Normal And Abnormal Refraction | Operating Microscopes

Normal And Abnormal Refraction
EMMETROPIA, AMETROPIA, HYPEROPIA, MYOPIA, ASTIGMATISM, PRESBYOPIA.
By EDWARD JACKSON, A. M., M. D.,
OF PHILADELPHIA.

DISTINCT vision, by which the existence and position of different objects are recognized, as contrasted with mere perception of light, depends on the assorting or focussing of the light that falls on the retina. Imperfect focussing of this light causes imperfect vision. To avoid this the accommodation may be strongly exerted, contraction of the pupil secured by a bright light, or the space between the lids narrowed. But efforts of this Kind to improve vision, if frequently or constantly resorted to, are liable to exhaust the endurance of the nervous system or disturb the nutrition of the eyeball and its appendages.

Errors of refraction lead either to imperfect vision or to eye strain. They may lead to both, but generally, in so far as the vision is imperfect, there has not been eve strain, and in so far as there has been eye strain the imperfection of visioii has been partly overcome. If the defect be great, the part of it overcome may cause eye strain, while beyond this some remains to render the vision still imperfect.

Normal and Abnormal Refraction: Emmetropia and Ametropia. Refraction may be regarded as normal when it gives, under the requirements to which the eyes are subjected, distinct vision without injurious effort. It is, for practical purposes, abnormal when distinct vision is prevented by imperfect focussing of light on the retina or is obtained only by excessive effort by eye strain.

In emmetropia light from distant objects (parallel rays) is accurately focussed on the retina without accommodative effort. Any departure from this optical condition of the eye constitutes ametropia. Emmetropia is the ideal state of refraction. In it not only are rays from distant objects perfectly focussed without effort, but rays from near objects are focussed upon the retina with the minimum exertion of accommodation; not only are distant objects seen distinctly, but the full extent of the accommodation is available for the distinct seeing of near objects. It is true that the myopic eye may be able to see objects still nearer to the eye, but the gain of a very few inches or a fraction of an inch of distinct near vision is more than balanced by the loss of distinct vision for everything beyond a certain very limited distance; and the gain in lessened accommodation required for near objects is more than balanced by loss through the increased need for convergence. Careful examination of large numbers of eyes, particularly among schoolchildren, shows that the actual experience of life fully supports the theoretical advantage of emmetropia.

The same observations show that exact emmetropia is comparatively rare.

The writer among 4000 eyes found the following proportions of ametropia of different kinds, and of emmetropia:

It may be asked, If emmetropia is the ideal state of refraction, why is it so rarely found? The answer is that the shape of the eve results from processes of growth resisting intraocular pressure, and cannot be a rigid, definite, mathematical form. The ideal form for any part of the body, and the ideal of proportion between different parts, are never found in life. The deviations of the eye are insignificant compared with the deviations of the other organs, but sufficient to cause errors of refraction of practical importance in a very large proportion of eyes. The study above referred to indicates that the largest number of eyes have low hyperopia; 62 per cent., show hyperopia of 1.5 1D. or less, including hyperopic astigmatism. The eye has been evolved to meet the requirements of life among the lower animals and savages, for whom myopia, even of low degree, would be a very dangerous defect. Deviation of the eye in that direction caused the extinction of individuals and families. The requirements of modern civilized life, however, rapidly extend in the direction of near eye work, so that hyperopia becomes a serious defect. Even the emmetropic eve may be unable to meet the requirements of close work; and as the power of accommodation diminishes with age, it loses the power of distinct vision at short distances, requiring optical assistance in all cases (presbyopia).

Eye strain. The symptoms arising from excessive efforts to prevent indistinctness of vision may be considered under this head. They are largely the same in different forms of ametropia, and may also arise from excessive eye work, insufficient light, or other unfavorable conditions, even though the eyes be emmetropic. Eye strain is caused by excessive use of the accommodation, from too long hours of close work, or by looking at small objects brought too near the eye ; or because of deficient vision, or in making good the defect of hyperopic eyes or by ordinary near work after the accommodation has diminished with age (presbyopia). It may also arise from excessive efforts to keep the eyes properly directed, as of convergence where objects have to be brought too close on account of uncorrected myopia, or from the effort of accurately co ordinating muscular movements, as those of accommodation and convergence. It may come by exhaustion of the visual centers in the effort to appreciate blurred and imperfect retinal images, or it may be due to the use of' eyes otherwise normal at a time when the general nutrition is impaired by wasting disease or exhaustion by effort in other directions.

Eye strain may be manifested by failure of near vision after use of the eyes (relaxed, accommodation) or by temporary blurring of distant vision (spasm of accommodation); by changes in the retina swelling and opacity, with dilatation of the retinal vessels and exaggerated reflexes ; by changes in the optic nerve redness, haziness, or opacity or swelling of the nerve head ; by changes in the choroid, including increased redness, or alteration of color by edema or atrophy ; and, secondary to the changes in the choroid, by opaci¬ties in the vitreous and the crystalline lens, and softening of the sclera with local bulging (posterior staphyloma).

The progressive changes in refraction, to be discussed under Myopia, are also symptoms of eye strain. Acute or chronic conjunctivitis may arise from the same cause. This may amount to a slight exaggeration of the irritation felt when the eves are tired, or it may develop into a chronic catarrhal con practically incurable even by removal of the original cause. When the conjunctivitis is severe, corneal disease may be associated with it, and if chronic it is apt to be attended with changes in the lids, marginal blepharitis, styes, etc. Eczema of the lids and neighboring parts has also been ascribed to eye strain and relieved by wearing glasses.

The symptoms manifested outside of the eye and its appendages are

Headache. This is often spoken of as reflex, but is better regarded as due to nerve exhaustion. It is commonly frontal, in some cases extending to the occiput or throughout the whole head. Sometimes it is strictly limited to one side hemicrania. It may be directly associated with the use of the eyes, or be apparently constant, or may occur at certain times, apparently not determined by any particular eye work, and yet in the latter case may be as completely cured by the careful relief of eye strain as when more evidently connected with eye work. The headache of eye strain is not sui generis. It has the same characters as headache arising from entirely different causes. In many instances it is partly due to eye strain and partly to the other causes. If the other causes can be discovered and removed, it may be cured without the wearing of glasses or any reduction in eve work. 'More frequently it is cured by the correction of ametropia or faulty habits of using the eyes. Sometimes, when removal of one factor has given temporary relief, but the headache returns, the discovery and removal of the other factor may be necessary to make the relief permanent.

Neuralgic pains in other portions of the body or attacks of migraine may arise from eye strain. Anorexia, nausea, vomiting, palpitation of the heart, and similar disturbances may be due to eve strain. Nervousness, which the patient speaks of as an intolerable desire to cry out or do some violent act, inability to keep quiet after prolonged eye work, peevishness and irritability of temper, are among its manifestations. For the rarer forms of disturbance the therapeutic test by relief from the strain will be necessary to establish the diagnosis. Eye strain may cause certain motor disturbances, as twitching of the lids, tonic blepharospasm, and in rare cases choreiform movements or epileptiform seizures, or it may be the most substantial cause of hysterical manifestations. With these, as with headache, eve strain is usually but one of two or more factors.

Hyperopia. hyperopia, Hypermetropia, or Farsightedness, is the error of refraction which arises when the retina is situated in front of the principal focus of the dioptric surfaces.

Fig. 158 represents a hyperopic eye able to focus parallel rays at f behind the retina and I the lens which, turning the rays toward r, the virtual "far point" of the eye, causes them to be focussed on the retina and corrects the hyperopia.

The hyperopic eye is adjusted for convergent rays, and these are not encountered in nature. Without accommodation it sees indistinctly at all distances. By the exertion of accommodation it sees clearly, but only by the exertion of accommodation exceeding (by the amount of its hyperopia) that required of the emmetropic eye; and, having to use some accommodation constantly, it is deprived of the periods of rest which come to the emmetropic eye when fixed on distant objects. The greater amount of accommodation required of it causes the hyperopic eye to suffer earlier from the diminution of accommodation by age, and afterward the further loss of accommodation deprives it of distinct distant vision. We have from hyperopia liability to eye strain and indistinctness of vision, either of which may become an indication for correction of the defect by convex lenses.

Causes, Varieties, and Course. Hyperopia is due in the majority of cases to antero posterior shortening of the eyeball, axial hyperopia. This is caused not so often by a flattening of the globe as by a diminution in all its diameters. The other causes for it are flattening of the curvature of the cornea or crystalline lens, hyperopia of curvature, and removal of the crystalline lens (accidental or operative), or its congenital absence or dislocation aphakial hyperopia. It is possible to conceive also of hyperopia due to a low index of refraction of the crystalline lens index h peropia.

At birth nearly all eyes are hyperopic. It is possible that during the first years of life there is some general tendency for hyperopia to diminish, although this is not proven (see p. 178). On the other band, from early adult life to old age there is a general tendency for hyperopia to slowly increase, due to the gradual increase in size of the crystalline lens. As Priestley Smith has shown, the lens, like other structures of epithelial origin, continues to increase so long as it continues healthy, increasing one third in bulk between twenty five and sixty five years of age. Increase in the size of the lens, supposing it to keep the same shape, causes an equal increase in its focal distance and a corresponding increase of hyperopia. This is independent of the apparent increase due to the failure of accommodation, and continues after all power of accommodation has been lost.

The varieties of hyperopia recognized in practical work are based on the relations of hyperopia to the accommodation. They can be best illustrated by an example Suppose a case of hyperopia of 10 D. in which the total accommodation is only 8 D. When the full power of accommodation is exerted, there remains 2 D. of uncorrected hyperopia. This, a part of the hyperopia which no effort of the accommodation can correct, is called the absolute hyperopia. It often happens where there is considerable hyperopia and good accommodation that the accommodation is not fully relaxed at any time when the eyes are used, even for distant vision. If this part of the accommodation amounts to 2 D., then so much hyperopia is always corrected when the eyes are in use; it is called latent hyperopia. Besides the 2 D. of accommodation that cannot be relaxed, there remains 6 D. of accommodation which can be relaxed or exerted, and which, therefore, can be used to correct an equal amount of hyperopia, but which hyperopia can be left uncorrected at will. This part of the hyperopia which can be corrected or not by the accommodation is called facultative hyperopia. The absolute hyperopia and the facultative, added together, give the manifest hyperopia. The manifest hyperopia, with the latent hyperopia, together constitutes the total hyperopia.

The relations of these different varieties or parts of the hyperopia may be better understood by the following diagram

The subject may be still further illustrated by considering what happens when successive convex lenses are placed before an eye with a hyperopia of 10 D., and a total accommodation of 8 D. Without any lens the vision of such an eye is imperfect. A weak convex lens improves it, and the improvement continues as the strength of the lens is increased up to 2 D., which corrects the absolute hyperopia, and, with all the power of accommodation added to it, focuses parallel rays on the retina, giving good distant vision. As the convex lens is made stronger the vision is not further improved, but the best vision is obtained with less exertion of' accommodation. Thus, with a 4 D. lens it is necessary to exert only 6 D. of accommodation, and with a 7 D. lens only 3 D. of accommodation. This continues until all the manifest hyperopia is corrected by an 8 D. lens, the vision remaining clear with only 2 D. of accommodation. If, however, a still stronger lens is placed before the eye, the accommodation being able to relax no farther, the 2 D. of accommodation, plus the lens, gives an over correction, blurring distant vision. The portion of the accommodation which cannot be relaxed has been indicated in the above diagram as involuntary, and the part that can be relaxed or exerted at will is voluntary accommodation. By the use of a mydriatic the total accommodation, both voluntary and involuntary, is relaxed and the total hyperopia revealed.

Absolute hyperopia only occurs after the power of accommodation for objects at a distance from the eye has fallen below the amount of hyperopia. In early life it is only seen in hyperopia of the highest degree. After middle age, the power of accommodation being lost, it appears in all hyperopic eyes, and when the accommodation is entirely gone all hyperopia is absolute'. Latent hyperopia may not be present. Many persons with strong accommodation are able to relax it entirely when looking at distant objects through convex lenses. In other eyes it is constantly present, and in still others is present only part of the time. The inability to relax the accommodation is often spoken of as spasm of accommodation: Such spasm is most likely to occur when the eyes are irritated or fatigued. The facultative hyperopia, lying between the latent and the absolute, varies with these, decreasing as either of them increases, and on the whole tending to diminish with age along with the diminishing accommodation. In measuring refraction without a mydriatic the important point is to get as much of the hyperopia manifest as possible, and to do this the two eyes must be tested together, as recommended on page 209.

With reference to these different varieties it is essential always to bear in mind that their relations to each other are not fixed that there is no constant ratio between the manifest and the latent hyperopia at any particular age or for the individual. The proportions may vary from day to day, or even from minute to minute.

Symptoms. Since hyperopia may be corrected by accommodation, only the highest degrees give rise to symptoms in early childhood. The earliest symptom is convergent squint, arising with the effort of accommodation. This effort being great, the nervous impulse overflows, causing additional muscular contractions in muscles closely associated with that of accommodation, and especially excessive contraction of the internal recti muscles. Convergent squint of this kind is apt to begin before six years of age, and is most commonly associated with hyperopia of high, but not the highest, degree. Squint occurs where the hyperopia can be corrected by great exertion of the accommodation. When this is too difficult imperfect vision is accepted. Such imperfect vision may be noticed by a careful observer in early childhood, but commonly is not detected until the child begins to read. It is then found that to increase the size of the imperfect retinal images the book is held very close to the eyes, as in myopia. This practice in early childhood quite as frequently indicates high hyperopia.

Under the influence of school work lower grades of the defect begin to cause eye strain. This often shows itself in local congestion and inflammation of the conjunctiva and lids, conjunctivitis, styes, photophobia, and frequent winking on account of the conjunctival irritation. In later childhood begins the liability to headache; young children rarely complain of ocular headache. During school life even the lower grades of hyperopia are liable to cause eyestrain, but afterward, most eyes being used to better advantage and not being so severely taxed, the low degrees of defect are less likely to cause trouble, although headaches established during childhood may be continued, and periods of poor health may cause the development of eye strain.

As the time approaches when even emmetropic eyes suffer from presbyopia hyperopic eyes manifest the same symptoms earlier, in proportion to the degree of hyperopia. These symptoms are failure of the vision for near work, particularly in the latter part of the day or when tired or working by poor light : print has to be held farther from the eyes in order to be read, and conjunctival irritation and inflammation again occur, often in repeated acute attacks that are ascribed to 11 cold." Still later, as the power of accommodation falls so low that it can no longer correct the hyperopia, indistinctness of vision is developed.

Treatment. While any departure of the refraction of the eye from the emmetropic standard constitutes an error or an anomaly of refraction, it is only when tinder the conditions of work imposed upon the eye such an error or anomaly causes interference with vision or strain, that the refraction is to be considered abnormal. Treatment, therefore, is not indicated by the mere existence of hyperopia, but by the fact that the hyperopia has caused symptoms. or is likely to cause them, under conditions of work to which the eyes are about to be subjected. Manv hyperopic eyes, therefore, do Dot require the aid of correcting lenses, but when symptoms arise that may with probability be ascribed in part, to this error of refraction the correcting lenses :should be used.

How they are to be determined has been sufficiently indicated in the pre section (page 198). The general rule should be to give the full correction that is, the lens which makes the hyperopic eye similar to the emmetropic eye, enabling it to focus parallel rays on the retina without any exertion of accommodation, and to focus divergent rays with the least effort of accommodation. To this general rule certain objections are offered which must be carefully considered, and certain exceptions which must be recognized.

It is urged that if some eyes continue normal with uncorrected hyperopia, may continue normal with their hyperopia but partly corrected, and the rule should be to give the weakest glass that will allow the use of the eyes with comfort. But it is impossible, except by trial, to know that any incomplete correction will be sufficient in the particular case. The full correction promises the greatest degree or the greatest probability of relief after the eye has once become accustomed to it. The inconvenience of wearing glasses is the same with a, partial as with a full correction; therefore, if the patient must wear glasses at all, he ought to have from them the greatest benefit or the greatest certainty of benefit obtainable.

The second objection to giving the full correcting lens is that if a portion of the hyperopia is latent and it is often incorrectly assumed that this is so in nearly all cases the wearing of the full correction renders distant vision indistinct. If the latent part of the hyperopia were a fixed amount, this objection would have more practical weight. As it is, one cannot correct the manifest hyperopia of to day and be sure that the same lens will not overcorrect it to morrow. As long as latent hyperopia is allowed it will vary, and, at certain times, lead to blurring of distant vision unless a very wide margin is left for such variation. On the other hand, it is only necessary to wear constantly the full correcting lenses to render the total hyperopia manifest. Sometimes this is accomplished in a few minutes or a few days; in other cases it may take weeks, but if the glasses are a true correction and are steadily worn, it can always be brought about. This manifestation of total hyperopia is doubted by some ophthalmologists, partly because of the failure of patients to wear their glasses constantly or always to look through them when worn, but chiefly on account of the inaccuracy of supposing that the correcting lens for a limited distance, 15 or 20 feet, is a true correction for greater distances. Such a lens causes a very perceptible blurring at greater distances, very annoying to persons accustomed to distinct vision, and never to be overcome by any amount of persistence in wearing glasses. The person who under a mydriatic sees perfectly at 4 in. with a I D. convex lens never will see perfectly at a longer distance with that lens never will accept such a lens with satisfaction, not because of any " spasm of accommodation," but because it is not his correcting lens for parallel rays ; it is 0.25 D. too strong. (See also page 209.)

A third objection is that even if finally accepted' the full correction is harder to become accustomed to than a partial correction. This seems plausible, but experience indicates that it is not the case unless the partial correction is so incomplete as to give a very diminished assistance to the eye. It appears to be easier for an eye to learn to relax its accommodation entirely than to learn the new partial relaxation that a partial correction of the hyperopia renders necessary. Some surgeons claim it is best to arrive at full correction by successively increasing partial corrections. The full correction may at first cause the greater trouble, but this is at its maximum during the first two or three days, and after that it rapidly diminishes; it is certainly less in the aggregate than is entailed by a series of increasingly stronger glasses, which, moreover, cause greater expense.

The wearing of correcting lenses should be constant. This should be the rule in hyperopia, although not so essential as in myopia and astigmatism. Some indications as to the constancy with which glasses should be worn may be drawn from the symptoms. Headache, particularly if continuous or occurring without apparent connection with any particular use of the eyes, is very much more likely to be relieved when the lenses are worn con The same is true of chronic conjunctivitis and marginal blepbaritis and of inflammatory changes within the eye. Where there is headache or irritation directly following special use of the eyes, as in reading or sewing, which quickly passes away when such eye work is suspended, it is likely that relief will be afforded by using the correcting lenses only during the periods of such work.

It is often necessary to have the glasses worn continually at first, until the headache or chronic inflammation has been entirely cured and the eyes have learned the habit of relaxing accommodative effort when not working. After this it may be quite enough to use the glasses only when the accommodation will be especially taxed. Again, many children have trouble from hyperopia, requiring the use of correcting lenses during school life, who, when they leave school, can lay aside glasses and continue free from any symptoms of eye strain.

Exceptions to the prescribing of a full correction are made first, in young persons with good accommodation and high degrees of hyperopia and with comparatively trifling symptoms, occurring only when the eyes are especially taxed; second, in cases in which it is impossible to persuade the patient to submit to some present inconvenience in the hope of future benefit. Under these circumstances the only thing to do is to give a very incomplete correction at first and increase the strength of the lenses slightly at short intervals. Patients who take this attitude are generally in a position to bear the increased expense, and if it is explained that the first glasses are only for temporary use and are to be changed after short intervals, perhaps changed several times at such intervals, the partial correction may be resorted to. Deficiency of convergence or marked exophoria may also be considered as an indication for not completely correcting hyperopia.

In cases of convergent squint the constant wearing of the full correction is always to be tried. Apart from the wearing of correcting lenses, there is no treatment for hyperopia; but the symptoms that arise from it may be relieved by diminished use of the eyes, especially for near work, or by improvement of general health, and by the influences and remedies that bring it about.

Myopia. Myopia, Brachymetropia, Short sight, or Nearsightedness, is the error of refraction existing when the retina is situated back of the principal focus of the dioptric surfaces, and rays of light to be focussed upon it must enter the eye divergent from some comparatively near point.

Fig. 159 represents a myopic eye focussing parallel rays at f in the vitreous, and requiring the lens 1, which will cause them to diverge from r, the far point of the eye, in order that they shall be focussed on the retina.

Causes, Tendency, and Varieties. Myopia may occur as the result of a simple congenital tendency to the formation of too long an eyeball or too great curvature of its dioptric surfaces, but the great mass of myopic eyes be regarded as pathological. They exhibit distinct, and often very grave, lesions of the ocular tissues, to which the myopia may be secondary, but which it tends to aggravate.

The sclera is distended by a normal intraocular pressure of 25 or 30 mm., of mercury. This pressure preserves the form of the eyeball and the proper relation of the dioptric surfaces to each other and to the retina. The normal sclera resists this pressure without yielding. Acute disease, diathetic impair of general nutrition, a local inflammatory process starting with congestion of the choroid from eye strain, or a congenital nutritive deficiency lowers the resisting power of the tissue, leaving it unable to withstand the intraocular pressure. Distention then occurs, commonly near the posterior pole of the eye, causing elongation of the antero posterior axis of the eyeball.

When such distention is started, anything tending to increase intraocular tension or to diminish the resisting power of the sclera favors it. Different writers attach different degrees of importance to the various possible factors. Some believe a diathetic vice of nutrition essential to the production of myopia ; some regard external pressure, dependent largely upon the form of the orbits and the width between them, as most important ; some consider inflammatory changes within the eye as the chief cause of distention ; some. ascribe an important influence to accommodation, and others to excessive convergence. The writer recognizes the possible influence of all these factors, but believes excessive convergence is by far of the greatest practical importance.

It is universally recognized that prolonged near work favors the occurrence and increase of myopia. Such near work causes physiological hyperemia, often exaggerated by poor light or excessive minuteness of the objects looked gg at; faulty position of the head, leading to venous congestion of the eyes ; con indoors to a sedentary occupation, which impairs nutrition ; strain of accommodation ; and excessive convergence which, sooner or later, increasing myopia renders necessary.

When the eye has become myopic its elongation makes convergence abnor¬mally difficult, and the continued use of the eye for near work, because it cannot be used for distinct distant vision, increases the amount of convergence required of it. With weakened sclera, with increased pressure of the extra¬ocular muscles from increased convergence effort, and the pressure abnormally continuous, the tendency is for distention to increase. Myopia tends to be progressive. Probably all cases of myopia are at the start progressive. Some myopias cease to increase when the requirements of excessive near work made temporarily or during school life are relaxed. Others become stationary, from increasing rigidity and resisting power of the sclera which seen] to come normally with increasing age. Still other cases continue progressive until conver¬gence becomes too difficult to be sustained, when the more, defective eye is permitted to deviate, and divergent squint, either intermittent or constant, is established. After this, the muscular pressure of convergence ceasing, the myopia ceases to increase. In a few cases, however, the sclera is so thinned, its resist¬ing power so low, that distention continues until the intraocular changes pro¬duce blindness. To these the term malignant myopia is properly applied.

Myopia reaches in Lich higher degrees than hyperopia, and the high myopias constitute a larger proportion of the cases; myopia of over 20 D. is as common as hyperopia of 10 D.

In speaking of degrees of myopia we may designate a , low myopia that of less than 2.5 D., where some accommodation is habitually employed for near work. Moderate myopia is from 2.5 D. to 5 D., where near work can be (lone without accommodation. High myopia ranges from 5 to 10 D., in which work is best (lone at the far point of distinct vision. Very high myopia is above 10 D., and is usually accompanied by great alteration in the shape of the eyeball and changes in its coats.

Symptoms and Complications. Myopia renders indistinct all objects situated beyond the far point of the eye. Such indistinctness is not always noticed if it begins in early childhood or come , on very gradually, although generally it is detected by the patient or bi , care takers, especially by inability to see letters on the blackboard at the ordinary distance. The indistinctness is removed by bringing the object closer to the eye, by placing before the eye a solid disk or card with a pinhole opening, or by looking through a concave tens. The changes within the eyeball often prevent full vision even with correcting lenses. The small moving specks or shadows due to points of haze or unequal refraction in the vitreous humor, noted in all eyes under certain optical conditions, are especially noticeable in myopic eyes. Such eyes are also especially liable to vitreous opacities, which give rise to more extensive clouds and shadows upon the retina.

Objectively, the myopic eyeball may appear noticeably enlarged and elongated, especially when' turned strongly toward the nose; and the lids over it prominent or widely separated. The pupil is often rather large, and apparently sluggish, because less often contracted in the act of accommodation or convergence. The myope has a vacant or even stupid look, due to inability to see and respond to expression on the faces of others, and shows a distinct inclination toward reading and other pursuits which do not require clear distant vision.

ophthalmoscope commonly reveals intraocular changes closely associated with the causation and increase of the myopia. The most characteristic of these, are alterations in the choroid, as congestion and edema, causing reddening, blurring of details, and lighter patches (" woolly," 11 fluffy," or " patchy " choroid), and changes in which the pigment in parts of the fundus is reduced , while it may be increased in others ("disturbed" or ' moth eaten ' choroid or choroidal atro 11 slight," " partial," or 11 complete "). These changes are most frequent at the outer side of the optic disk, usually taking a crescentic form the myopic crescent represented in Fig. 160. An eye may present two or three well marked successive crescents, the one next the disk characterized by nearly or complete cho atrophy, the next showing partial atrophy, and the outer one mere conges¬

The continued succession of such areas, to complete atrophy, gives rise to a trian the temporal side of the disk, the socalled conus. The disturbed area may extend around the disk, forming a ring of atrophy usually broadest to the temporal side (see also pp. 192, 193).

Such an area is often the seat of softening of the sclera, with consequent distention and thinning posterior staphyloma. When this occurs at the tem side of the disk the optic nerve entrance is tilted, so that it is seen more obliquely. The disk appears a narrow oval. The vessels, drawn upon by the distention, pass more directly outward ; the temporal side of the opening in the sclera through which the nerve enters is made prominent as a white, crecent, also a " myopic crescent." The whole appearance is well character a "dragged disk." If these changes occur at the lower, nasal, or upper side, the disk is found "dragged " in that direction. In the later stages of high myopia similar lesions of the choroid are to be found in other parts of the fundus, especially about the macula, where a small lesion may cause great impairment of vision. In the earliest stages, and later if under the influence of eye strain the myopia is increasing, general hyperemia and disturbance of the choroid may be noticed. Late in the course of high myopia vitreous opacities, cataract, especially nuclear and posterior polar, and detachment of the are liable to occur.

Course. The best statistics of eyes examined at birth show that practically none are then myopic. But high myopia is sometimes encountered in early childhood, and probably sometimes does exist from birth. In the great mass of cases it certainly develops later. In a very few it seems to occur through a healthy development of the eyeball, to go on without choroidal changes or other evidences of disease up to adult life, and then to become stationary. In the great mass of cases axial myopia begins in a period of marked ocular congestion accompanying near work; then at times it becomes stationary; at other times, those of especial strain, it rapidly increases. When the myopia is arrested during early life it continues for some years stationary ; later, by the slow growth of the lens, referred to under Hyperopia, it may be lessened or finally disappear entirely. In a few cases myopia begins during adult life or old age in connection with degenerative changes in the choroid and sclera, and may be a symptom of diabetes. Curvature myopia may begin at any time of life after disease causing corneal distention, conical cornea, or after injury causing partial dislocation of the lens. Index myopia comes in old age as a precursor of cataract, the so called second sight.

Treatment. The indistinctness of vision is remedied by concave lenses. Permanent avoidance of near work will usually check the progress of myopia, but it is generally necessary to check its progress while near work is continued, and fortunately this also is possible for the great mass of cases by the use of correcting lenses. Two factors in near work that might tend to increase myopia are accommodation and convergence; but accommodation is far more tasked in hyperopia, and hyperopic eyes show no such tendency as the myopic eyes to distention of the eyeball. On the other hand, hyperopia is an obstacle to straining convergence, 'while myopia favors or compels it. The tendency of myopia to increase does not disappear when by its progress accommodation is reduced to a minimum or becomes unnecessary; but it does often cease when, binocular vision being given up, convergence is no longer required. If excessive convergence causes myopia and keeps it progressive, the first indication for its treatment is its optical correction, that, the patient may, have distinct vision to induce him to turn his attention toward distant objects, and to free him from the necessity of excessive convergence.

The correcting glasses for myopia should be worn constantly. Wearing them only for distant vision greatly lessens their usefulness. It is most important for a voting person to use the correcting lenses constantly, so that in the requirements made on accommodation he shall have a constant check to excessive convergence. The fear that accommodation may prove injurious has frequently led to the use of a partial correction only for near work. This rarely proves permanently satisfactory. Convergence to a near working point without some accommodation is impossible; and this accommodation makes it necessary to bring the object still closer and further tax the convergence.

The fear that normal accommodation is bad for a myopic eye has led to the prescription of lenses strong enough to greatly improve distant vision, yet weaker than the full correction. Such lenses may be very dangerous to the myopic eye. Looking obliquely through them increases their effect and renders dis¬tant vision more distinct. The patient discovers this and avails himself of it. But looking obliquely through a lens gives, besides the increased power of the spherical, the effect of a cylindrical element and aberration, which vary with the direction and amount of obliquity, and which subject the eye to a strain similar to that caused by uncorrected astigmatism a strain all the harder upon the eye because it is inconstant. Glasses which may be made thus tic) approximate the full correction for myopia are the most dangerous that can be worn. Yet because their use has often resulted disastrously many surgeons hesitate about giving the stronger lenses of a full correction, although these would be really free from such a danger. If for any reason something less than the full correction is given, it should be carefully considered whether its use is liable to be thus perverted and cause injury.

general rule is, in myopia give correcting lenses for constant use. To this there are certain exceptions. With presbyopia it becomes necessary to give weaker lenses for near work. Again, when binocular vision has been given up, strain of convergence, the chief indication for the use of correcting lenses is removed, and a full correction may induce a renewed effort of convergence to restore binocular vision. On this account it will generally be better not to give a correcting lens for the worse eye. Persons who have reached middle age or later life without the use of lenses often find it difficult or impossible to become accustomed to them. Improved vision will often not compensate for the discomfort and inconvenience given, so that these cases must be made exceptions. With very high myopia a lens slightly weaker than the full correction gives an image more like that to which the patient has been accustomed, and which is, therefore, preferred. When this is the case, there is no temptation to get an increased effect by looking obliquely through the lens. Some persons object to the diminished retinal images caused by strong concave lenses, and prefer very much weaker lenses. If one weak enough to entail no strain when looking through it obliquely answers the purpose without any risk of excessive convergence, it may be wiser to give it. Occasionally, too, the full correction may be given for distant ¬vision, and something deducted (1 or 2 D.) from the glass for near work, until the habit of accommodating normally for near objects has been formed. Patients should be warned of the dangers of looking obliquely through concave glasses.

Besides using correcting lenses, the myope must learn to keep his near work as far from his eyes as possible. The lenses are chiefly useful by enabling him to have a greater working distance, and no benefit as regards the progress of the myopia or the health of the eye can be expected unless the opportunity to diminish the strain of convergence is utilized. As an aid to a greater working distance, good light and the avoidance of reading very fine print or prolonged looking at other minute objects must be attended to. Care must be taken to avoid protracted near work. It should be interrupted by frequent intervals, during which the convergence may be allowed to relax and the eyes to fix on some distant object. The position of the bead is also important, particularly in young persons. Reading while lying down or in a bent Posture, causing pressure on the veins of the neck, favors ocular congestion, and should especially be avoided. Use of the eyes during period’s impaired nutrition, as from acute disease, during great physical exhaustion, may also be dangerous. Outdoor life, besides demanding distant rather war vision, acts by improving general nutrition. When choroidal congestion is marked, the influence of complete rest of the eyes for some days under the influence of a mydriatic may promptly check a process that tends to soften and rapidly distend the sclera. When increase of myopia does occur the lenses should be promptly changed accordingly.

The operative treatment of myopia, by removal of the crystalline lens by discission, followed by extraction if the patient's age makes it necessary, is claimed not only to improve vision by removal of high myopia, making com weak glasses necessary, but also to exert an influence in checking the progress of the myopia, and actually to cause a diminution in the antero axis of the eyeball. In the judgment of the writer it is not proper to resort to it in any case where the progress of the myopia can be arrested by the wearing of correcting lenses and ordinary hygienic precautions. But where glasses cannot be comfortably worn or with them the myopia continues distinctly progressive, it is proper to extract the crystalline lens. This operation may also be resorted to in cases of high myopia in one eye and in myopia with commencing lens opacity. In such eyes cataract often remains incomplete for many years, and grows no easier of extraction it may even become more difficult to remove because of the larger nucleus when ripe than when the opacity begins to interfere with vision. The reduction in myopia by extraction of the lens varies in different eyes, usually between 15 and 20 D. Generally, it will Dot be exactly corrected by the removal of the lens; glasses for both near and distant vision will be required, accommodation being lost with the removal of the lens.

. Its Nature and the Vision of Astigmatic Eyes.Astigmatism is always an ametropia of curvature. It is a defect in which rays from a single point do not after refraction tend to meet at a single point.

In irregular astigmatism the curvature is irregular and the refraction differs in the different parts of the pupil.

In regular astigmatism the refraction is the same in different parts of the pupil, but differs at the same point in different directions. This depends upon inequality of curvature of the dioptric surfaces in the different directions.

A familiar illustration of the kind of surface causing it is found in the curve of the edge of a watch. The curve in the plane parallel to the face of the watch is weaker than the curve in the plane perpendicular to the face. The inequality of curvature causes the rays to be refracted more strongly in the direction of the stronger curve, and in that plane to come to a focus before they have reached a focus in the plane of the weaker curve. Instead of being focussed to a single point, they are focussed successively to two lines at right angles to each other and separated by a certain interval.

In most cases of regular astigmatism the fault depends chiefly upon inequality of' curvature in the cornea, although there is usually also some inequality in curvature in the crystalline lens. It is common to speak as though the astigmatism were due entirely to the corneal curvature, but it should be remembered that this is only exceptionally the case.

In considering the refraction of the astigmatic eye it is only necessary to follow the course of the rays as regards two meridians, called the principal meridians viz. the meridian of greatest curvature or greatest refraction, and the meridian of least curvature or least refraction. In regular astigmatism these are always perpendicular to each other. In some eyes they are not perpendicular, but in such eyes the astigmatism is not regular, or if a part of it be regular, there is present also some irregular astigmatism, which cannot be corrected by any lens. (See page 206.) When the refraction has been corrected in the principal meridians all of the regular astigmatism, all the astigmatism that is corrigible, is corrected for all meridians.

The focussing of light by the astigmatic eye may be illustrated by Fig.161, in which the circle represents the cornea as seen from the front; a a represents the principal meridian of greatest refraction, and b b the principal meridian of least refraction. By the vertical curvature all rays entering the upper half of the cornea are brought down to the level of the central ray when they reach the point f and all rays entering the lower half of the cornea are brought up to the central ray at the same point. At f all the rays have been brought to the level of the central ray but they have not been focussed to a point, for in the meridian of least refraction, b b, they have been less turned from their original course, and therefore from side to side are still spread out the distance ff. Not until they have travelled on to the point g are those from the right half of the pupil and from the left half of the pupil all collected to the center line of the pupil. By the time they have been thus collected from side to side they have begun to spread downward and upward, so that they occupy vertically the distance g g. A horizontal line, ff, into which all the rays are collected, is the focus for the vertical meridian, the first focal line; and g g a vertical or second focal line in which all these rays are afterward collected, is the focus for the horizontal meridian or hori curvature of the cornea. The interval between ff and g g, depending on the difference of curvature in the directions a a and b b, called the focal interval of Sturm, shows the amount of astigmatism.

To f and g the rays from a single point outside of the eye are collected, forming at each a focal line; at all other distances behind the cornea they Spread out, making an area of diffusion which is commonly an ellipse, though at cone point between f and g it becomes a circle. The focussing of the rays from a point outside of the eye upon a line of the retina gives rise to the peculiar defect of vision produced by astigmatism. This defect is such that lines running in the direction of the focal line on the retina are seen clearly, except that their ends shade off gradually, but the lines running in other directions appear blurred, as in Fig. 162.

a represents lines running in three directions, as seen by an emmetropic eye. b represents the impression such lines make on the retina of an astigmatic eye : I shows them running in the direction of the focal lines on the retina, so that these overlap each other, giving the impression of a distinct line ; 3 shows them running at right angles to the focal lines on the retina, so that they overlap the spaces on either side, giving the greatest blurring; and 2 shows them running obliquely, so that the overlapping causes blurring, but less than that for 3. All lines looked at by the astigmatic eye are seen in one of these ways at any given time. The eye may, by change of accommodation, so vary its refractive power as to bring first one and then another focal line upon the retina, making the lines clear at first in one direction and then in the other.

Symptoms of Astigmatism. Generally lines can be seen clearly only when they run in some one direction, and this direction is that of one of the principal meridians. This necessarily occasions a certain indistinctness of vision, which is peculiar in that, when tested by the test letters, some of these on account of the direction of their characteristic lines are more blurred than others. The patient may miscall several of the letters of a certain size, and yet recognize others of but half that size. In general, the indistinctness due to astimatism is not more than half as great as that produced by myopia or hyperopia of equal amount.

It has been stated that the astigmatic eye seeks to overcome indistinctness of vision by unequal contraction of different parts of the ciliarv muscle, causing unequal convexity of the crystalline lens in different meridians. It has not been certainly proved that this occurs. But the indistinctness may be partly overcome by rapid changes from one state of accommodation to another, causing first the one focal line and then the other to fall upon the retina in such quick succession that their impressions may aid in a single mental perception. Either use of the accommodation leads to eye strain with all its possible manifestations pain, congestion or inflammation of the eye and its appendages, headache, and other manifestations of disturbance of the general nervous system. In childhood the difficulty of the imperfect images hinders the development of the powers of visual perception, and even of the general mental processes. Indistinctness of vision, though present from early life, may somewhat diminish as the patient learns to use his eyes, but increases again when age has caused the impairment or complete loss of accommodation. High astigmatism, especially myopic, with the greatest defect in the vertical meridian, is quite as likely to cause partial closure of the lids, with secondary disturbances of the cornea, as is myopia.

Varieties. Astigmatism with the rule is astigmatism with the meridian of greatest refraction vertical or nearly so, as it is in a large majority of cases.

Astigmatism against the rule means that the meridian of greatest refraction is horizontal or nearly so. The number of cases of this kind is comparatively small, but they grow more frequent after middle life. The astigmatism that follows cataract extraction, iridectomy, and similar corneal sections is usually of this kind, because such sections are generally made in the upper margin of the cornea, and their influence is to flatten the cornea in the meridian perpendicular to their length. Astigmatism against the rule has also been noted as a forerunner of glaucoma.

Oblique astigmatism means that the direction of the principal meridians departs much from the vertical and horizontal, and approaches rather to 45 and 135 degrees. Some writers believe that astigmatism against the rule and oblique astigmatism are most likely to cause inconvenience, or to cause more inconvenience than astigmatism with the rule of equal amount. This may be explained by the fact that only lines parallel to the principal meridians can be perfectly focussed on the retina, and that the greatest number of lines looked at are either vertical or horizontal.

While the amount of astigmatism and the direction of its principal meridians are independent of the position of the retina, the relation of the retina to the focal lines determines the variety under which astigmatism is classified; thus, in Fig. 163, suppose c represents the cornea, the solid lines represent rays as refracted in the vertical meridian, and the broken lines the rays as refracted horizontally, f to be the position of the anterior focal line, and g the position of the posterior focal line. When the retina passes through f the defect is called simple hyperopic astigmatism hyperopic because as regards the meridian of least refraction and the focal line g the eye is hyperopic simple because it can be corrected by the simple cylindrical lens which corrects the meridian of least refraction.

When the retina is situated at h the astigmatism is called compound hyperopic. The eye is hyperopic for both meridians, for both focal lines, and it can be corrected only by a compound cylindrical or sphero cylindrical lens.

When the retina passes through g the defect is simple myopic astigmatism, the eye being myopic for the meridian of greatest refraction and the focal line f, and capable of correction by a simple cylinder correcting the meridian of greatest refraction.

When the retina is at m the astigmatism is compound myopic, the eye being myopic for both focal lines and meridians, and its ametropia is only corrected by a compound cylindrical or sphero cylindrical lens.

When the retina is situated between f and g the eye is hyperopic for g and the meridian of least refraction, and myopic for f and the meridian of greatest refraction; the astigmatism is called mixed, and requires for the correction of the ametropia a lens convex in one meridian and concave in the other. It is evident that simple increase in the antero posterior axis of the eyeball by distention will cause the same case of astigmatism to pass from compound hyperopic to simple, then to mixed, afterward to simple, and finally to compound myopic. In case of astigmatism becoming myopic these changes successively occur in the course of the progressive distention of the eyeball (see also pages 127 and 128).

Correction of Astigmatism. This is effected when rays, instead of being focused to two focal lines, are focussed to a single point. The correction of the ametropia present requires that for parallel rays this point shall fall upon the retina. But the astigmatism may be fully corrected, although a certain amount of other ametropia (hyperopia or myopia) remains uncorrected. Astigmatism is corrected by any cylindrical lens or combination of lenses that makes up for the difference of refraction in the two principal meridians. Thus a convex cylinder with its curve parallel to the meridian of least refraction, and equal in strength to the difference between the two principal meridians, will correct any ease of astigmatism. A concave cylinder with its curve parallel to the meridian of greatest refraction, and strong enough to make the difference between the two meridians, will correct it equally well. Or a convex cylinder correcting a part of the astigmatism may be placed with its curve in the direction of the meridian of least refraction, and a concave cylinder strong enough to correct the remainder of the astigmatism with its curve parallel to the meridian of greatest refraction.

In general, any case of astigmatism may be corrected by one of three combinations of lenses. Take, for instance, a hyperopic astigmatism in which the horizontal meridian is hyperopic 4 D., and the vertical meridian hyperopic 2 D. The astigmatism may be corrected (1) by a convex 2 D cylindrical lens placed with its curve horizontal (axis vertical), and the additional hyperopia corrected by combining with this a convex 2 D. spherical lens. This astigmatism may be corrected (2) by a concave 2 D. cylindrical lens placed with its curve vertical (axis horizontal). This would have the effect of increasing the hyperopia of the vertical meridian, and to correct the hyperopia a convex 4 D. spherical lens would be required. It would also be possible (3) to correct the astigmatism with a convex 4 A cylinder with its curve horizontal (axis vertical) and a convex 2 D. cylinder with its curve vertical (axis horizontal). The one cylinder would bring the posterior focal line on the retina without affecting the anterior focal line, and the other cylinder would bring the anterior focal line on the retina without affecting the posterior line. In this way both focal lines, brought to the same distance from the cornea, would become a single point, and the astigmatism would be corrected, and with it also the hyperopia.

For the one case of astigmatism any of the following lenses might be chosen, the correction being optically as good with one as with another :

(1) + 2 D. sph. <> 2 D. cyl. axis 90' (vertical);
(2) 4 D. sph. 2 D. cyl. axis 180' (horizontal)
(3) 2 D. cyl. axis 180' <> 4 4 D. cyl. axis 900.
Looking at these, it will be seen that (1) has on the whole the weakest surfaces. It is theoretically possible with it to get the thinnest lens and the one having usually the least aberration. It is also the lens most commonly selected in testing the eye with trial glasses, and the one most frequently prescribed.

It will be observed that (2) has one convex and one concave surface. The spherical surface has to be stronger than that of (1), and therefore causes more aberration; but this is a matter of very little importance. It is of greater importance that by placing the concave surface toward the eye and the convex surface away from it something of a periscopic effect can be obtained by this second lens allowing the eye to be turned in different directions without causing so much obliquity of the visual axis to the lens surfaces. On this account (2) will prove on the whole the most satisfactory for a large proportion of cases.

With reference to (3), it will be noted that it includes two cylindrical surfaces with their axes exactly perpendicular. Such a lens is very hard to grind sufficiently accurate for practical purposes, and impossible to grind with theoretic accuracy. Its surfaces, too, are stronger, and therefore cause more aberration. On every account this form of lens, the crossed cylinder, is to be avoided. It has rarely been used except for mixed astigmatism, where it gives weaker surfaces than either of the sphero cylindrical lenses. But this does not compensate for the increased expense and necessary inaccuracy of crossed cylinders, and it is better never to employ them.

The following formulas will illustrate this subject as regards mixed astigmatism :
(1) 1 D. sph. 2 D. cy]. axis 90' ;
(2) 1 D. sph. 2 D. cyl. axis 180';
(3) + I D. cyl. axis 90' I D. cyl. axis 180'.
In compound myopic astigmatism the same thing holds, as the following equivalent formulas will indicate:
(1) 2 D. sph. 2 D. cyl. axis 180'
(2) 4 D. sph. + 2 D. c~l. axis 90' ;
(3) 4 D. cyl. axis 180' <> 2 D. cyl. axis 90'.
In simple astigmatism the correction for the better meridian is 0 ; and one element of formulas (2) and (3) becomes 0, so that the two become alike. In simple hyperopic astigmatism we would have the following:
(1) or (3) + 2 D. cyl. axis 90' ;
(2) + 2 D. sph. 2 D. cyl. axis 180'
from which one may choose the simple cylinder, which is the cheapest lens, or the sphero cylindrical lens, which gives the better periscopic effect. In simple myopic astigmatism the formulas are thus:
(1) or (3) 2 D. cyl. axis 180' ;
(2) 2 D. sph. + 2 D. cyl. axis 90'.

Wearing Glasses for Astigmatism. The whole treatment of astigmatism consists in the wearing of glasses. Since astigmatism interferes with distinctness of vision at all distances, and since it entails, when uncorrected, a use of the accommodation entirely different from that of emmetropic, hyperopic, or myopic eyes, it is important that the lenses correcting it should be worn constantly. This is essential in all cases at first. Sometimes a patient, by wearing glasses constantly acquires the habit of using the accommodation normally and can continue to so use it by sacrificing something of distinctness of vision on laying aside his correcting lenses at times when the eves are Dot to be especially taxed. Such persons, after the constant use of cylinders for some time, are able to do without using them constantly when the eyes are not employed on work requiring distinct vision. In general, however a patient having much astigmatism may be warned that he will always require the help of correcting lenses.

Cylindrical lenses, contrary to what is sometimes expected, are often difficult to become accustomed to, especially if they are strong, if the patient is advanced ill years, and if the axes of the cylinders before the two eyes must be turned in different directions. Strong cylinders are never satisfactory at first. With some persons, especially when past middle life, the difficulty of becoming accustomed to them is so great that they are very likely to give up the attempt. This should be carefully considered before ordering glasses. Any cylindrical lens changes somewhat the shape of the retinal images and, therefore, the apparent shape of objects looked at. When the axes are turned in different directions the distortion of the retinal images, corresponding to the directions of the axes, differs in the two eyes, so that it becomes difficult to fuse the two impressions they make and secure binocular vision. These unpleasant effects may be diminished by wearing for a time an incomplete correction of the astigmatism or by bringing the lenses particularly close to the eyes.

Aberration. A spherical lens does not perfectly focus the rays passing through it. In general it acts toward the edge as a stronger lens. This may be illustrated by the following diagram, which shows the course of the parallel rays as refracted by a convex spherical surface (Fig. 164). The rays passing through the center are focussed at f, the principal focus of the lens, and those passing through the margin are focussed closer to the lens. The unequal distribution of light in the circle of diffusion, its concentration to a ring at the edge and a point at the center of that circles may be studied with a strong convex lens focussing light upon a card.

In the human eye the periphery of the crystalline lens is more convex than the center, and acts, therefore, as a stronger lens than the center, just as in the ordinary spherical lens. The periphery of the cornea, on the other hand, is always more or less flattened. Within the pupil, in the majority of eyes, the increased convexity of the crystalline lens predominates, so that they present a stronger refraction, higher myopia or lower hyperopia, at the periphery of the pupil than at its center. This condition the writer has called positive aberration. When the opposite occurs the refraction is stronger, the myopia higher or the hyperopia lower at the center of the pupil than near its margin, constituting negative aberration.

Aberration plays an important part in skiascopy, determining the form and size of the light area in the pupil, causing reversal of the movement of light in the periphery (in positive aberration) to be perceived closer to the eye than the movement of light at the center, where it is of more practical importance.

When aberration is confined chiefly to the extreme periphery of the pupil, where it is shut off by the pupillary contraction in a strong light or during near work, it has no influence on the working power of the eye. When it begins Dear the center of the pupil, causing the eye to be more hyperopic when the pupil is contracted by a strong light or for close work than when more dilated, it has an important influence in producing eye strain, and may be a cause of error in the selection of lenses. An eye with positive aberration will often select with the undilated pupil a convex lens 0.25 D. stronger, or a concave 0.25 D. weaker, than it will accept while the eye is fully under the mydriatic.

Aberration is to be recognized 1)), skiascopy and considered in the choice of lenses. It cannot be exactly corrected by any particular lens, but is sometimes an indication for the wearing of a stronger lens than one which will allow of perfect distant vision, such a lens being found in these cases decidedly more helpful. High negative aberration is sometimes due to increased refractive power in the nucleus of the lens incipient senile cataract or to conical cornea.

Irregular astigmatism is recognized by skiascopy, causing appearances represented in Fig. 165 A and B. Traumatism or disease of the cornea, leaving irregularities of its surfaces (Fig. 165, A), tissue changes in the lens preceding cataract (Fig. 165, B), and occasionally faulty development of the cornea or lens, cause irregularities of refraction that prevent the perfect focussing of light to a point by the dioptric media. Such defects are not capable of correction by lenses. The eye, however, often presents within the area of the pupil small areas in which the refraction is comparatively uniform, which areas may be corrected by some combination of lenses, and the vision and comfort of the patient thus be greatly improved. The practical thing to do is to study these cases carefully by ophtlialmometry and skiascopy, and to correct the most regular portion of the cornea.

In a few cases, where Do lens can render much service, it may be worth while to try a stenopaic spectacle. This is an opaque disk in front of the eye with a narrow slit or, more commonly, a single pin hole opening in it. Such an apparatus often gives a noticeable improvement of vision, but it is rarely found very serviceable because it interferes with the visual field.

Anisometropia. Some inequality in the refraction of the two eyes is the rule, and occasionally this is such as to render one eye hyperopic and the other myopic or one astigmatic, while the other is free from astigmatism. Such a Difference constitutes anisometropia. The importance of the difference depends entirely on its degree, and not on whether it amounts to a difference in the kind of ametropia.

The general rule when the difference is Dot great is to give each eye its exact correction. If the difference between the two correcting lenses is very great, they affect the size of the retinal images, so that binocular vision be difficult. When one lens is much stronger than the other, looking through the periphery produces a correspondingly different prismatic effect, causing objects to be seen double, or the effort to use the images falling on the two retinas causes strain of the extraocular muscles.

For the above reasons the full correction of anisometropia cannot always be practised. It is generally safe to prescribe the correcting lenses for both eyes when these differ less than I D. If they differ not more than 2 D., they will generally be accepted, although this cannot always be assumed. If they differ more than 2 D., the patient will find it very difficult or impossible to use them for satisfactory binocular vision, although a few persons will prefer to have anisometropia of 3 or 4 D. fully corrected. When the difference of refraction cannot be fully met by difference of glasses, the rule is to correct the better eye and to allow the worse eye the full correction of its astigmatism, with a spherical lens equal to that of the better eye or a little stronger. Sometimes, if both eyes have good vision, but cannot work together, one may be corrected for distant vision and the other given a lens that will adapt it for near seeing. Congenital anisometropia often gives little trouble, but anisometropia coming on from change in the refraction, as in progressive myopia, is likely to be very annoying. The similar effect produced by glasses not accurately suited to the eyes is also very annoying. Acquired anisometropia, particularly from 0.5 to 2 D., is especially liable to give rise to squint, and its correction is indicated to preserve or restore binocular vision.

. I The failure of accommodation with age leads finally to complete inability to change the optical condition of the eye, so that only rays of a certain convergence or divergence can be focussed upon the retina. In the great majority of eyes, which are hyperopic, this renders necessary the use of convex lenses for near vision. For this purpose the need of lenses is felt the eye is presbyopic as soon as the power of accommodation has diminished so that it is unequal to the task of keeping the crystalline lens convex enough to focus rays accurately oil the retina when the eye is engaged in ordinary near work. When this occurs either symptoms of strain, such as congestion and pain in the eye, conjunctivitis, or headache, arise, or after the effort has been sustained for some time the ciliary muscles suddenly relax and all near objects become blurred. If the eyes are now rested for a minute, the power of distinct near vision returns, but if the near vision is continued, it again fails, and, persisting in the attempt, such failures become more and more frequent until the effort is given up.

The failure is first for objects at the shortest distance from the eye, as small objects or fine print that needs to be brought close in order to be seen. Objects that may be held farther away, or the same object in a strong light which will render it distinguishable at a greater distance, may still be clearly seen, the patient noticing only that he requires good light and has to hold things farther from the eyes than formerly. Presbyopia is caused first by the increasing rigidity of the crystalline lens, which limits its tendency to become more convex when tile tension of the suspensory ligament is removed by contraction of the ciliary muscle. Subsequently the ciliary muscle also becomes weakened or undergoes atrophy, and the power of accommodation is completely lost.

Presbyopia is relieved by supplementing the insufficient focussing power of the crystalline lens by a convex lens of the necessary strength placed before the eye. In choosing such a lens it is to be borne in mind that we have to enable the eye not only to see clearly at the required distance for an instant, but to sustain distinct vision at that distance over periods of continuous use. The maximum contraction of a muscle is always one that cannot be long sustained; hence the lens giving the patient a near point where he wishes to work will be insufficient for continuous work. With most persons only two thirds of the accommodation can be long kept up. A few can sustain three fourths of it, but others, particularly young persons suffering from weakness of accommodation, can comfortably sustain only one half of the full amount.

In correcting presbyopia, then, we not only find the near point of distinct vision, but from that near point and the refraction of the eye calculate the total power of accommodation. Then assuming that two thirds of this accommodative power is available for continuous work, the difference between that available accommodation and the accommodation required for the sort Of Dear work to be done is the strength of lens that should be given to correct the presbyopia. This may be illustrated by examples of different errors of refraction.

Suppose, first, a case of presbyopia in emmetropic eyes. The nearest point of distinct vision being 18 inches (45.5 cm.), corresponding to 2.25 D. of accommodation, two thirds of this, which may be assumed as available for near work, equals 1.5 A Now, if the patient wishes to use the eyes for ordinary reading, writing, sewing, etc. at a distance of 13 inches (33 cm.), where 3 D. of focussing power will be required, 3. 1.5 D. = 1.5 D. will be the strength of the convex lens that should be given to supplement accommodation to correct the presbyopia. If the patient has been weaving such a lens or one nearly as strong, and still shows evidence of undue strain of the eyes for near work, it may be that be cannot sustain two thirds of his total accommodation, but requires the presbyopic correction to be made somewhat stronger, as 1.75 or 2 D. On the other hand, if such a patient has been reading without any lens and without much inconvenience, it may be assumed that he can sustain more than two thirds of his total accommodation, and therefore a weaker lens, as the I or 1.25 D., may be given.

in another case the patient has hyperopia of 2 D., and a near point of distinct vision of' 16 inches (40 cm.), corresponding to 2.5 D. of focussing power, to which is added the 2 D. needed to correct the hyperopia, making 4.5 D. of total accommodation. Two thirds of this accommodation, or :1 D., would only correct his hyperopia, and leave I D. to adapt the eye for near Vision at a distance of I in. If such a patient is to work at 13 inches (33 cm.), where 3 D. of focussing power is needed, lie will require the help of a lens equal to 3 D., 1 D., or 2 D. The increased use for accommodation will cause the hyperopic eye to suffer earlier from presbyopia if it has not the help of correcting lenses for the hyperopia. It will also be noted that with a certain near point the hyperopic eye requires a stronger supplementary lens, since that near point represents, with a greater amount of accommodation, a greater need for it. The lens required in the above case might be found by correcting the hyperopia with a 2 R. convex lens, when it would be found that the Dear point was at 9 inches (23 cm.) (4.5 D. of accommodation), and that two thirds of this accommodation, 3 D., would be sufficient for work at 13 inches (33 cut.). Hence no further correction for presbyopia would be required, the correction of the hyperopia causing the Presbyopia to disappear.

By myopia the need for a presbyopic correction is postponed and diminished. Thus, an eye with myopia of 3 D. will be able to work at 13 inches (33 cm.) without any lens and without accommodation, and for that kind of work will never suffer front presbyopia. Take another case, where the myopia is I D. and the near point found at 22 inches (57 cm.), corresponding to 1.75 D. of focussing power; subtracting from this I D. of myopia leaves 0.75 D. as the total accommodation. Of this two thirds, or 0.5 D., being available for near work, is to be added to the 1 D. of myopia, making 1.5 D. of available focussing power, and for work to be done at 13 inches there will be need in addition for a convex lens of 1.5 D. That is, in myopia of 1 D., with only 0.75 D. of accommodation, the same help is required as in emmetropia with accommodation of 2.25 D. With myopia, as with hyperopia, the total accommodation may also be found by first correcting the myopia and then taking the near point.

In astigmatism the accommodation can only be accurately determined by taking the near point after the correction of the astigmatism, and the amount of convex spherical to be added for near work on account of presbyopia will then be determined as though the eye had been originally emmetropic. Sometimes in giving lenses for presbyopia with astigmatism, while the concave cylinder is better for distance, the convex cylinder with its axis turned at right angles is better for near work. Suppose a case of simple myopic astigmatism requires for its correction 1.5 D., cylinder axis 180', and with this correction before the eye the near point is 18 inches (46 cm.), the accommodation 2.25 D. The spherical to be added for near work at 13 inches (33 cm.) would be 1.5 D., and a convex 1.5 D. spherical, combined with the concave 1.5 D., cylinder axis 180', is the optical equivalent of the convex 1.5 D., cylinder axis 900. For distant vision such an eye may be given 1.5 D., cylinder axis 1800, and for near vision + 1.5 D., cylinder axis 90'.

Course. Presbyopia usually begins between the ages of forty and fifty. With hyperopia, which may have given no earlier evidence of its presence, it begins younger; with myopia, later or not at all. Even with emmetropic eyes the increasing rigidity of the lens may require the use of convex glasses before the age of forty and with a few the need of a presbyobic correction is deferred until after the age of fifty.

In all cases after it has begun presbyopia is progressive. The power of accommodation continues to diminish until it is entirely lost, and such diminution causes the necessity for increasing the strength of the supplementary lenses the presbyopic correction. Generally, the lenses should be changed often enough to have a difference of not more than 0.75 D., or about once every two or three years from forty five to fifty five. Most patients require the same correction for presbyopia for both eyes. In a few cases this is not so, the accommodation failing faster in one eye than in the other, and requiring a correspondingly stronger supplementary lens. In such cases the eyes should be repeatedly tested to make sure that there is actually a differ between them, and the tests repeated at short intervals.

The Mounting of Glasses. Lenses are commonly supported before the eyes by spectacle or eye glass (pince nez) frames. The former have the advantage of more rigidly fixing the position of the lens before the eye. The latter are more readily removable when the lenses are not required for constant use. The proper adjustment of the frames is a matter of much importance, since the right lens in the wrong position does not have its proper effect, and may be entirely unsatisfactory (see pages 236 240).

The Period of Adaptation. Weak lenses, less than 1 D., may prove satisfactory and comfortable from the start or within a few days after beginning to wear them. Children may become accustomed to even strong lenses in a very few days. Correcting lenses will generally be accepted without complaint when the eye is kept for 'some time under the influence of a mydriatic. But, apart from these exceptions, lenses are rarely accepted with entire comfort at first.

The period of adaptation during which the first discomfort diminishes and passes away may last from two to six weeks, or even longer; during this period convex lenses are likely to cause blurring of distant vision, concaves render near work noticeably more fatiguing, and cylinders cause distortion of objects and an indefinite discomfort. These unpleasant effects may from the start be more than balanced by the benefits experienced, yet it is prudent in all cases to warn the patient that some weeks must elapse before the glasses can be expected to do their best. With such a warning most people encounter the necessary difficulties without loss of confidence. But if permitted to put on the glasses expecting immediate satisfaction, they become disappointed, lose faith in the prescriber, and are likely to refuse to give them a fair trial. The good of the patient and the reputation of the surgeon both demand that a careful explanation of the period of adaptation should be given when the glasses are prescribed.

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