Page:The New International Encyclopædia 1st ed. v. 18.djvu/188

* SIGHT. 150 SIGHT. the eyes in the stooping position; (3) congestive processes in the base of the eye, which, leading to softening, give rise to extension of the mem- branes; (4) the shape of the orbit in broad faces, causing excessive convergence, the trouble occur- ring especially in such persons. That in increased l)ressure the extension occurs principally at the posterior pole is explained by the want of sup- port from the muscles of the eye at that part. Now, in connection with the causes mentioned, the injurious effect of fine work is, by imperfect illumination, still more increased: for thus it is rendered necessary that the work be brought closer to the eyes, and that the stooping position of the head, particularly in reading and writing, is also increased. Hence it is that in schools where, by bad light, the pupils read bad print in the evening, or write with pale ink, the founda- tion of myopia is mainly laid. On the contrary, in watchiiiakers, although they sit the whole day with a magnifying-glass in one eye, we observe no development of myopia, undoubtedly because they fix their work only with one eye. and. therefore, converge but Httle, and because they usually avoid a very stooping position. So far from short-sightedness improving in ad- vanced life, as is popularly believed, it is too fre- quently a progressive affection; and every pro- gressive myopia is threatening with respect to the future. Those cases in which the myopia de- velops slightly in young persons and practically becomes arrested are called simple or statioiianj myopia. Progressive myopia is the form which increases steadily with degenerative changes in the choroid and "other deep structures. Persons with uncorrected myopia of any severity have a characteristic vacant expression from constant inability to see any other than near objects. 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 nnich 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 10 to 18 inches. Jlore- 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 slojnng desk, with good but 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 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, 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 oerilist of reputation should always, if possible, be 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 mjopia, is congenital and often hereditary. As the hyperopic eye is obliged to acconunodate for parallel rays, it is con- stantly strained unless corrected by proper con- vex glasses. If uncorrected it leads to symptoms of asthenopia or eye-strain, frontal and occipital headaches, pain in the eyes, congestion and liurning sensations in the lids and eyeballs. Presbyopia ( derived from the Greek words TTpiafivt, an aged person, and wj/,ops, the eye), or old sight, is a change which naturally occurs in every eye between the 40th and 4.5th 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 e3'e 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 continues 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 Str.!I.smis ). the vision of the most distorted eye is almost always imper- fect : and it is well known that impressions on the two retintT are similar in kind but dissimi- lar in form. Tlie 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 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 retina". "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. Color-blindness is noticed inider its own name. Night-hlindness, 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 entirelv and almost instantaneously blind as soon as twilight commences. It is seldom encountered in this country except among sailors just re-