Our Eyes, and How to Take Care of Them: Ii
HYPEROPIA, OR OVER-SIGHT.
THE condition termed hyperopia is the opposite of short-sightedness, and consists in abnormal flatness of the eyeball from before backwards, with, in some cases, a positive smallness of the globe in all its dimensions. This malformation may be seen at the outer extremity of the orbit by separating the lids while the eye is turned towards the nose, − the eyeball having somewhat the shape of a turnip.
The antero-posterior axis of such eyes being too short, their refractive power is not sufficient to bring even parallel rays to a focus upon the retina, but is adapted for convergent rays only. It is therefore evident that convex glasses, which by rendering parallel rays convergent compensate for this deficient refractive power, must be the only effectual means of relief. This condition is not to be confounded with old sight, where the refractive power is perfect and distant vision good.
Where the hyperopia is of moderate degree, the exercise of the accommodative function in aid of the refraction is sufficient to give clear vision of distant objects, and even for reading during childhood and youth ; but glasses will be required for reading at an earlier age than they are needed by normal eyes, and in the mean time such eyes should be carefully used, and never employed for a long time upon small objects, especially in a feeble or artificial light. When the hyperopia is of greater amount, almost any continuous use of the eyes is painful, and convex glasses are indispensable, even sometimes from the age of six or seven years.
This imperfection of the eye is often undetected for a considerable time, during which a child thus affected experiences great annoyance and gets very little sympathy. The child is able, as a general rule, to see at a distance, and perhaps, by using nearly all his accommodative power, obtains even clear images of large objects. But if on a dark day, or when confused by the presence of strangers, he is asked to read a fine or blurred print, his sight soon becomes confused and he ceases to distinguish the words. He stammers and hesitates. His parent or teacher, knowing that he has read the same lesson well enough at other times, thinks him stupid or wilful, and reproaches or punishments are perhaps administered, which, by disturbing the nervous system, lessen yet more his power of accommodation and increase his visual inability. The child is himself at a loss to understand why he cannot then see what he knows he has seen.
It is perhaps accidentally noticed that such a child sees well with his grandmamma’s spectacles; but this discovery is often only received with an outcry of astonishment, the glasses are snatched off, and he is denied this assistance, the only means which can be of real use to him.
Instances are now and then met with where glasses are even more necessary at six or eight years of age than they are to the majority of healthy eyes at sixty years. But this extreme hyperopia is less frequent than moderate degrees of this imperfection, occurring in youths and young adults. These have been able to use their eyes during childhood with little difficulty by making constant use of their accommodative power; but as this begins to lessen from the age of ten years, they at length observe symptoms of what is termed asthenopia, or weak sight, and feel pain in the eyes or above the brows after long-continued use.
As is well known, convex glasses have the property of bringing parallel rays to a focus at a distance from the glass corresponding to its degree of curvature. Thus they assist hyperopic eyes by rendering rays so far convergent before they enter, that even with its deficient refractive power the eye is able to form a distinct image on the retina without any further exercise of its accommodative faculty than is required in a normal eye. It is evident that such glasses are the only rational and efficient means of relief for hyperopic persons. They may be worn both for distant and near vision, enabling the eyes to refract parallel rays sufficiently while keeping the accommodative power in reserve, as in the normal eye, for the concentration of the divergent rays which proceed from small near objects.
The glasses should be such as afford the clearest and most comfortable vision ; but it is often necessary to wear at first a weaker number than is subsequently needed, or than will entirely neutralize the hyperopia ; because the eyes have been so long accustomed to exert their accommodative power for all purposes, that it is difficult at once to relax this effort when looking at distant things, although the glasses supersede the necessity for it.
SQUINTING AS A CONSEQUENCE OF HYPEROPIA.
It was first shown by Professor Donders of Utrecht that nearly all the cases of squinting towards the nose, “ cross eyes ” as they are sometimes termed, are accompanied by, and result from, hyperopia. This convergent strabismus, or squint, is caused by excessive use of the muscles which turn the eyes inwards, in the endeavor, by increase of the accommodative effort, to obtain distinct vision.
It is very important that this defect should be remedied in early childhood, as, if allowed to continue, vision frequently becomes so much impaired in the eye which is most deviated that it is not regained after an operation performed at a later period. When occurring later in life, where glasses can be intelligently used to correct the hyperopia, the strabismus may be relieved by this means. Glasses are often required, after an operation, to increase and maintain its good results. But in matters so important, competent advice should be obtained, without heeding the counsel of friends who advise delay.
ASTIGMATISM.
Astigmatism usually depends on a difference of curvature in two meridians of the cornea, so that rays passing through one meridian are brought to a focus sooner than those passing through a plane at right angles to the first.
Persons having this defect of refraction sometimes observe that they see certain lines more clearly than others ; vertical lines, for instance, will appear well defined, while those which are horizontal are indistinct, or vice versa. In many cases, however, their attention has not been directed to these phenomena, but they are only conscious that they see with difficulty and pain if the eyes are much used. In some instances they have tried convex or concave glasses with little benefit. Frequently so much irritation and congestion of the eyes has been induced, that these symptoms are at first supposed to constitute the disease, the primary affection being overlooked.
Astigmatism may be present in an eye otherwise normal, or may coexist with hyperopia or myopia. Its degree or its meridians of greatest variation may be different in the two eyes, which should therefore be separately tested. It can only be relieved by glasses ground upon cylindrical instead of spherical surfaces, so as to refract rays passing through one meridian while those at right angles to it are unaffected. The glasses must be accurately fitted and their frames carefully adapted to the eyes, as any deviation of the axis of the cylindrical glass from its proper direction with regard to the faulty meridian of the cornea lessens or even nullifies its corrective power.
Immense relief is often found in wearing these glasses, and unbounded gratification is sometimes expressed by those who, after many fruitless endeavors, see by their aid for the first time with real distinctness.
Convex-cylindrical and concave-cylindrical glasses are now kept for sale by many opticians, and where other glasses fail to give relief the eyes should be tested with parallel vertical and horizontal lines. The selection of cylindrical glasses is, however, often a question involving nice adaptation to complicated conditions of refraction, and in the mixed and compound forms of astigmatism it is sometimes necessary to have glasses ground of different curvatures upon their two surfaces, to suit each case.
ACCOMMODATION OF THE EYE.
Thus far we have considered the eye as an organ possessing refractive powers only. But it has other capabilities as an optical instrument in its admirable power of self-regulation, by which it is able to adapt itself spontaneously for seeing distant or near objects. This is termed the faculty of accommodation.
In looking at distant objects the normal eye is in a state of rest, and the parallel rays which enter it from such objects are brought to a focus so as to form a distinct image upon the retina, by the refractive power alone, without calling into play the accommodative function.
Rays proceeding from near objects are no longer parallel, but diverge from each other and require an increased focal power for their concentration to form a clear retinal image. This increased power is supplied by accommodation.
Accommodation of the eye for vision of near objects is obtained by two distinct but intimately associated muscular efforts. The eyeballs are turned towards each other, by the internal recti muscles, so that the diverging rays may enter each eye in the direction of its axis and not obliquely; and at the same time the ciliary muscle, within the eye, acts upon the crystalline lens and increases its convexity, augmenting in so doing its refractive power, and thus giving to the divergent rays the same direction towards the retina as if they had entered the eye as parallel rays.
The accommodative power may be weakened or lost from various causes.
OLD SIGHT.
To many persons the discovery that they do not see as well as they once did is the first intimation of receding youth. Infirmities, wrinkles, they may have none ; but they suddenly become aware that they sometimes cannot thread a needle or read fine print without fatiguing effort.
At first such a person finds that he can still read any print by placing the book farther from his eyes, thus rendering the rays less divergent, or by holding it near a light, so as to obtain a better illumination of the page, and thus increase the number of luminous rays which enter the eye. At length, however, he finds that neither holding the book at arm’s length nor going nearer the light will give him his accustomed vision, especially in the evening or on a cloudy afternoon. Fine print appears blurred ; and if read at all, it is slowly and with difficulty. If he writes in the evening, he perceives the next day that he has written larger than his ordinary hand.
Meanwhile, perception of distant objects is as clear as ever, and many an individual, puzzled to account for the loss of his former minute vision, struggles in vain to continue some of his favorite pursuits and to read his evening newspaper comfortably, until perhaps he tries on a convex glass, and his rejoicing eyes at once regain all their faculties.
One of the parts principally concerned in accommodation, the crystalline lens, gradually increases in hardness ; and, in most eyes of previously normal accommodation, this hardness attains such a degree at about fortyfive years of age that the ciliary muscle can no longer effect the change of form in the lens which is requisite for the concentration of divergent rays ; or, if this can be done for a short time, the eye soon becomes conscious of a fatiguing effort and is forced to abandon it. This state of things is presbyopia, or old-sight.
It is evident from this explanation of the changes in the lens that a suitable convex glass, which lessens the divergence of the rays before they enter the eye, and thus calls for less effort of accommodation, must be the sole means of relief.
The advice often given to those who begin to experience symptoms of presbyopia, to put off the use of glasses as long as possible, is injudicious ; and the assertion that persons who decline to use glasses for a certain time will be able always thereafter to dispense with them is wholly erroneous, so far as regards normal eyes. It is generally made by those who are themselves short-sighted, and for that reason are able to see small objects without glasses at and after the age when others require their assistance.
But the use of convex glasses may be postponed for a while, without injury to the eyes, in deference to the reasonable wish of a lady to appear young as long as possible, or from any motives of convenience or preference ; provided the eyes are used but sparingly for small objects, especially when the light is dim.
Such glasses should be chosen as render objects clear without much enlarging them. At first they may be needed only in the evening or on a cloudy day. But as each year lessens the accommodative ability of the eye, it follows that glasses will be more and more constantly required. After a time their focus must be increased because of the renewal of the original symptoms, the lens having undergone yet further hardening and become less capable of accommodative change.
LOSS OF ACCOMMODATION AFTER ILLNESS.
After certain diseases, among which diphtheria, measles, and scarlatina may be especially mentioned, the accommodative power is often partially or almost wholly lost. In diphtheria this loss of power in the nerves supplying the ciliary muscle is often associated with partial and temporary paralysis of some other nerves, particularly those of the throat. For the time being the person is more or less unable to see small objects, to continue reading, etc. But, although the recovery of these delicate nervous functions is often gradual, they may be restored by appropriate treatment as the system gains strength. Every care should be taken to avoid prostration of the nervous system, and the eyes must be sparingly used until they regain the ability to work without fatiguing effort.
INSUFFICIENCY OF THE INTERNAL RECTI MUSCLES.
Besides the change of form in the lens, we have also, in accommodation for near objects, a convergence or turning of the eyes towards each other. This is effected by the action of the internal recti muscles, which turn the eyeballs inwards towards the nose. Rays from an object thus enter the two eyes in such a direction as to fall upon corresponding portions of their retinæ and form there images which harmonize with each other. If these converging muscles act too feebly, although the accommodation may be good as regards either eye when used alone, the other eye being covered, there will be a want of harmony in the images formed in the two eyes when used together, so that a confused impression will be conveyed to the brain. The efforts made by the enfeebled muscles to maintain their accustomed action cause a feeling as of strain at the inner side of the eyeball near the insertion of the muscles, the discomfort often extending to the forehead above the eyes.
Continuous use of the eyes, when insufficiency is present, brings on a sensation similar to that experienced when any other muscle is kept too long upon the stretch, as, for instance, when the arm holds up a heavy weight. Frequent intervals of rest should therefore be allowed such eyes.
A disposition to turn outwards is often observed in very near-sighted eyes, but is then generally associated with serious internal changes which claim the first attention.
EYE-GLASSES.
The use of glasses becomes a necessity or convenience, at some time in their lives, to a large proportion of the people of civilized communities. If short-sighted, they require glasses in youth, as well as in age, for distant vision. If possessing normal eyes, they need assistance, with advancing years, for seeing near objects. The comfort and safety of the eyes often depends on a proper selection of these auxiliaries.
The glasses in most common use have their two surfaces ground of the same curve, convex or concave upon each side. Periscopic glasses, in which the two surfaces are of different curvature, are sometimes worn, as giving rather more range of vision without turning the head ; but their optical qualities are in some other respects less perfect than those of the usual form.
To avoid the trouble of changing from one pair of spectacles to another, two different foci are sometimes combined in the same glass, − the lower portion being ground to the focus adapted for reading, and the upper part to that suited for distant vision. The same result is also obtained by setting two halves of lenses of the two different foci in the same frame.
Cylindrical and prismatic glasses are intended only for certain special conditions of refraction, more common than was formerly supposed, and which when existing are but slightly relieved by the ordinary forms of convex and concave glasses.
Near-sighted persons should select the lowest number of glasses which make vision clear at a distance without rendering objects smaller and unnaturally brilliant. If any difficulty is met with in finding such glasses, the eye should be examined by some competent authority, to determine if any unusual combination of lenses is required, or ascertain the presence of disease if the difficulty in suiting the eyes arises from this source.
When convex glasses are required by hyperopic persons for distant vision, they should be of such power as to render everything distinct. Those used for reading should make print clear at the usual distance, without magnifying much. Persons who need glasses of different foci for near and distant sight should not wear their reading-glasses when looking at a distance, for if they do so they will find them less serviceable in reading or sewing, and soon require a higher power.
The frames of glasses may be round, oval, or of any form and of various material, according to fashion or preference. As a rule, light materials and large glasses are best. If the nose is so shaped that eye-glasses can be kept in place without having too strong a spring, they may be worn if preferred rather than spectacles. But if the spring is too strong it often causes pain by its pressure on the nerves around the eyes. Cataract glasses should generally be mounted as spectacles, for they are worn almost continuously, and their weight makes it difficult to keep them upon the nose if framed as eye-glasses.
Whatever style of mounting is preferred, the frames should be adapted to the form of the bridge of the nose and to the distance between the eyes : so that, as a rule, the centres of the glasses shall be in front of the pupils Men generally require frames with longer bridges than women, because of the greater space between their eyes. Silver frames are perhaps the most economical for the poor, since if bent or broken they can be repaired.
Pebbles, as they are called, are often praised as having qualities excelling those of ordinary glasses. But this is not the case to any considerable extent, even when they are made with care and at a much higher cost than other glasses.
Tinted glasses, or those having wiregauze around their border, are often worn as protectors against light and dust. When used to defend sensitive eyes from light, a mild blue is generally more grateful than a neutral tint; but a neutral or French gray may be worn if more acceptable to the eye. Green glasses are to be avoided in most cases, as they do not absorb or neutralize the irritating rays in the spectrum of light as it passes through them. Inflamed eyes often find most relief from glasses surrounded by wire gauze, which exclude wind and dust as well as light ; but these should not be worn so closely as to keep the eyes heated.
Very thick plate glasses, set in spectacle frames, are sometimes used as a safeguard by stone cutters, machinists, etc., whose eyes when unprotected are often fatally injured by the penetration of particles of metal driven with great force into the interior of the eyeball.
THE OPHTHALMOSCOPE.
In looking into an eye, our unaided vision usually penetrates but a little way beyond the pupil; but by means of the ophthalmoscope, invented a few years since by Professor Helmholtz of Heidelberg, we are able to explore the depths of this organ and detect the smallest variations from a healthy condition.
The room being darkened, the rays from a light placed near the head of the person to be observed are reflected into his eye as if they came from the eye of the observer, and the latter, by looking through the central aperture in the instrument, can examine the illuminated interior of the eyeball, perceiving every detail of healthy structure or morbid change as accurately and clearly as we can see any part of the exterior of the body. All this is done without injury or discomfort to the eye looked at; and the diseases of its internal parts, heretofore hidden mysteries, can be studied and understood perhaps more perfectly than those of any other organ of the body.
The benefits derived from the ophthalmoscope are not limited to the better knowledge and earlier discovery of morbid affections of the eye itself. This means of observation also enables the physician to detect diseases of distant organs by the manifestations of their presence exhibited in secondary changes in the deep-seated tissues of the eye, and to avert impending mischief to the brain by timely discovery of its indications in the optic nerve and retina, in season to counteract the subtle influences threatening life or reason.
DEFECTS OF SIGHT FROM MALFORMATION OR FROM CERTAIN STRUCTURAL CHANGES.
In a considerable number of cases where the eyes of children appear healthy upon an ordinary inspection, there is more or less want of acuteness in the sight, which is little if at all relieved by any glasses. Examination with the ophthalmoscope shows in some cases an imperfect development of the retina and choroid ; in others there is commencing cataract; in others the cornea is slightly hazy from previous ulceration, or it is conical in its form.
A frequent symptom in these cases is a disposition to bring objects quite near the eyes ; but even then they are but dimly seen, and not as they would be were the child merely near-sighted, with great clearness.
Such children should not be required or allowed to apply their eyes closely to small objects, and they should be carefully examined by a skilful professional man to determine the precise condition of the eye and its proper management.
Complete or partial blindness may occur during pregnancy or while nursing ; but this should not cause too much alarm, as, if unaccompanied by permanent morbid changes, it may be expected to disappear, slowly, after termination of the causes which gave rise to it. But careful inquiry should be made as to the presence of any complicating circumstances, for if such exist the spontaneous recovery of sight cannot be so confidently hoped for.
Henry W. Williams, M. D.