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 objects, spectacles must be pro vided which render vision distinct at from 16 to 18 inches. Moreover, 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.
2. The optical remedy for short-sight obviously consists in concave glasses of a focus suited to the individual case. At first sight it might be 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 (for example, in the double eyeglass, which is only at intervals held before the eye), or when the affection is slight, and the eye is otherwise healthy, perfeot neutralization is admissible; but so many cir cumstances forbid the complete neutralization of the myopia that an oculist of reputa tion should always, if possible, be consulted as to the choice of spectacles. Glasses, if injudiciously selected, usually aggravate the evil they are intended to remedy ; and id connection with this subject we must warn our readers against the prevalent habit of employing a single eye-glass; it is most prejudicial to the eye which is left unemployed, and often leads to its permanent injury.
Long-sight and presbyopia (derived from the Greek words presbys, an aged person, and ops, the eye) are usually considered by English writers as synonymous terms. Donders, who is now universally accepted as the highest authority on this department of eye-affec tions, maintains that " the term presbyopia is to be restricted to the condition in which, as the result of the increase of years, the range of accommodation is diminished, and the vision of near objects is interfered with." As from youth up to extreme old age, the vision of near objects becomes progressively more and more difficult, it is impossible to fix any limit as the commencement of presbyopia. In practice, however, a word is required which indicates the condition in which the eye, at an advanced period of life, and sometimes sooner, requires convex spectacles for distinct near vision, as, for exam ple. for reading, and this word is presbilopia. In this state, the nearest point of distinct binocular vision is found to lie about 8 in. (or double the ordinary distance) from the eye, and at this point Donders arbitrarily places the commencement of presbyopia. This condition, which is as natural a concomitant of advanced life as gray hairs or wrinkle, is occasionally met with in young persons. In these cases, it generally arises from intes tinal irritation, and may be a precursor of amaurosis; hence such cases should be care fully watched. In ordinary presbyopia, the defect is at once remedied by the use of
glasses of low convex power, as of 30 or 24 in. focus, which should, however, only be worn during reading and writing, and not constantly. Although the improper use of convex glasses is not by any means so dangerous as the inconsiderate use of concave glasses, the advice of a good oculist regarding the choice of spectacles is well worth his fee.
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 singly being perfect; or there may be double vision with one eye only. The first form may occur (1) in cases of squinting, or (2) in cases of paralysis of one or more of the muscles of the orbit. In cases of squinting (q.v.), the vision of the most distorted eye is almost always imperfect; and it is well known that impressions on the two retinm are similar in kinds but dissimilar in form. Time 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 case when the squint is not very well marked, double vision resultsi whenever both eyes are employed together, in consequence of images of nearly equal intensity falling on non-corresponding parts of the two retime. This variety of double vision, although annoying, is perfectly harmless. When double vision arises from mus cular paralysis, disease of the brain of a serious nature is to be apprehended. although the affection sometimes appears to arise from exposure to cold. 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.
Color-blindness is noticed under its own name.
Right-blindness, or hemeralopia (from the Greek, signifying "day-sight "), is a peculiar form of intermittent blindness, the subjects of which see perfectly with an ordinary light, but become entirely and almost instantaneously blind as soon as twilight com mences. It is seldom met with in this country except among sailors just returned from tropical regions. It is frequent among the natives of some parts of India, who attribute it, as our own sailors do, to sleeping exposed to the moonbeams. The most probable cause of the affection is, however, exhaustion of the power of the retina from over excitement from excessive light, so that this organ is rendered incapable of appreciating the weaker stimulating action of twilight or moonlight. All that suggests itself in the way of treatment is to protect the eyes from strong light during the day, and to prescribe quinine and a nourishing mixed diet.
must be regarded as an allied affection to the preceding.