Defective eyesight : the principles of its relief by glasses / by D.B. St. John Roosa.

  • Roosa, D. B. St. John (Daniel Bennett St. John), 1838-1908.
Date:
1899
    CHAPTER VII GENERAL REMARKS AS TO LENSES General remarks as to spectacles and eyeglasses. — Periscopic and toric lenses. — Use of glasses m the amblyopia of strabismus. — Anisometropia. — The dioptric system for measuring glasses. — Bifocal or Franklin glasses. — Kind of artificial light that should be used for close work. — Summary of method in testing defective vision. When the proper lenses for the correction of an error of refraction have been prescribed, the work of the optician begins; but the practitioner who leaves all the details as to whether eyeglasses or spectacles should be used, the size of the lenses, and so forth, to him who manufactures and sells the lenses, will sometimes be disappointed in his results. Even in large cities, where opticians and good opticians abound, the prescribers of glasses should, in some instances, see the glasses over the patient's eyes. Mistakes in the numbers of the glasses are seldom made by the opticians. They have too many checks for that, but they are not always fully alive to the necessity of seeing that the glasses are properly centred; that is to say, that the centre of the glass is immediately in
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    notions of comfort and appearance as to specta- cles or eyeglasses, in all but exceptional cases. It is perfectly easy in adults, and even in some large children, to adjust eyeglasses instead of spectacles for those who use them only for reading, or occupations on near objects. When they are to be worn constantly, even, eyeglasses may be adjusted to most eyes, so that they will not blow off in the face of a strong wind, while riding on horseback, or walking on the deck of a ship, and the like. Some authorities lay stress upon very large glasses ; but there is really no im- portance to be attached to a large lens. What is called No. 1 by the opticians, is large enough for adults, while children may have a size smaller. The advantage of larger glasses is altogether a fancied one, while they become much heavier by the increase in size, and are unsightly. Gold is the best material for frames and nose-pieces, sim- ply because of the durability of the material, and its not being acted upon by wet and cold. Iron and steel are also serviceable, while nickel-plated steel frames do very well. Patients should be in- structed, when beginning to wear glasses, as to how to take care of them. It seems elementary
    teaching to say to a patient that, in order to see well through a glass, it should be kept clean; but such instruction is often necessary. Japanese paper or chamois skin, are perhaps the best means of cleaning glasses. Besides rubbing the glasses every day, frames and glasses may occasionally be washed in a weak solution of aqua ammonia. Eyeglasses should not be kept doubled, but at full length. Cases are now generally provided that this may be done. Very few persons wear- ing eyeglasses require a cord or chain to keep their glasses on. So many improvements have been made of late years in the nose-pieces of eyeglasses, that this is seldom necessary. Periscopic and Toric Lenses Periscopic lenses, or those with one surface con- vex while the other is concave, have a great ad- vantage over those that are convex or concave on both sides, biconvex or biconcave. It is well to see to it that opticians furnish peri- scopic glasses even when low numbers are used. But in cases of compound or mixed astigmatism of a high degree, the periscopic principle in a so-called toric lens is oi great importance in enabling the