessary to give both systems, and to show their relations to each other. According to the old, then, the number gave in inches the radius of curvature of the lens. It will be seen by referring to the formula for calculating the focal distance of a lens from its radius of curvature and index of refraction, that, when the index of refraction is 1, the focal distance is equal to the radius of curvature. Now, this is approximately the index of refraction of spectacles in general use, and therefore the number of a glass gives also its focal distance. In calculations for combinatio...
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