Spectacles: their uses and abuses in long and short sightedness; and the pathological conditions resulting from their irrational employment ...
Sichel, J. (Jules), 1802-1868. n 86839662
 fct t • • i 1 1 • <i * « ■ » • 4 • I / 4 m i> » • . » . v» • ». : ' • -l • .*•» • * Ik.lii: llhktill !•< * . . . ...» b| V .... »»«• » f »»•'•••. .  Internet Archive in 2018 with funding from Wellcome Library https://archive.org/details/b29301932 SPECTACLES : THEIR USES AND ABUSES IN LONG AND SHORT SIGHTEDNESS; AND THE PATHOLOGICAL CONDITIONS RESULTING FROM THEIR IRRATIONAL EMPLOYMENT. BY J. SICHEL, M. D., OF THE FACULTIES OF BERLIN AND PARIS ; CLINICAL PROFESSOR OF DISEASES OF THE eye; officer of the legion OF HONOR, etc. etc. etc. TRANSLATED FROM THE FRENCH, BY PERMISSION OF THE AUTHOR, BY HENRY W. WILLIAMS, M. D., FELLOW OF THE MASSACHUSETTS MEDICAL SOCIETY, ETC. ♦ BOSTON: PHILLIPS, SAMPSON AND COMPANY, 110 Washington Street. 1850. Entered according to Act of Congress, in the year 1850, by PHILLIPS, SAMPSON & COMPANY, In the Clerk’s Office of the District Court of the District of Massachusetts. William H, Floyd COLLECTION. stereotyped by HOBART & ROBBINS; NEW ENGLAND TYPE AND STEREOTYPE FOUNDERY, PREFACE. The publication of these Clinical Lessons on Specta¬ cles, and the pathological conditions consecutive to their irrational use, was commenced in 1845, in the Annates d’ Oculist iqne, at Brussels. The present work contains many new and very impor¬ tant considerations, and a description of several frequent and grave maladies, of which no one had previously spoken ; such as presbytic amblyopia , congenital pres - bytic amblyopia , amblyopia produced by the use of too powerful glasses , muscce volitantes caused by the abuse of spectacles , etc. These affections may be prevented, arrested in their development, or completely cured, by the use of properly selected spectacles. For these reasons I may recommend the perusal of these pages, not only to physicians, but also to opticians. The latter have it in their power, yet more than the former, to diminish the number of these maladies by judicious advice, or to aug¬ ment it by the unseasonable concession of too powerful glasses. Unfortunately, they very generally continue the habit of commencing by too powerful numbers in the selection of spectacles, although this practice is entirely contrary to their own interests. I beg them to read and reflect on what I have said on this point, particularly in §§ XI. and XXX. SlCHEL, D. M. Paris , 1848. 4 * ■ TRANSLATOR’S PREFACE. The catalogue of our Medical literature contains no work on the important subjects discussed in this treatise — Ocular Hygiene — and the large class of diseases, in the production or amelioration of which spectacles exert an important influence. I have, therefore, great satis¬ faction in availing myself of the kind permission of its distinguished author, and publishing in this country a translation of a work emanating from the highest author¬ ity in this department of medical science. The question of the appropriate uses and the disadvan¬ tages of spectacles, is one, which, in the absence of all authority, has been little understood by many practi¬ tioners who have occasion to give advice respecting their adoption ; it is, therefore, with a full conviction that this monograph will be generally welcomed, and will prove a guide of the highest utility, that its valuable pages, the results of mature observation, cultivated intellect, and wide experience, are offered to the Profession in America. The author has stated his intention of publishing “ Lessons upon the Inequality of Focus of the two Eyes, and upon Cataract Glasses,” at some future time ; but the present work is perfectly complete and independent, including everything relating to long and short sighted¬ ness, and all those considerations which are of most gen¬ eral importance to the physician, to opticians, or to those devoted to literary pursuits. On the appearance of this second work, I hope to have the pleasure of publishing a translation. 10 Essex Street, Boston, Mass., 1850. H. W. W. - ' CONTENTS SECTION. I. II. III. IV. V. VI. VII. VIII. IX. X. XI. XII. XIII. XIII£. XIV. XIV£. XV. XVI. XVII. XVIII. XIX. XX. XXI. PART I. PAGE Compass of Vision, . 7 Faculty of accommodation of the sight to different distances, . 8 General causes determining the compass of vision, . 12 General considerations upon Presbytia and Myopia, 14 Spectacles in general, and the numbers which denote the power of the glasses, . 17 The form of Spectacle glasses and their frames, the manner of wearing them, and some other acces¬ sory points, . 21 Influence of the faculty of accommodation upon the preservation of a proper focus in the presbytic and myopic, . 24 PRESBYTIA, OR PRESBYOPIA. Hygienic precautions to be observed by the presbytic, 26 Degree of light necessary to the presbytic, ... 30 Use of Spectacles in Presbytia, . 32 Manner of choosing Spectacles for Presbytia, and of progressively augmenting their power, ... 36 Sudden development of Presbytia, . 41 A particular species of Amblyopia produced by Pres¬ bytia and too assiduous employment of the naked eye, . 46 Considerations upon the history and the disputed nature of Presbytic Amblyopia, . 52 Treatment of Presbytic Amblyopia, .... 58 Application of Ocular Myotomy to Presbytic Ambly¬ opia, or Kopiopia, . 62 Employment of Spectacles in Amaurosis, ... 67 Varieties of Presbytic Amblyopia, . 72 A variety of Presbytic Amblyopia, in which the patient sees no better with Spectacles than with the naked eye, . 73 Two varieties of Presbytic Amblyopia which rapidly augment, 75 Complications of Presbytic Amblyopia, and their treatment, . 82 Complication of Presbytia with Conjunctivitis, and the apparent Amblyopia which results from it, . 86 Presbytia as a cause of Ocular Neuralgia, ... 89 VI CONTENTS. SECTION. PAGE XXII. Myodopsia produced by Presbytia, . 93 XXIII. Injurious effects of too powerful convex glasses, espe¬ cially when they are employed for seeing at a dis¬ tance, . 97 XXIV. Treatment of Amblyopia caused by the abuse of convex glasses, . 108 XXV. A species of Congenital Amblyopia complicated with Presbytia, and ordinarily taken for a very high degree of Myopia, (Congenital Presbytic Amblyo¬ pia,) . 112 XXV Treatment of Congenital Presbytic Amblyopia, . .115 XXVI. Varieties and complications of Congenital Presbytic Amblyopia, . 121 XXVI. a. Acquired Myopia, or Presbytia changed to Myopia, 122 XXVI. b. First variety. Simple Acquired Myopia, without weakness of vision, . 123 XXVI. c. Spontaneous return of the ability to work without convex glasses, after having worn them, . . . 126 PART II. MYOPIA. XXVII. Second variety of Acquired Myopia. Myopia com¬ plicated with Amblyopia, . 131 XXVII^. Congenital or Non- Acquired Myopia. Degrees, . .140 XXVIII. Myopia symptomatic of ocular disease, .... 148 XXIX. Pules of Ocular Hygiene to be observed by Myopes, . 153 XXX. Choice of Spectacles in Myopia, . 159 THE TROUBLESOME CONSEQUENCES PRODUCED IN MY¬ OPES BY THE INFRACTION OF HYGIENIC RULES, AND ESPECIALLY BY THE IRRATIONAL USE OF CONCAVE GLASSES. XXXI. Progressive augmentation of Myopia, .... 163 XXXII. Myodopsia, considered as one of the most constant effects of the use of too strong concave glasses, . 168 XXXIII. Kopiopia symptomatic of a high degree of Myopia, or consecutive to the abuse of concave glasses, . 171 XXXIV. Myopic Amblyopia and Amaurosis, .... 175 COMPLICATIONS OF MYOPIA, BY WHICH IT IS AUGMENTED, EITHER IN REALITY OR APPEARANCE. XXXV. Opacities of the Cornea, . 187 XXXVI. Myopia complicated with Ambtyopia produced by causes not depending on the compass of vision and the use of Spectacles, ...... 189 XXXVII. Problem in regard to Myopia, . 191 XXXVIII. Use of convex glasses in Myopia, . 193 SPECTACLES. PART I. § I. COMPASS OF VISION. Every one knows that the compass of vision is different in different individuals ; but it is only the extremes which have received a particular designa¬ tion: presbyopia , or long-sightedness, from n^aSvi, old person , because the reach of vision tends to increase with age ; and myopia , from to wink, on account of the instinctive movement of con¬ tracting the opening of the eyelids made by those persons in whom the focus of vision is at a very short distance, in order to admit only the rays nearest to the visual axis, and to exclude the peripheric rays, which, too strongly refracted by the media of the myopic eye, unite in front of the retina, and produce only a confused image. The differences comprised between these two extremes are so numerous that it is difficult to de¬ termine a precise standard of normal vision. This difficulty results from the insensible transition be¬ tween long and short sightedness. 8 SPECTACLES I § II. FACULTY OF ACCOMMODATION OF THE SIGHT TO DIFFERENT DISTANCES. We have said that the point of normal vision cannot be established with precision, because it varies infinitely in different individuals. Even for each person this point is not fixed, but is without exactly circumscribed limits. By an experiment easily made, we may convince ourselves that the same object is visible to us with equal clearness at various distances, often widely differing. If we place, for instance, printed characters of ordinary size at the shortest distance at which we can see them with perfect distinctness, and then remove them by insensible gradations from one to several inches, we can still continue to read them with facility. It is the same for larger objects, which we can remove several feet without ceasing to perceive all their details. Certainly, the extent of the space which may be passed over by the object without the loss of a clear perception of it, will vary very much in a given number of per¬ sons enjoying normal vision, but few will be found in whom visual perception will not remain clear in an extent of several inches for bodies of small dimensions, and of several feet for those more voluminous. The less any individual, by the nature of his occupations, has been forced to main¬ tain himself always at the same distance from the object he observes, — the more he has been accustomed to fix his eyes alternately on near and distant objects, — the more the extreme limits of his sight will be extended. Those who shall have THEIR USES AND ABUSES. 9 most varied these distances, will preserve the greatest extent between the positions in which they can perceive objects; for such it will some¬ times be difficult, according to the views generally entertained upon presbyopia and myopia, to de¬ termine in which of these varieties their sight should be placed; for, like the myopic, they read at a very short distance, and, at the same time, they are very far-sighted, like the presbyopic. This simple consideration suffices to prove the great latitude which the extent of vision may pos¬ sess ; the numerous and insensible shades by which presbyopia and myopia approximate towards each other, and the power possessed by the eye of adapting itself to different distances. This power is now generally admitted, and proofs of its existence are furnished by physiology, comparative physiology and anatomy, and pathology. Those furnished by the latter, the least studied, and in part completely ignored, appear to us the most important ; it is those which we shall principally consider in these lessons, which are to be altogether practical. If certain eminent physiologists have denied the existence of this faculty, it is probably on account of their ignorance of these practical facts, and also, in part, because they have them¬ selves possessed this power of accommodation in only a limited degree, or have lost it by want of exercise, or by working too assiduously upon small objects at limited distances, or by occupying them¬ selves too much in microscopic researches. Ac¬ cording to the laws of optics, this power of accom- 10 SPECTACLES .* modation of the eye to different distances ought to exist; without it we should not clearly perceive objects of the same dimensions, except when placed at the same distances. It is probably exe¬ cuted by the aid of interior changes in different parts of the globe of the eye. The axes of this organ can probably extend or shorten themselves ; its constituent parts, as the cornea, the crystalline, the vitreous humor, the retina, &c., can probably approach one another, or change their dimensions or curvature. The changes of the diameters of the pupil, the inequality of convexity of the two faces of the crystalline, its lamellar structure, the increase of its density from without inward, and, finally, a crowd of other circumstances, concur to produce this marvellous power, unpossessed by any optical instrument. The action of the muscles of the eye exerts a considerable and, perhaps, a principal influence, as has anew been proved, of late years, by ocular myotomy, after which the focus of vision sometimes changes to a notable degree. Also, when the gaze is suddenly changed from a near ob¬ ject, especially if it is of small dimensions, to a distant one, with an effort to distinguish it as clearly as possible, a painful sensation in the eye is immediately perceived. Sometimes even, after having regarded small objects a long while, as in observations with the microscope, the power of distinguishing distant objects is lost during some instants. These two circumstances, with the easy experiment before mentioned, form two arguments, THEIR USES AND ABUSES. 11 ad hominem , capable of producing a first degree of conviction in those who have not given special attention to this subject. If we reflect, also, on the prodigious extent of vision of birds of prey, who, from an immense height, recognize a small animal moving upon the soil, and, descending upon him with the quickness of lightning, seize him in a single instant with astonishing accuracy ; if we consider, also, the special organ, the ciliary processes, which, from its structure and its inser¬ tions, can but be destined to bring the crystalline and the corpus ciliare nearer to the retina, we shall be most firmly convinced of the faculty of adjustment of the eye to different distances. It is impossible to occupy ourselves here with the proofs of the existence, and the physical and ana¬ tomical explanations of this power, for which we must refer to works on optics and physiology. The second order of proofs, drawn from the pa¬ thology of the eye, is alone to be considered in these lessons. These also refute, in our opinion, the theory of the late Treviranus, who, by a de¬ tailed mathematical demonstration, believed that he had proved the structure of the crystalline, composed of concentric lamellae increasing in density from the exterior to the centre, to be sufficient to explain the distinctness of vision at so many various distances. In taking the faculty of accommodation as a starting point, we hope to demonstrate that its notable diminution, or complete loss, is the cause of presbyopia and myopia, and of the maladies 12 SPECTACLES .* which follow the ill-judged use, and the abuse, of spectacles; — we hope, also, to throw some new light upon this interesting point of physiology. § III. GENERAL CAUSES DETERMINING THE COM¬ PASS OF VISION. The sight of man, in a natural state, has gener¬ ally a somewhat extended range ; we might even say, that the greater number are born with long sight, and that, in his primitive condition, man is presbyopic. All travellers cite more or less astonishing instances of the extent of the power of this faculty in most savage tribes ; some even deny their ca¬ pability of discerning small objects placed near them, — but there is much exaggeration in this assertion, and, in those to whom it is applicable, it depends on a want of attention, or a defect of intelligence, rather than upon an impotence of the eye to exercise this faculty of accommodation, or upon any particular imperfection of the organ of vision. For others, the difficulty of discern¬ ing small objects when brought near, results from the habit of always observing larger and more distant objects, which is followed by a loss of the faculty of accommodation to short distances and small dimensions. Among civilized nations, primitive long-sight¬ edness is also the general rule ; and when par¬ ticular tastes, as that for hunting, for travelling, especially by sea voyages, solicit the frequent THEIR USES AND ABUSES. 13 employment of this sense, it soon acquires a very great power. Among those who reside in the country and devote themselves to agricultural employments, short-sightedness is rare. When, on the other hand, by inclination or necessity, the sight is assiduously, and from early life, exercised upon small and near objects, as is almost alwaj^s the case in our state of civilization, the sight, forced to accommodate itself to these distances, becomes, very early, shortened. Even before the infant can walk, objects are given him for play¬ things which he is obliged to bring near his eyes in order to see all their details. A few years later, he is placed at school : there, sometimes to distinguish the too small print of his book, some¬ times to write in a fine character, he is forced to incline forward towards his paper or his books. Born among the working classes, the child is none the more favored on that account in the education of this sense ; as soon as may be, he is placed in apprenticeship, and not only is often con¬ strained to exercise his sight upon minute details of form, but the fatigue of his arms forces him to bring the objects yet nearer to his eyes. Habit¬ uated continually to adjust his sight to these lim¬ ited distances, he finally ends, necessarily, by losing the faculty of accommodation to distant objects, and becomes myopic. 2 14 SPECTACLES .* § IV. GENERAL CONSIDERATIONS UPON PRESBY¬ OPIA AND MYOPIA. Since it is impossible exactly to determine the normal range of vision, the same difficulty must recur when we attempt to establish the precise limits where presbyopia and myopia commence to manifest themselves. In fact, we find long-sighted persons, whose vision at a distance has an extent double that ordinarily enjoyed by the long¬ sighted ; whilst others can bring small objects within half the distance at which others can see them, who, however, possess nearly the same vision of remote objects. It is the same with the short-sighted, of whom some see much further, or much more near, than others. All the ordinary standards of the range of vision, proposed to serve for defining the limits of long or short sight, are insufficient. The employment of convex or con¬ cave glasses forms a better and more sure crite¬ rion. All that can be said, in a general way, of these two varieties of vision, is this : the presbyopic sees distant objects distinctly and without fatigue ; he sees much less clearly small and near objects, and his eye easily becomes fatigued when he is forced to fix his attention upon them during a long time. That he may see them well, it is requisite that they should be in a strong light; but if he observes these objects with the aid of moderately powerful convex glasses, he sees them more distinctly, without finding them much mag- THEIR USES AND ABUSES. 15 nified, and without being compelled to bring them as near the eyes as a short-sighted person would be forced to do. Aided by these glasses, especial¬ ly if they are not too powerful, his sight is less fatigued than when it is unassisted. If he ob¬ serves distant objects through concave glasses, their impression is more or less clouded, smaller, and always less distinct than with the naked eye ; these symptoms augmenting in direct proportion to the concavity of the glasses. The myopic, on the contrary, by the aid of these last-named glasses, sees distant objects bet¬ ter than with the naked eye, and of a natural size. With the naked eye, he sees them indistinctly. They gain in clearness when observed through any diaphragm, as through a tube, through the hand nearly closed, or placed as a shade above the eye-brows, or, better yet, through a small hole pierced in an opaque body, as, for instance, a card. This is the reason that he habitually winks in regarding at a distance ; from which circum¬ stance his designation is derived, and not from mios, small, and ops, vision, (vision of children,) as a compiler has ridiculously stated. The wink¬ ing has doubtless, moreover, another effect; the contraction of the eye-lids, in compressing the globe of the eye, augments the power of accom¬ modation. Every short-sighted person may easily feel that the simple approximation of the eyelids is not sufficient, when he desires to see much further than ordinarily, but that there takes place in his eye, during the greater or less constriction 16 SPECTACLES .* of these membranous curtains, something extraor¬ dinary, which indicates a change in the habitual relations of the various parts of the organ. Look¬ ing at small objects through convex glasses, the myopic finds them considerably magnified. To see them with dimensions nearer their veritable size, he is forced to place them infinitely nearer than the presbyopic would do. These conditions are more marked with an increase of power of the glasses. Most persons, and many physicians, believe that the range of vision of an individual may be recog¬ nized from a certain conformation of the eyes. This opinion is far from being well founded. Often, it is true, we find myopic persons in whom the globe is voluminous and projecting, the ante¬ rior chamber large, the iris brown, the pupil very changeable and contracted, etc. The presbyopic have more frequently small eyes, a flattened cornea, a small anterior chamber, a blue iris, and indolent and dilated pupil. But all this is not constant : very often, of two individuals, in whom the confor¬ mation of the eyes is absolutely the same, one is long, and the other short, sighted. This is easily explicable by the inequality of curvature which may exist in the external and internal constituents of the organs of vision. The too great convexity of the cornea or sclerotica, and the exuberance of the refractive media, may be compensated, or more than compensated, by the less density of the aqueous humor and the vitreous body ; by a differ¬ ence in the axes of the globe, and in the action THEIR USES AND ABUSES. 17 of its muscles ; by an adjusting power more or less considerable, etc. Here would naturally place itself the question whether an individual may be at once presbytic and myopic ; but we adjourn for the moment its consideration, its solution requiring certain ele¬ ments which we have not yet considered. § V. SPECTACLES IN GENERAL, AND THE NUMBERS WHICH DENOTE THE POWER OF THE GLASSES. Beyond the loss of the faculty of accommoda¬ tion, which may produce myopia and presbytia, the immediate cause of the latter is a too feeble refractive power of the transparent media of the globe of the eye, or a too short antero-posterior diameter of the organ. In these two cases, rays coming from a not distant point, and being, in consequence, too divergent, unite too late, and form their image behind the retina. In order to concentrate them in a normal focus upon the membrane charged with the transmission of the impression to the brain, — collective, that is to say convex, glasses, of exactly sufficient power to supply the deficiency of refractive power in the eye, become necessary. The remote causes of the insufficiency of this power are somewhat numerous : either a too slight curve of the parts constituting the dioptric apparatus ; or the too small quantity of the vitreous or aqueous humors, as well as their thinner consistence ; or again, an o# 18 SPECTACLES : excessive muscular action, are capable of destroy¬ ing the normal convergence of the rays. The contrary is true in myopia : the refractive power of the eye is too great, or its antero-poste- rior axis too long ; as a consequence, the rays coming from distant points, which are very slight¬ ly divergent and almost parallel, will be brought to a focus in front of the retina, and, in order to paint their image upon this membrane, they re¬ quire the aid of concave glasses, of which the dis¬ persive power should be equal to the excess of refraction. The organic conditions of the eye opposite those which produce myopia give rise to presbytia. It is wrong to believe, in one or the other of these cases, that these particular disposi¬ tions must betray themselves by manifest and un¬ varying external signs. On the contrary, it is only by exception that the exterior conformation of the eye, by an excess or a want of convexity of the cornea and of the globe in general, permits us to form an opinion as to its being myopic or presbytic. In most cases, it furnishes no indica¬ tion in this respect ; the internal parts, by their disposition and structure, alone determining the compass of the sight. We generally employ bi-convex and bi-concave lenses ; that is, those of which the two surfaces have the same curve ; they are the glasses of greatest power, the most simple of fabrication and of foci most readily calculated, because it is equal to the radius. Each of the surfaces of these glasses is in fact a THEIR USES AND ABUSES. 19 segment of a sphere of greater or less diameter. The shorter the diameter, the greater will be the convexity or the concavity of the glass. The ra¬ dius of their curve, which announces at the same time the focus where the image will be produced, is employed to indicate their refractive or disper¬ sive power, and thus establish the relations be¬ tween the different glasses of the same species, and to graduate their use. The radius is calcu¬ lated in inches of the ancient French measure. The system of metres has not yet been adopted, and we do not know that any advantage would result from its application to these computations. If, for instance, we speak of a convex glass, No. 48, it is understood that each of its surfaces forms a segment of a circle of 48 inches radius. If the same number of a concave glass is spoken of, the radius is the same, but we have two segments, concave on one of their faces, and placed with their convexities back to back, thus presenting one concavity towards the eye, and another to¬ wards the object. The curvature of the arc of a circle increasing in proportion to the diminution of its radius, it follows that the numbers of glasses, reckoned by their focus, diminish with their power, the lens of the lowest number being the strongest, and vice versa. The highest number, which serves as a starting-point, and the manner of successively descending to more powerful glasses, are not everywhere uniformly regulated. At Paris, twelve years since, numbers much too strong were employed by patients when com- 20 SPECTACLES ! mencing their use. Experience has permitted me to establish that there is much to be gained by com¬ mencing with numbers of the most feeble power, and I have introduced the use of glasses which formerly were employed but rarely and almost accidentally, such as the numbers 96, 80 and 72 ; and have advised that these should be made use of at the outset, especially by the presbytic ; as we shall explain in treating of that affection. This custom has been generally accepted at Paris, although at first opticians ridiculed these elevated numbers, because they regarded them as ineffica¬ cious, or without any influence. In France and upon all the continent, the num¬ bers of spectacles are regulated according to this rational principle which we have explained. In England and in America, these numbers are merely conventional. Thus, for instance, the No. 1, corresponds to our No. 48 ; the No. 2, to No. 36, etc. Beyond the inherent inconveniences of an arbitrary and irrational scale of numbers, this sys¬ tem has also two other very grave faults, viz : that of starting from too powerful a number, as we shall hereafter see ; and that of a too rapid progression. We have said that bi-con vex and bi-concave glasses were those generally used. When there exist particular reasons for employing convex-con¬ cave, concave-convex, or plano-convex, the focus may be found by calculation, and after having made its reduction, it is to be designated by the number which would be proper to glasses of double curvature and an equivalent power. THEIR USES AND ABUSES. 21 § VI. THE FORM OF SPECTACLE GLASSES AND THEIR FRAMES, THE MANNER OF WEARING THEM, AND SOME OTHER ACCESSORY POINTS. We shall devote here a few words only to these practical questions, which are treated of ex profcsso in special treatises. Spectacles are usually manufactured of an oval form and small size, to render them more ele¬ gant ; but, as regards their utility, it is infinitely preferable that they should be large and round, covering not only the globe of the eye, but also a part of its vicinity. This is especially necessary for colored glasses employed to mitigate the im¬ pression of light, in the cases of photophobia, and congestion and chronic inflammation of the internal membranes. The border of such glasses should extend to the margin of the orbit ; otherwise, the light, especially that which is reflected from the ground, will strike upon the circumference of the globe, the centre only being protected by the darkened glass ; and the impression thus produced is doubly injurious on account of the contrast. Something similar is true of lenses, when they are oval and too small ; refraction takes place only for objects placed in front of the eye, whilst those placed above, below, or laterally, especially during the movements of the organ, present their natural image. A very disagreeable confusion and inequality of vision, and sometimes diplopia, results from this. These effects are more marked 22 SPECTACLES : when the glasses are bi-convex or bi-concave ; for then their diminished curvature at the circum¬ ference causes vision to be less clear than in look¬ ing through the centre. To obviate this incon¬ venience, periscopic glasses, that is to say, in the meniscus form, may be advantageously employed, convex-concave for the presbytic, (with predomi¬ nance of convexity,) and concave-convex for the myopic, (with predominance of concavity.) As to the glasses of cylindrical surface, I have not yet been able to form a conclusive opinion in regard to them. In general, it has appeared to me that they have no appreciable advantages, and that, if they are to be used, they should be chosen of a number a little more feeble than other glasses. The frame-work of spectacles should be light and of proper dimensions. If it be too large and broad, their immobility is lost, and the eyes are fatigued by the vacillation of the image ; if it be too narrow and heavy, the temples are com¬ pressed, pain and a feeling of uneasiness are pro¬ duced in the parts near the eye, and secondarily in this organ, and the sight is thus affected. The glasses should be neither too near, nor too far from each other ; if this consideration is not attended to, diplopia and other anomalies of vision may result. In wearing spectacles, they should be carefully placed parallel, and not obliquely to the iris ; for the oblique incidence of rays impairs the clear¬ ness of the image. If brought too near the eye, THEIR USES AND ABUSES. 23 they hinder the movements of the lids, or the transparence of the glasses is destroyed by the contact of the cilia, of tears, and of mucus. Equal care should be observed not to remove them too far, and place them more or less low upon the nose, which changes their mode of refraction, and gives them a different power from that which their number indicates. To speak generally, they should be placed as near the eye¬ lids as may be without causing them to come in contact with the cilia. In this respect, the con¬ formation of the nose, of the eyes, and the edge of the orbit, may occasion difficulties, which should be vanquished by the optician, in giving to the frames the particular form which the circum¬ stances require. Tears, transpiration, the vapor exhaled in the respiration, and that contained in the air, are deposited more or less upon the glasses of specta¬ cles. They should, therefore, be occasionally taken off and carefully wiped with a piece of fine linen, or, what is better, fine wash-leather. When they are laid aside, the surface of the glasses should not be brought in contact with the objects on which they are placed, for, especially if con¬ vex, they are easily scratched by the contact of dust, foreign substances, and the inequalities of the surfaces with which they are brought in con¬ tact. They should be placed open, on their bor¬ der; or folded, with the branches placed under¬ neath to protect the glasses. It is yet better to place them, each time they are laid aside, in their 24 SPECTACLES : case, of which the cavity should be lined with a soft material, and should have such a form and such dimensions that the glasses should not rub in entering. Before replacing them on the eyes, they should be wiped. If these particulars are neglected, the glasses are scratched, lose their polish, and become opaque in spots or striae ; and these defects, if not early perceived and remedied, alter and enfeeble the sight. The same result ensues if they are used when dim with dust, vapor, the impress of the lingers ; or if the glasses were originally imperfect, — being scratched, of uneven surface, or containing bubbles of air, mi¬ nute foreign substances, or other defects. The purity of the material of the glasses and the polish of their surfaces are essential ; they should there¬ fore be very carefully made from the hardest glass, or from crystal. § VII. INFLUENCE OF THE FACULTY OF ACCOMMO¬ DATION UPON THE PRESERVATION OF A PROPER FOCUS IN THE PRESBYTIC AND MYOPIC. Whatever may be the compass of vision, it is very important that the faculty of accommodation should be assiduously exercised ; but not in a man¬ ner too constant, or too long continued in the same direction, that is, not always at short or at long distances. It is necessary to exercise the sight alternately upon near and distant objects. The reasons for doing so are easily explained. THEIR USES AND ABUSES. 25 In the first place, all exercise of the accommo¬ dating power is more or less fatiguing, since it is analogous to muscular action, and perhaps even consists in a kind of contraction of the muscles of the eye, as may be readily perceived in one’s own person upon looking fixedly at very near or very distant objects. All excessive and irregular mus¬ cular action becomes fatiguing and injurious to the organ. On the other hand, an adjustment too long continued to distances always identical, whether at minimum or maximum, deprives the organ of vision of its power of adaptation to less or more considerable ranges, and may prolong or shorten the focal distance beyond due bounds, and permanently. Let us explain by giving some examples. The exercise of the adjusting power is particularly fatiguing to the presbytic, when he is desirous of employing it beyond its just limits. If, for instance, he reads a fine print, during too long a time or at too short a distance, his sight becomes dim, he no longer discerns clearly. He feels in the eye the sensation of a painful effort. If he persists a long time in this exercise, and repeats it often, his sight will lose its natural range for ob¬ serving distant objects, or will become enfeebled, as we shall hereafter see. These troublesome effects will be the more marked in the presbytic, for the reason that presbytia augments naturally with age, and therefore allows an adaptation to short distances less readily and in a less extent than myopia. The latter, in fact, diminishing with the progress of years, permits this accommo- 3 26 SPECTACLES .* dation, without inconvenience, for distances which more or less surpass the ordinary focus. These considerations will cause the hygienic rules, which we shall lay down for each of the varieties of vision, to be better understood, at the same time that the}r will aid to make their impor¬ tance appreciated. PRESBYTIA, OR PRESBYOPIA. § VIII. HYGIENIC PRECAUTIONS TO BE OBSERVED BY THE PRESBYTIC. Since civilized life requires the frequent exer¬ cise of the sight at a nearer distance than the focus of the presbyopic will, as a general rule, permit, — in order to avoid the consequences which may follow this neglect of the physiological laws of the sense of vision, those affected with presbytia should follow certain hygienic rules. Whilst reading, writing, or engaged in other occu¬ pations requiring minute attention, the objects should be removed to the longest possible dis¬ tance at which their distinctness shall be unim¬ paired. This distance may easily be determined by moving the object backwards and forwards, and should always be maintained during assiduous employment. During other less pressing occupa¬ tions, — those rather of leisure than professional, — they should, on the contrary, exercise their sight at less distances, in order to preserve the faculty of accommodation, and to accustom themselves to see objects within their normal focus. But, during THEIR USES AND ABUSES. 27 both these kinds of occupation, they should be careful to interrupt themselves frequently, in order to change their sight to the most distant objects they can perceive. Every presbytic would do well, after five or ten minutes’ work upon minute near objects, to raise the eyes and turn them for an instant to the most distant parts of the room, or even, if he be near a window which is not entered by full sunlight, to look out from his apartment and endeavor to distinguish the details of distant objects. This exercise, by alternately lengthening and abridging the range of vision, preserves the faculty of accommodation and the normal focus, at the same time that it prevents the enfeebling of vision. An interruption of a few instants, often repeated, is sufficient to ensure all the salutary effect of this exercise. I have often known all the menacing fatigue felt by the presbytic after long uninterrupted labor, though sometimes already preluding amblyopia, to yield after using this means only. This fatigue is expe¬ rienced by them whenever they continue their employment during a long time without reposing the eyes. It seems that Taylor had already fore¬ seen the profit to be drawn from so simple a rule. This oculist of the middle of the last century, who, though charlatan, was neither wanting in quick perception nor experience, though his knowledge and skill were far below what has been ascribed % to him by his biographers ; this ambulatory ocu¬ list already knew, and made great account of, the utility of this alternating exercise. I read, some 28 SPECTACLES ! time since, in a work of Eschenbach, published in German in 1752, and now rare, (a very important work for the just appreciation of this man so little known even by his biographers, and who has usurped so great a celebrity,) that he sold at a very high price a pretended secret for preserving and fortifying the sight, — a secret which consisted only in frequently removing the eyes from near objects to those at a distance. During the more than ten years since observation and reflection have induced me to recommend this prophylactic means, I have often been answered with the objection that it would be troublesome to inter¬ rupt so often the course of avocations ; but I re¬ peat, that a cessation from them for a half minute, or even a shorter time, is sufficient, — and the habit once fixed, the interruption is no longer even perceived. The better these rules shall be observed, the more the eyes will endure usage, and the later the necessity of wearing spectacles will be felt. In proportion as one is obliged to occupy himself in minute details, will it be impor¬ tant for him to constrain himself to obey them. 