Eyestrain (commonly called asthenopia) / by Ernest Clarke.

  • Clarke, Ernest, 1857-1932.
Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: Eyestrain (commonly called asthenopia) / by Ernest Clarke. Source: Wellcome Collection.

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    necessary, dividing the effect equally between the two eyes. For instance, if the prism that corrects the deviation is 2° base in, and the lenses are + 3I>, we have 8.7 X 2 3 = 5.8 mm. so that each lens will have to be shifted 2.9 mm. inwards. If the lenses were purposely decentered to correct insufficiency of the external or internal rectus muscles, we apply the same test, and if deviation exists, ascertain by prisms base in or out whether the decentering is too much or too little. The plane of the glasses should be perpendicular to the visual axis when in use, hence reading glasses should be inclined from the vertical, the upper end being tilted forward. The glasses should be so fitted that they are as near as possible to the eyes, and yet not near enough for the lashes to touch them. Concave glasses are weakened, and convex glasses strengthened, by removing them from the eye, and vice versd, Jackson has pointed out that a cylindrical lens becomes more cylindrical, and a spherical one sphero-cylindrical, by being placed obliquely before the eyes.^ The bridge of the frames should fit the nose well, and not be liable to slip down. It is always best to have spectacles and not folders—folders never fit; nevertheless they are very useful to the presbyope, and the myope may be allowed to use them ; but under no circumstances should they be given to young people or to astigmatics. The so-called astig- matic pince-nez which do not fold, and work with a horizon- tal spring, may be allowed in astigmatism, but are inferior to spectacles. Except in the case of monocular amblyopia beyond any hope of cure, or monocular blindness, single eyeglasses should be strictly forbidden. • Oph. Rev., vol. vi., p. 247.