In the treatment of myopia the principal ob jects are: (1) to prevent its further develop ment and the occurrence of secondary disturb ances; and (2), by means of suitable glasses, to render the use of the myopic eye easier and safer.
(1) To effect, if possible. the first object. the patient must look much at a distance. but as we cannot absolutely forbid his looking at near ob jects. spectacles must be provided which render vision distinct at from 16 to 18 inches. More over, it is desirable that at intervals of a half hour work should be discontinued for a couple of minutes, and no working in a stooping position should be permitted. The patient should read with the book in the hand. and in writing should use a high and sloping desk, with good hut not too strong light from behind. If the myopia in creases, all near work should be given up for life out of doors.
(2) The optical remedy for short sight ob viously consists in concave glasses of a focus suited to the individual ease. At first sight it might he supposed that glasses with a concavity exactly sufficient to neutralize the defect in the eye would always suffice; and when the glasses are used exclusively for distant vision. or when the affection is slight, and the eye is otherwise healthy, perfect neutralization is admissible; but many require different glasses for distance and reading. An oculist of reputation should always, if possible, he consulted as to the choice of spec tacles. Glasses, if injudiciously selected, usually aggravate the evil they are intended to remedy; and in connection with this subject may be men tioned the prevalent habit in foreign countries of employing a single eyeglass: it is most preju dicial to the. eye which is left unemployed, and often leads to its permanent injury.
Far-sightedness, hyperopia. or hypermetropia is an error of refraction in which parallel rays are brought to a focus behind the retina, usually on account of shortening of the eyeball, sometimes from diminished convexity of cornea or lens, ab sence of the lens, or changes in the media. It is more common than myopia, is congenital and often hereditary. As the hyperopic eye is obliged to accommodate for parallel rays, it is con stantly strained unless corrected by proper con vex glasses. If uncorrected it leads to symptoms of asthenopia o• eye-strain. frontal and occipital
headaches, pain in the eyes, congestion and burning sensations in the lids and eyeballs.
Prcsbyopia (derived from the Greek words 71-pea(3t's, an aged person, and co, Ups, the eye), or old sight, is a change which naturally occurs in every eye between the 40th and 45th year. On ac count of loss of elasticity of the lens, the power of accommodation is diminished. The stated time for the occurrence of presbyopia has been arbitrarily fixed, as that is the period at which the near point, the nearest point to the eye at which ordinary print can be easily read, has re ceded to nine inches and some discomfort is ex perienced. The near point really begins to recede at ten years of age, and eoutinues to do so through life. if uncorrected there is difficulty in reading, blurring of print, and symptoms of eye-strain as in hyperopia. Correction is secured by convex spherical glasses, which bring the near print to a comfortable distance with respect to the person's occupation. Allowance must be made for coexisting myopia, hyperopia, or astig matism, and the strength increased at intervals.
Double vision, or diplopia, is of two kinds. It may arise from a want of harmony in the movements of the two eyes. the vision of each eye being perfect: or there may be double vision with one eye only. The first form may occur in cases of weakness or paralysis of one or more of the muscles of the orbit, which results in squint ing. In squinting ( see STRABISMUS), the vision of the most distorted eye is almost always imper fect: and it is well known that impressions on the two retina> are similar in kind but dissimi lar in form. The mind takes cognizance only of the former: so that a person with a bad squint sees objects with the sound eye only. But if the sight of both eyes is nearly equal. as often is the ease when the squint is not very well marked, double vision results whenever both eyes are em ployed together. in consequence of images of near ly equal intensity falling on non-corresponding parts of the two retinae. This variety of double vision can be corrected by suitable glasses. The second form of double vision—viz, double vision with a single eye—is a much more rare affection than the preceding one, and depends upon some irregular refraction of the cornea or lens.