IMAGE EVALUATION TEST TARGET (MT-3) 1.0 I.I 1.25 1J£ 1^ m 1.4 II— M 20 1.6 v: ^ % /w w/ % o ^l. //a / 7 y Photographic Sciences Corporation 23 WEST MAIN STREET WEBSTER, NY HS80 (716) 872-4503 i^ How we Heey or the Healthy Eye. • THE vision of the naturel healthy oye ranffes from infinity to a so-called near point, which gradually becomes more remote from the cornea as life advances, but which should not be farther away than eight inches. A pair of such eyes possess clear vision of all objects over the specified range of distance, that is, from eight inches to the horizon or the fixed stars. ; and they can continue the act of seeing at any point over the whole of this range for any reasonable time without interruption. There are three kinds of defective sight owing their origin to the faulty formation of the eve- ball— ^ ls«.— Myopia, or Near Sight, in ^Aich the length of the eye-ball from front to back is in excess. 2ncZ.— Hypermetropia, or Far Sight, in which the length of the eye-ball from front to back is less than the standard for a normal eye ; and 3rc2.-- Astigmatism, or Peculiar Sight, which is due to a want of symmetry of the cornea, which instead of being spherical in its outline, is some' what differently curved in two opposite direc- tions. s Myopia, or Near Sisbt* m a THE vision of the myopic eye is conditioned by space, in not being able to see clearly any objects which lie beyond a definite distance. Hence we see short-sighted people hold objects to be examined close to the eyes. When a person is able to read small print easily and fluently in the hand, and has not the normal standard for distant vision, we may at once infer that he is a subject for Myopia. When Myopia is once established, and unless its influence is speedily counteracted, it provides for its own increase by the effect of convergence effort, and it may attain a considerable degree before even its verj existence is discovered by parents of only ordinary observation. The best thing that be done in the treatment of children so suffering is at once to stop their reading and all use of their eyes for near objects as much as possible, and make them use their eyes for distance. Encourage them in out door exercises, and after a time, when the improve- ment which is sure to follow this treatment ceases, and the patient's health is good, a proper pair of glasses r in be worn and the studies gra- dually resum<^c!. Great care will, however, have to be taken that children do not become too studious. Out-door exercise must fitiD. be taken, and the eyes well- used in looking at distant objects. ^ 8 The use of glasses for reading, "not that the patient may see better, but that he may see jar- ther off] " is absoluteljr necessary, and should always be enjoined during school life, or periods of close study, as the only means of preventing an increase of the Myopia or near-sightedness. It is the more necessary to render this clearly understood, because patients are naturally most disposed to prize and to use glasses for doing what can not be accomplished without them, that is, for seeing distant objects. They are often unwilling to use them for near V. ork alleging, and for a time with perfect truth, that they can see better and more comfortably without them. It is not uncommon, indeed, for short-sighted people to say, when asked if they have used glasses for reading, to assume a tone almost of self-righteousness in their denial of the imputation. They say, " Oh, no, I have never done that I " and are often gi^eatly exercised in their minds when the urgent necessity for a total change of their habits in this respect is explained to them. If we consider for a moment what must be the state of a person who has grown up to manhood or womanhood with an uncorrected Myopia of only a slight nature, we shall not fail to perceive the great importance of its correction. A person with healthy sight may produce the condition artificially by placing magnifying spec- tacles before the eyes. The Artificial Myopia, or near-sight, thus produced, weuld be deprived of W <\'> -^^ H, A .A' ^w. 9 half its inconvenience by the previously-acquired knowledge of the exact forms and characters of numerous objects, which would be only dimly seen • but the subject of it would find, foi* exani- ple, that instead of being able to tell the hour by an ordinary drawing-room clock from any part of the room, he would have to approach within three or four feet of the dial in order to" perceive dim indications of the hands. He would lose all the play of expression on the faces of persons with whom he was engaged in conversation. A curious commentary on the state of life passed by Myopia persons is daily seen by me, by the pleasureable expressio/ls of my patients, when these defects of vision have been properly remedied, by the use of proper Spectacles. ^1 ' I Hypermetroptlai ar Far Sin^lit* THE presence of Hypermetropia is generally made known, even in early life, by the fati- gue, pain, and speedy dimness of vision which attend the employment of the eyes over any matters requiring dose application. The patient will read fairly well for a time, but after a period, varying with the degree of 10 the defect and with the strength of the muscular system, there comes to be a conscousness of effort in the act of seeing. The eyes feel strained, and the letters become somewhat blurred, and are only restored to clearness by distinct and often strenuous offort. There is an instinctive desire to rest the eyes, close them firmly for a moment or two, and often to compress the closed eyes with the hand. After doing this a fresh start is made, only to terminate in another compulsory stoppa- ge after a shorter ]3eriod than the first. In some instances, a habit is acquired of unduly approxi- mating the book or other object of vision,* in so much that the condition has actually been mis- taken for the opposite one of MyopK, and the wrong lenses have been prescribed for its relief. If the subject is compelled to work continuous- ly, as by the demands of some occupation which cannot be laid aside, the eyes are apt to become red, blood-shot and irritable, and to suffer from obstinate forms of inflammation or irritation. The symptoms not unfrequentiy undergo sudden increase, as an effect of the general debility by some form of enfeebling illness. • It is essential to the complete relief of Hyper- metropia that spectacles should be worn always, but it will often happen that the patient cannot bear full correction of his defect at first. It is therefore often necessary to correct only a portion of the defect in thq first instance, and wait for the retura of some of the old symptoms, when the lenses first used may be strengthened ir- I J' 11 either to the extent of full correction, or by an- other step towards it ; the fall power sometimes not being used except by three gradations. Hypermetropia is sometimes the cause of weak sight m children. It is also the usUal cause of squint or cross eye, and this can often be cured in the beginning by the use of proper spectacles ; but if it is of long standing it can only be cured by a surgical ope- ration. Astigmatism, or Peculiar Sight. ASTIGMATISM has been already described as due to a want of symmetry of the eye-ball. A very small amount of Astigmatism is proba- bly the natural condition of the human eye, and the defect must exist in an unusual degree in order to be disturbing to vision. Tliis visional defect may be recognized by a conspicuous difference in the distinctness with which lines drawn in different directions are seen from the same point of view and with the same adjustment of the eye. This defect is of course a great hindrance to clear and easy vision in reading, as letters are made up of linen run- ning in different directions. w m V }} AiBitibepbpla, or; Weak.Slsiit* N the Asthenopia or weak-sighted ejre there may be limitations of vision within given distances, but the limitation chiefly complained of is one of time. The patient can see, if not perfectly, vet suffi- ciently well for many purposes, jor a given period only, and when this period is over, past, the sight becomes blurred, or the eyes become painfull, use of them in either case becoming difficult or impossible. If the symptoms of pain, confusion of vision, dread of light and of all use of the eyes, continue, and no measures are taken to ameliorate or remove them, there is danger of the eyes not only becoming nearly useless, but that they may finally get so sensitive and irritable that ordina- ry daylight cannot be borne without pain and discomfort. When weak sight has been allowed to progress for years, it sometimes becomes so severe that one finds a darkened room the only really comfortable place, dark-coloured glasses even not affording a sufficient protection from the light. Severe cases of weak sight are tedious and dif- ficult to cure, and particularly so if of long stan- dinpr, but recent cases are curable. ^ en prescribing spectacles for Asthenopia oi giuat severity or long standing, we shall often find that the use of the eyes is as painful or dif- ficult as before. For a tina^ this will bo so in t'% 1 ■AMiMHHJMEM f^ ^-'^.> -^v"- \ ■ 13 many cases ; and it is well to warn patients that their glasses will call upon the eyes to work un- der new conditions, which, although better than those which they supersede, may yet be irksome so long as they are new. They must not, therefore, expect immediate relief as a rule. We often have to deal with the effect of that most pernicious of all recommendations, the re- commendation to ** rest the eyes, " and conse- quently, with organs which have been brought- by disuse into a state of debility and excitability. Such conditions can only be relieved by careful strengthening of the weakened muscles. This is begt accomplished by selecting a book of good type, and to read regularly with the prescribed glasses three times a day. The patient must determine by trial the number of minutes he can read without discomfort. The first reading must not be until half an hour after breakfast, the second about noon, and the third finished before dusk. The periods of reading must be regularly increased from day to day. In cases where dis- comfort occurs in less than five minutes, the increase should not be more than half a minute per day until ten minutes are reached. In other cases, the patient may increase one minute until he can read thirty minutes three times a day without pain. If this can only be done with pain the patient must persist notwithstanding. Should the pain continue from one period to the next, it is evident that he has gone beyond ^ ■( 14 I the maximun of his ahility, and he should fall back to a period at which he can read without discomfort, and proceed as before, gradually lengthening the periods of exercise. The patient who jfinds that he can read for ten minutes without distress is very likely to go on for twenty minutes ; but to do this is to invite relapse. It is better to submit quietly to a period of self-restraint, than to lose time by fruitless endeavours to hasten a process whicn depends entirely upon the gradual strengthening of the muscles. In many obstinate cases of Asthenopia, we should find some derangement of the health. Under such circumstances the patient must not expect, by the employment of spectacles, to render unnecessary all the other resources of the healing art. The condition of eyes may often be greatly improved by treatment specially directed to them ; but complete restoration is hardly to be hoped for, except in conjunction with a corres- ponding degree of improvement in the general health, and to this end the patient must call to his aid the advice of the general physician. ^> yr.. { wm 15 fall [lOUt ally rten on ivite 3riod itless ►ends f the a, we 3alth. j^not 5S, to Wthe •eatly id to to be orres- neral all to ^^ Presbyopia, or 014 Siglit. THE vision of the old-sighted eye is also con- ditioned by space, in not being able to see clearly any objects which lie withm a definite distance. ^ The changes which result in Presbyopia, or old sight, may be said to commence from the age of twenty years, when the near point begins to recede, and at about the age of forty or forty-five, and sometimes perhaps rather earlier in life, we notice a desire for a good strong light in reading or threading a needle, and a disposition to push fine reading or sewing farther away from the eyes. Then follow more or less fatigue and con- fusion of sight in reading and sewing, in the evening especially, if tired, and it is found com- fortable to favour the eyes by frequent rest and change of occupation, very soon a like difficulty in reading fine print is noticed in the day time. Hence we see old persons hold objects to be examined far off from the eyes. This ^s due to a partial loss of the accommo- dative power of the eyes. Hence, in looking at a distance, the accommo- dative or adjusting power not being required, the eye sees as well, or nearly as well, as ever 5 but in near vision, the accommodative power being necessary, and not being available, vision is im- perfect. The loss of the accommodative power is not from disease of the eye at all, but is chiefly ^'i 16 w) owing to the natural increase of the hardness ot the lens and consequent loss of its elasticity. i It is surprising how frequently we find persons trying to postpone what they look upon as an evil dajr, and delay obtaining the help of proper- ly-suiting spectacles until they have fruitlessly tried to do without them. This is a great mistake ; nothing is gained by waiting, and much may be lost. We lose, in the first place, a great deal of amusement and Ins- truction from the necessity of giving up our eve- ning reading and neglecting fine work of all kinds. In the beginning, these interruptions in our ways of passing time are not so serious and frequent as to give much annoyance, but as old sight increases, they become of importance. In the second place, and of greater moment, is the risk we run of fa.tiguing and straining the eyes, and so causing weak sight. Nothing is gained by postponement ; the eyes are not made stronger, and are likely to be made weaker. At the i)resent time there need be no fear that the adoption of glasses will be interpreted as a confession of old age. I have frequently to pres* cribed for young people lenses as strong as those usually worn by people of advanced age. Most men and women do the serious and best works of their lives after the age of forty, and some even after the age of sixty. n '> 1 r' ^V* tM ri •> iWim^ammMmt^m ^»aim w K W .•iilTi 1 i^4 It 17 f 1 Cataract. > CATARACTj or opacity of the lens, is duo to changes m the structure and com position of the lens fibres. These changes seldom occur throughout the whole lens at once, but commen- ce first in some one part, such as the centre, whilst in certain forms of Partial Cataract the disease remains permanently confined to some well-cir- cumscribed parts. Cataract is manifested by a whiteness in the pupil, which is naturally of a clear black, and vision is more or less impaired, according to the greater or less opacity of the crystalline lens by which the rays of light are proportionately inter- cepted. Persons of all ages are subject to Cata- ract, but those of an advanced age are more so than at an earlier period of life. | • Opacity and dimness of sight usually commen- ce very slowly, and gradually increase in extent and degree till vision becomes materially obs- tructed and total blindness ultimately ensues. Usually, one eye becomes affected first, and after- wards the other, and it is very rare that both are affected at the same time. "* The impairment of sight produced by Cataract presents several peculiarities. It is greater for distant than for near objects. ! In the early stages of the disease the patient will turn his back to the light when trying to do his best in the way of seeing, and in time he comes to protect his eyes from the light as much 18 as r)0ssible, knitting the brows, bending the head, and even putting up the hand as a shade ; vision is better in a dim light. Persons suffering fi om Cataract, therefore, come into a room with a gait which is usually peculiar and often characteristic, and which widely differs from that of persons in whom, from any cause, there is impairment of light-percep- tion. The former will shade the eye and bend down the head ; the latter will have eyes wiaely open and head ^rect. Cataract can only be cured by operation. It may be months or even years before the disease arrives at such a state as not to be assis- ted by the use of spectacles, and even after an operation has been performed, the use of Cataract lenses becomes indispensable. < ^^Z Coloti!r«Bliiidiiess$« I CGLOtJR-BLTNDNESS may be congenital or acquired. In the latter case, it denotes some disease of the optic nerve. It also occurs as a symptom in certain functional disorders. 'Congenital Colour-blindness is not often iden- tified unless specially looked for. According to recent and extended researches in various coun- tries, a proportion, varying from about 3 to 6 cent, of the males, are colour-blind in various degrees. *^ w^ 19 There is a reason to believe that it is commo- ner in the lower classes than amongst those who are better educated. These facts show the import tance of carefully testing all men whose employ- ment renders good perception of colour indispen- sable, such as railway signalmen and sailors. * Colour-blindness is usually partial, that is, for only one colour, but is occasionally totaL The commonest form is that in which green is confu- sed with various shades of grey and red ; blind- ness for blue and yellow is very rare. Congenital Colour-blindness is very often here- ditary, but nothing further ift known of its cause. It is scarcely ever seen in women. It always af- fects both eyes. Niglit-Blliid]iess« NIGHT-BLINDNESS may be caused by tem- porary exhaustion of the retina from pro- longed exposure to diffused bright light. ♦ Note.— Since writing the above, the Bengal Govern- ment have taken up the subject of Colour-Blindness, and havepubhshed rules for the examination of Masters, Mates, and Engineers of Foreign and Home trade ships, who appW at Calcutta for certificates of competency. It would be well if these rules were also applied to the ejcamiriation of Rail- way Engine-Drivere and Guards. f 1 t • « • & ' 20 The circumstances under which it comes on are such as imply not only great exposure to light, but lowered nutrition of the system. Pos- sibly iti actual cause may be some constant defebt in diet, such as want of fresh meat or vege- tables. Sleeping with the face exposed to bright moonlight is believed to help in its production. Thus it is commonest in sailors after a long voy- age, with hardships in the tropics, and in soldiers after long marching under a bright sun. It may be caused by inhalmg noxious vapours. Snow-blindness appears to be essentially the same disease with the addition of considerable congestion, pain, and intolerance of light. These, changes are thought to depend on the effect of the rarified atmosphere in which, in mountain- eering cases, the exposure occurs. Night-blind- ness frequently attends scurvy in tropical clima- tes. The sight might become similarly injured by prolonged work on minute objects in a bright light. There are, however, Certain other cases of night- blindness the cause of which are obscure, and on whose course no treatment has any influence. The disease generally progresses slowly to almost complete blindness. The sight is often better one day than another, but fails altogether by candle or gas-light, till at length the desease affects the eye by day light, and the sight becomes perma- nently impaired. '■ >^'- *^« f-^'-^V i 21 Maseee Yolitantes, or Floating: iSpeaks* jrs r « T^**. MWJE VOLITANTES appear to the patien under various forms, floating about in the field of vision. They are often very annoying, but do not interfere with the perfection of sight. They appear at times to consist of slender rings, which seem to ascend from the lower part of the field of vision and then to fall down again. In other cases, they take the form of pearly strings, which twist and twine about in all directions, or they may be seen as fine bands hovering about in the visual field. They are most distinctly seen when the patient looks at,some clear bright object, as, for instance, at the sky or a white wall ; in a dim light they are probably not visible. After over working the eye, they are very apparent, and also if the di- gestive organs are out of order. Muscse are by no means a symptom of any great consequence, and are often observed by persons whose eyes are perfectly healthy. Short-sighted persons are very apt to sufier from Muscse in consequence of the increased circles of difi*usion cast by the minute bodies on the retina. It will often be found that cases of Muscse depend on gastric derangement, or at any rate occur when the stomach or liver is out of order, and a little attention bestowed on those organs will do much towards removing the Muscat. In r 22 other cases, rest and a tonic plan of treatment are of service. Lastly, it should be remembered that Muscse sometimes remain stationary for years, and then disappear of themselves. Tinted lenses often prove useful to patients suffering from Muscge. Having already noticed the more serious opti- cal imperfections of the eye, and enabled our readers to discover what, if anything, is faulty in their vision, it remains now to be said that all eyes, including such as are considered perfect, are, optically considered, far from perfect. 9; Spectacles* *i!SV* li^ OF the many advantages Optical Science has conferred on us, we are certainly most indebted to Spectacles. The pleasure and advan- tage derived from their use are appreciated by all classes of society. By their help the sight of the age is greatly assisted, and that of the weak strengthened. The determination of the proper spectacles for any defect of the eyes is undoubtedly a matter of the greates-t importance. This importance is inereas*ed, on the one hand, by the number of patients requiring such assis- tance ; a number that, throughout the civilized 4,^r^i ^'4^W*fi ^: */ 23 world, may be reckoned by millions ; on the other, by the actual injury inflicted by unsuitable glasses ; and lastly, by the customary sale of most spectacles by persons who have no adequate knowledge of the conditions of their utility. Since the visional process is not an absolutely simple action, but is compounded of several acts which are originally independent, and which, through the unity of the sensorium, come secon- darily to influence one another, it follows that, in defects of vision, the determination of the ne- cessary spectacles can never be a perfectly sim- ple procedure. It requires a due consideration of all the ocular functions ; and the neglect of any one of them may produce a strain which will act injuriously and will impair the eyes ; while the proper spec- tacles adapted to correct the defect would be pre- servative and beneficial. The injury produced by wearingr improper spectacles is not always immediatoly percei- ved ; but -when, after long use of them, the sight conies to be sensibly impaired, it is con- sidered as the natural consequence of age — although the greater part of the evil be occa- sioned by using lenses carelessly adapted to the eye. With the healthy eye, on looking at a distance, the rays of light that enter the eye are focussed on the retina without effort ; but in regarding near objects, as in reading or writing, there is an effort of the eye to focus the rays of light, other 24 wise they would reach the back of the eye with» out being brought to a point at all. The reason of this is, that rays of light from small objects, as in letters for example, approach the eye not in parallel lines, but in jines that di- verge as they pass into the eye. To converge such lines to a point on the retina requires more focus- sing power than if they entered the eye ia paral- lel iignes. This focussing power is furnished by a muscle, and when it is brought into use the lens is made more convex or full, and thus a greater focal power is given to the eye. If we look at distant objects, the muscle relaxes, because the use of the accommodative power is unnecessary ; but always in reading, writing, sewing, and every kind of occupation requiring fixation of sight on near objects, this accommodative apparatus is brought into use. Though involuntary, the accommodation or ad- justment of the eyes for near objects is an effort. Hence the relief that comes from looking up and off from near work, so especially grateful to fati- gued and sensitive eyes. Another factor in the act of adjustment of tho eyes for near objects is convergence. By this is meant the turning inward of the eyes so that both may be directed to the same object. The conver- gence of the eyes is also an effort, and is brought about by a muscle on the outside of each eye-ball attached to the white of the eye, on the side ?i // .1 "J ;ir- 25 )i^ » i u -r^J^ towards the nose. In health, both these muscles work together in perfect harmony. If the cornea, or the crystalline humour of the eye, or both, be too flat, the rays that pass through the pupil will not converge upon the retina, but behmd it, and thus the sight of the object must, of course, be imperfect and indistinct. To remedy this evil, a convex lens of a proper focal distance must be placed before the eye, to refract the rays towards the centre ray, and so cause them to meet sooner than they did before, and thus form the proper image on the retina. If, on the other hand, the cornea or crystalline, humour be too convex or protuberant, the rays passing through the pupil will meet in the vitri- ous portion of the eye before they reach the retina, and an equal indistinctness of vision will be produced. This is to be remedied by placing a concave lens before the eye, which, by diverging the rays, will prevent their meeting until they arrive back at the retina. The different distances at which different persons, or the same persons at different periods of life, can see distinctly, or the length of their sight, must depend on the elasticity of the lens and its accommodative power. Lenses are usually constructed from various material, such as pebbles or glass. It must not be supposed that all those made from pebbles* are equally good ; on the contrary, their only slight advantage of hardness is usually more than neu- tralized by their not having been cut from the f*