Refraction of the eye : its diagnosis and the correction of its errors : with chapter on keratoscopy / by A. Stanford Morton.

  • Morton, Andrew Stanford.
Date:
1882
Catalogue details

Licence: Public Domain Mark

Credit: Refraction of the eye : its diagnosis and the correction of its errors : with chapter on keratoscopy / by A. Stanford Morton. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

    62/70 (page 54)
    Presbyopia. 1 hough the patient does not require glasses for distant V., he may, as age advances, find it more and more diffi- cult to read without them. This defect is called Presbyopia and is caused chiefly by failure in the power of accom- modation, but is also accompanied by a flattening of the crystalline lens. It is indicated by a recession of the near point, and it is said by Donders to have commenced when this is further from the eye than 22 cm. (9 ins.), i.e., than the focal length of a lens whose refracting power is 4-50 D. ff. As we have already seen, (Chap. IV.) such a lens would, in E7nmetropia, bring rays from the “ near point to a focus on the retina in the absence of all accommodation. The difference, then, between this lens and one whose focal length equals the distance of any given receded near point, denotes for Emmetropia the amount of accommodation which is deficient, and the lens necessary to enable the patient again to read at 22 cm. e.g.—With near point receded to 40 cm. (16 ins.) we must deduct from 4-50 D. ff), the lens, whose focal length is 40 cm. (16 ins.) viz., 2-50 D. fff) and have, as the neces- sary lens, + 2 D. fff). When all accommodation is lost, even an eye which was originally ejnmetropic may require a convex lens for parallel rays (Donders). The strength of this lens must, of course, then be added to 4-50 D. Iff) in order that such a patient may read at 22 cm. (9 ins.)