On correction of the myopia the facultative divergence amounts to = 26.

Here one might easily have concluded the asthenopia to be a result of fatigue of the interni, but this opinion was refuted by the effect of the treatment. The double tenotomy of the externi performed on January 2nd was first followed by convergent squint with h.o.m.onymous double images, which were united by a prism of 12 with the base inwards. In the course of a few days single vision was again restored. A fortnight after the operation, on correction of the myopia, patient could see singly to 3 mtr.: towards both sides h.o.m.onymous double images were still present, and in fixation to 30 cm. relative divergence on exclusion of one eye. Six months after the operation, on correction of the myopia and application of red gla.s.s to one eye, crossed double images occur close together, which become h.o.m.onymous by means of a prism of 3 with the base inwards. Patient sees double images always, without being much disturbed by them, yet they cannot be united by means of prisms. The habit of binocular single vision has also gradually been lost. In reading (without correction) a movement of convergence takes place (it cannot be determined whether this answers exactly to the distance of the object). If, on the other hand, one asks the patient to fix binocularly larger objects, such as a pencil close to her, she is unable to do it, relative divergence occurs then, as well as on exclusion of one eye. The asthenopic disorders remain unchanged and are not removed even by prismatic spectacles. Despite all reasons then for the supposition the asthenopia was certainly not of a muscular nature.

The uncertainty as to diagnosis is still greater in those cases which, according to v. Graefe, were to be designated as dynamic relative divergence; cases in which with parallel visual axes a disturbed balance is not present but occurs on convergence in such a way that the interni only perform their destined work with difficulty, and are nevertheless urged on in the interest of binocular single vision, till they give way in painful fatigue.

According to v. Graefe the diagnosis of this condition must be carried out in the following way. First of all the convergence must be fixed on a near object in the median line; if one eye remains behind in the movement it may be accounted for in various ways, for example, the impediment of movement caused by the change in form of the eye in myopia or the faulty innervation of the interni mentioned on p. 54. In both cases for the most part no dynamic, but manifest relative divergence is present in viewing near objects. It may also happen that the patient does not converge sufficiently, merely because accommodation is absent.

This experiment does not then prove the presence of dynamic relative convergence, and v. Graefe came to the conclusion, therefore, that a normal position of the eyes obtained only by the habit of binocular single vision must be relinquished so soon as we cause binocular single vision to cease. Just as under these circ.u.mstances dynamic absolute divergence is manifested in the observance of distant objects, so must this be the case in dynamic relative divergence in the observance of near objects. One eye is first excluded while looking at an object about 25 cm. distant, to determine whether it still remains in a proper position for fixation. We have reason to believe that the position which occurs in the excluded eye answers to the given conditions of tension of the muscles. Still it is not necessary to cause binocular vision quite to cease, it is sufficient and even more advantageous, simply to make binocular single vision impossible, which we are able to do by means of prisms. If, for example, a point be fixed lying at the usual distance for work of 25 to 30 cm., or, according to v. Graefe, a large spot intersected by a vertical line, and one then applies a vertically deviating prism to one eye, the influence of binocular single vision on the ocular muscles is removed, as the fusion of the double images standing above one another is impossible; and nothing prevents the a.s.sumption of a relative position of divergence instead of a proper convergent one; as a result of this the double images show a crossed lateral position as well as the difference in height produced by the prism. The extent of this lateral deviation may be measured by means of prisms, which being applied to the eyes with the bases inwards place the double images again perpendicularly above one another. Von Graefe holds it to be of importance to determine the strongest prisms which can be overcome for the given distance by means of convergence and by the outward movement of the eyes.

On the strength of this method of inquiry there is a prevalence of opinion that the asthenopic disorders common in myopia are caused by over-exertion of the ocular muscles; indeed people believe this so strongly that they a.s.sume the presence of muscular asthenopia even in individuals in whom the habit of working with relative divergence is already firmly rooted. Relative divergence may perchance cause annoyance through double images, though this really seldom happens, but it can never cause muscular asthenopia, for the internal recti muscles protect themselves by means of relative divergence from any stronger exertion.

Asthenopic disturbances are certainly frequent in myopia, but the above method of inquiry does not at all prove that their cause lies in the ocular muscles, for those appearances from which one concludes dynamic relative divergence and muscular asthenopia, are found in almost all myopes, even when the latter have no asthenopic troubles, for they owe their origin to the nature of the myopia. Myopes learn to converge to the distance of their far point, without exerting the accommodation; if we now cause a point at this distance to be fixed and then exclude one eye, or make binocular fusion impossible by means of vertical prisms, what imaginable reason is there for the excluded eye to remain in proper fixation? In emmetropia the habitual relation between accommodation and convergence will be able to ensure that the excluded eye also remains covering the fixed object, convergently as well as accommodatively; in myopia, every discretionary relative divergence up to parallelism of the lines of vision is perfectly justified, because no effort of the accommodation takes place. How in the world can it be held to be pathological that a movement of convergence does not occur, when one has just artificially removed all those physiological conditions which could possibly have brought it about? If one now likes, as v. Graefe proposes, to determine the prisms, which can be overcome by means of the outward movement, there is no doubt about the fact, that with the aid of prisms the lines of vision may be made parallel or even divergent, the retinal images indeed, always retaining the same distinctness, in so far as they are not injured by the prismatic diffusion of colours. There is just as little reason why the convergence usually attainable should not also be restored by the aid of prisms with the bases outwards, the retinal images are not only impaired by the prisms, but the accommodation united with the convergence, no longer corresponds to the real distance of the fixed point.

Enough, all these incidents, which are to prove the presence of muscular asthenopia in myopia, occur when the investigation is carried out as usual in the region of the far point, entirely on a physiological basis, and must not therefore be held to be pathological without further proof.

The proof of muscular asthenopia in slight degrees of myopia, emmetropia, or hypermetropia, is somewhat more certain; a deviation from physiological laws is certainly present, if we find that the corresponding convergence does not unite itself with the accommodation for a near object, we must be quite sure that an exact accommodation for the fixed point is also really present. It by no means follows because one causes a large black spot to be observed at a distance of 25 to 30 cm., that an exact accommodation takes place; one can see these things even with circles of diffusion, the retinal images are already dimmed by means of the prisms, and one can easily convince one's self that, on the renunciation of clear retinal images, normal eyes can reach every attainable convergence or relative divergence by means of prisms.

Insufficient accommodation and defective convergence are, however, easily caused by all painful sensations situated near the eye, which make the accommodation uncomfortable and fatiguing. This applies to every common head- or tooth-ache, and in the same manner to disturbances arising in the conjunctiva, or which depend on the stretching of the collective tunics of the eye in myopia, or which allow any other so-called "nervous" origin to be suspected.

We must place the same claims to the diagnosis of muscular asthenopia as to that of the accommodative form. Just as the latter is only detected if convex gla.s.ses really give the expected relief, so the proof of muscular asthenopia is only furnished when relief to the interni is brought about by means of the appropriate remedies. For myopes, who do not fall back on the aid of relative divergence, notwithstanding that they possess a clear field of vision only attainable with difficulty through convergence, it is the simplest plan to remove the far point to about 25 to 30 cm. by specially adapted concave gla.s.ses. If only slight myopia or none at all is present, but the relation between accommodation and convergence is disturbed, the latter can be corrected by means of prisms with the bases inwards--to be sure, only in a slight degree, as prisms of more than 4 are scarcely suited for spectacles, partly on account of their weight and partly on account of the diffusion of colours. Prisms may be ground with concave or convex surfaces, according to the requirements of refraction or accommodation.

Finally, if an elastic preponderance of the externi can be proved by means of considerable facultative divergence, the same may be lessened by tenotomy of one or both externi; still after my own experience I cannot advise the performance of this operation unless prisms of at least 16 are overcome by absolute divergence, for I have seen many patients in other practices who have acquired convergent squint and diplopia for distance as the sole result of the operation, while the asthenopic troubles for near objects continue. The proof that it is not a case of muscular asthenopia is sometimes only obtained by the operation.

BINOCULAR VISION IN SQUINT.

The fact that those who squint do not as a rule have diplopia, while squints depending on paralysis of the ocular muscles are combined with diplopia, was difficult to explain as long as the view was adhered to of identical retinal areas founded on anatomical construction. The first explanation hit upon was that a false ident.i.ty became established, an inequality of the retinae; were this the case diplopia must of necessity occur on correction of the squint by tenotomy.

Commencing with the a.s.sumption of a congenital ident.i.ty which led under all circ.u.mstances to the occurrence of diplopia as soon as the images of the same object fell in both eyes upon non-identical points of the retinae, the hypothesis was next advanced that the image of the squinting eye was not perceived, that a constant suppression of the sensations in the squinting eye took place. Suppression of sense-impressions does take place; as soon as our attention is entirely engrossed upon anything, we are in a position to disregard the impressions upon all other organs of sense; they do not reach our consciousness. That visual sensations are easily disregarded may be proved by experiments. Hold a small plane mirror obliquely before one eye, with the brim pressed into the angle of the nose so that the objects lying at the side and behind are seen in the mirror. If the other eye is now used to read with, it is quite easy to disregard the objects seen in the mirror provided that our attention is not attracted to places by a particularly bright light. No doubt those who squint also possess this physiological power, and it is therefore certain that they make use of it under certain circ.u.mstances; but the suppression theory necessitates that they should constantly and always do so, since diplopia is bound to occur directly they do not do it.

The absence of double vision is in fact the only evidence that can be adduced in favour of the exclusion theory; this negative fact, however, proves nothing, and is, moreover, capable of other explanations, as soon as one abandons the theory of congenital retinal ident.i.ty. The examination of those who squint demonstrates the untenability of this theory. People who squint seldom complain of diplopia, but double images can be rendered apparent in a comparatively large proportion of cases, usually with the greatest ease, by covering the best eye with a red gla.s.s and squinting with a vertically deviating prism. Many squinters now admit the presence of double images, but their position by no means corresponds to the ident.i.ty theory, their lateral displacement is far too slight, or patients find themselves unable to localise the position of the image. It sometimes happens that alternating vision with both eyes is mistaken for diplopia, the images are then invariably specified as h.o.m.onymous; however, with attention it is easy to distinguish this alternating vision from the simultaneous perception of two images of one and the same object.

There can be no doubt that in most cases the position of the double images does not correspond to the principle of ident.i.ty, and just as little doubt that one to whom double images are easily made apparent cannot possess the confirmed habit of always suppressing the image of the squinting eye. A certain number of cases remain in which it is impossible to produce diplopia; that these, however, do not constantly suppress the image of the squinting eye may be proved in the very simple way I have indicated. An object of fixation is placed in a darkened room, on one side of and behind the squinting eye is placed a small flame, the reflection of which, by means of a plane mirror before the squinting eye is thrown upon its retina. The reflection of the flame is seen on the cornea of the squinting eye, by slight rotation of the gla.s.s it can be brought into the area of the pupil, and at the same instant the patient sees the light, the reflection of which can easily be made to coincide with the image of the fixation object seen by the other eye.

The experiment has then an entirely objective basis, it always succeeds, a fact on which I lay special stress, even in eyes whose vision is very defective; therefore here also the habit of suppression of the retinal images of the squinting eye is not present.

That the squinting eye really possesses its full share of the visual field can easily be proved (especially in divergent squint) by the aid of a perimeter. The best eye is covered with a red gla.s.s, so that the objects projected from the fixation point, as well as the excentric field of vision of this eye, appear red. As soon as the test object moves towards the side of the squinting eye and enters the visual area covered by the latter, it appears in its natural white colour, and this in most cases before it has reached the centre of the retina of this eye.

Another proof that the squinting eye is really used for vision appears to me to lie in the fact that persons who squint, provided of course that the vision of the eye concerned is not very defective, do not show that uncertainty in the estimation of distance, which is apt to prove so troublesome to those who have only monocular vision.

[Ill.u.s.tration: FIG. 1.]

If, then, the view of the constant suppression of the retinal images of the squinting eye is untenable, how is it to be explained that squint as the result of paralysis of the ocular muscles causes diplopia, while concomitant squint does not? The answer to this question is clear as soon as we abandon the supposition of a congenital retinal ident.i.ty, and look instead upon the relation of the eyes to each other as harmonious; ident.i.ty, or co-ordination as something acquired. Central fixation is congenital and depends upon anatomical conditions, for as the macula lutea is anatomically the most perfect part of the retina, it is natural that the new-born child soon learns to place this part of the retina opposite objects which attract its attention, and therefore those relations of the eyes to each other are naturally developed. For instance, if both eyes (Fig. 1) are directed to the distant point _a_, the image of point _b_, situated at the same distance, will fall on the inner half of the retina of the left eye; the left eye will now learn by experience to refer inner retinal images to objects lying to the left of the fixation point; at the same time, however, with binocular fixation, the right eye learns to seek the images of the temporal half of its retina in the left field of vision, and _vice versa_. From this it is easy to trace the laws of binocular diplopia. For example, let _a_ in Fig. 2 be the fixation point, while at the same time the image of _b_ belongs in both eyes to the temporal half of the retina. Now, as we have already seen, the right eye has learnt to refer temporal retinal images, to objects lying to the left of the fixation point, while for the same reasons the left eye projects temporal images to the right. While then point _a_ is seen binocularly singly, point _b_ appears double, and certainly the image of the right eye is projected to the left of the fixation point, and that of the left eye to the right of it, in other words, crossed diplopia is present. But the eyes are divergent relative to point _b_; double images then which occur as a result of divergence (whether relative or absolute) must appear crossed, and one will easily be able to infer that for the same reasons those double images which occur in consequence of convergence, must be h.o.m.onymous. All this, however, only with the presupposition, that the habit of binocular fixation is already fully developed; any disturbance of the same, in whatever way (by prisms, mechanical displacement of the eyes, paralyses of the ocular muscles, or by those forms of squint which arise after childhood is past) causes the double images to ill.u.s.trate the law above explained. Certainly diplopia may be absent even then, but only in very rare instances. Now and then this happens in objectively proved ailments of the ocular muscles, where the patients complain of disturbed vision, which disappears immediately on the exclusion of one eye (see Case 12), a method of relief they usually discover for themselves; thus the indistinct vision is seen at once to be a disturbance of binocular vision. Many such patients learn to see the double images which formerly escaped them, after they have been instructed how to do so during the examination. With others, all efforts are in vain, it is impossible to render them conscious of the double images, notwithstanding that the presence of the binocular disturbance of vision proves that the habit of binocular fixation exists. This apparent contradiction is explained, if one reflects that the physiological basis of vision rests on a series of conclusions. The first thing which strikes us as a result of binocular fixation is, that the images of the centres of the retinae may be referred to one and the same region of the room, and this experience will be retained, even if the images on the centres of the retinae represent different objects in consequence of paralysis of the ocular muscles; the images are notwithstanding referred to one and the same part of the room, all objects are thrown together promiscuously, and the consequent embarra.s.sment is of course removed directly one eye is shut.

The experience of those patients whom it is impossible to render conscious of double images, despite the habit of binocular fixation, reaches up to this point. A second conclusion belongs to diplopia, and for that it is necessary to seek out from the confusion of objects, the two retinal images belonging to one and the same object, and the majority of people, though not all, take this second step also. It is seen at the same time that the opinion held by Donders, that diplopia is absent in squint, does not suffice, for this reason, because the image in the deviating eye is too excentric. What becomes then of the image lying in the centre of the retina?

[Ill.u.s.tration: FIG. 2.]

The absence of diplopia in squint may be explained quite simply by the fact that the habit of binocular fixation has not been learnt or has been forgotten; one can learn nothing that cannot be again forgotten.

The normal fusion of the visual fields can only develop in consequence of binocular fixation, and diplopia is only possible when some kind of binocular fusion exists. If no binocular fusion exists, then all possibility of diplopia is excluded. And why should those who squint from their earliest childhood not see well with both eyes, but yet with each separately, just as is the case with animals with laterally placed eyes? For example, in Fig. 3 there is convergent squint of the left eye, the right eye fixes the point a, whose retinal image is cast at _a_' in the left eye; the direction outwards in which these images are projected is discovered by drawing a straight line from _a_ to _c_ (the optical centre of the eye); suffice it to say that point _a_ is seen by each eye in the direction in which it really stands.

[Ill.u.s.tration: FIG. 3.]

But although both eyes see at the same time, yet the close relation which in binocular fixation develops between the centres of the retinae does not occur in squint; firstly, because the retinal area in the squinting eye which corresponds to the fixation point is too excentric, and secondly, because the angle of the squint often changes. In binocular fixation, the fixation point of one retina answers to the corresponding point of the other; in squint, on account of the varying size of the squint angle, if a like relation develops between the eyes, the fixation point of one retina must correspond to a larger area of the other. Possibly this explains a fact that is often to be observed. In those cases of squint where diplopia can easily be caused by covering one eye with a red gla.s.s and the other with a vertically deviating prism, the double images disappear on rotation of the prism round the axis of vision, as soon as the angle of the prism reaches an angle of about 45. The occurrence of double images shows that there exists for the upper and lower parts of the retinae a community of vision by no means coinciding with the ident.i.ty principle. The disappearance of the diplopia can be explained by the fact that the variations of the squint angle take place chiefly in the horizontal direction. Therefore the area in the squinting eye that corresponds to the fovea centralis of the fixing eye must be more extensive in the horizontal than in the vertical direction. Alfred Graefe has designated this phenomenon as "regional exclusion." Whilst then a sort of community of vision exists for the upper and lower parts of the retinae, the sensations of the retinal area lying in the horizontal plane of the macula lutea of the squinting eye must be suppressed. The physiological occurrence of a suppression of the retinal images, as far as we are able to investigate it, always refers to the whole retina; however, the possibility of a "regional exclusion"

should not be excluded to begin with; but in the inductive sciences it is for us to ask first, whether an incident really happens, and not whether it is possible. The fact from which Alfred Graefe draws his inference is not, as we have just seen, to be explained in any other way, and the ophthalmoscopic test described on p. 65 proves that also in these cases of "regional exclusion" both eyes are used for vision.

In many cases of periodic squint the condition of binocular vision is very interesting. Binocular fusion may be quite absent even in normal position of the eyes; on the other hand the non-occurrence of diplopia in squint does not prevent the occurrence of perfect binocular fusion with a normal position. In periodic outward squint I have sometimes seen binocular fixation without the existence of binocular fusion; the excluded eye deviates outwards, but as soon as it is free it puts itself into fixation, whilst neither with prisms nor stereoscope can anything other than alternating vision be proved, _i. e._ neither binocular diplopia nor fusion.

If squint arises when the habit of binocular single vision has become confirmed, diplopia is always present, at least at first; even children of six to seven years old make this statement uninvited, but they soon get accustomed to the new relations, and after a short time it is impossible to make them see double images (see Case 42). Habits cling more closely in adults, therefore that form of convergent squint in particular, which usually develops quickly in myopia of average degree, causes annoying diplopia to last for a longer time. For just when these patients want to employ binocular vision in order to estimate distance correctly, diplopia occurs to hinder and confuse them.

It is otherwise with the relative divergence which is developed in consequence of myopia. At first diplopia is present here for a short time; in this case circ.u.mstances are specially favorable to a temporary suppression of the deviating eye; the fixing eye receives large distinct images to which the attention is directed. Meanwhile the relatively divergent eye is usually turned to other more distant objects that furnish indistinct retinal images, from which the attention is easily diverted. The habit of suppression may become so dominant that binocular fixation continues to exist for distant objects and the presence of binocular fusion is easily traceable, while for near objects, which are monocularly fixed with relative divergence, it is impossible to render the patient conscious of the images of the deviating eye.

Considerable squint is by no means necessary for the cessation of normal binocular single vision; slight, frequently recurring deviations are quite sufficient, as in those cases where want of control renders physiological innervation for convergence more difficult. Double images are present here, although not in a troublesome way, as is usual in relative divergence, but binocular single vision does not exist even for distance. The reason for this does not lie in the impossibility of fixing the same object simultaneously with both eyes, for the objectively proved deviation may be extremely slight. A union cannot be obtained even by prisms. If crossed double images are present close together, a prism of a few degrees base inwards suffices to make them h.o.m.onymous. The habit of binocular single vision is lost, in consequence of that disturbance to the innervation of the interni which is designated as insufficiency of the same.

The stereoscope, as well as the prism, is useful for testing binocular single vision, especially when it is suitably modified for the purpose.

The prismatic gla.s.ses usually attached to stereoscopes are here quite superfluous. The advantage of the prismatic deviation consists solely in the fact that the centres of the images fixed for the macula lutea on each side can be removed farther from one another than the distance apart of the eyes amounts to, so that a greater extension of the visual area is rendered possible. Ordinary stereoscopic pictures are quite useless for testing binocular vision; it is a question here of employing diagrams, which contain on the one hand very prominent identical figures stimulating binocular fusion but which, on the other hand, offer for each eye special attractions not present in the visual field of the other. Further, it is desirable to regulate the stereoscope so that the gla.s.ses are not firmly inserted, but that gla.s.ses from the trial case may be applied according to the condition of refraction of the patient and the distance of the stereoscopic images.

The stereoscope is generally used with the greatest advantage in those cases where there is no conspicuous deviation, and by testing binocular vision conclusions may be drawn as to whether normal binocular fusion exists or has disappeared in consequence of the squint.

It is desirable to use both methods of investigation, that with the stereoscope as well as prisms, as each test has its own value. One who at once combines the stereoscopic fields of vision certainly has binocular single vision; in other cases this is only so far lost that the stereoscopic combination does not take place at once but only after some trouble. Care must be taken, especially when one eye has defective vision, that the corresponding visual field contains objects sufficiently large and easily recognisable, as very small objects which do not correspond to the lowered visual acuity are easily overlooked. It sometimes happens that both fields are seen at the same time, but that there is no fusion; finally it happens frequently that there is complete suppression of one visual field. In testing with prisms it may appear doubtful as to whether binocular fusion or suppression of one eye exists; however, the stereoscope at once gives us certain information.

It must not be forgotten that the altered relations between the eyes, which are always possible in squint, also appear at the same time; he who sees double with prisms, may yet be able completely to suppress the stereoscopic visual field of one eye. Binocular fusion, suppression of the squinting eye and simultaneous vision with both eyes without binocular fusion can alternate in the same individual. Von Kries has come to the same conclusion, and if our colleague is unable to explain all the phenomena of binocular vision that he could observe in his own case, we need not be astonished if we sometimes hear from our patients statements that appear incomprehensible and unphysiological.

At any rate it is evident that the absence of diplopia in squint can easily be understood, without adopting the arbitrary idea of a constant, habitual suppression of the image of the squinting eye.

VISUAL ACUTENESS OF THE SQUINTING EYE.

Whether the state of refraction or the condition of the muscular equilibrium is held to be the chief cause of squint, defective vision of one eye will always have to be acknowledged as one of the most important favouring circ.u.mstances; in order to cure squint it is important to have regard to the visual acuity of both eyes, and not only to the defective condition. But this is no easy matter.

First it is to be observed, that most cases arise at an age when an objective determination of refraction is possible, but when the visual acuteness cannot be determined. Even in children who have received slight instruction, it is frequently difficult to distinguish whether imperfect knowledge of the letters or faulty visual acuteness is the cause of the non-recognition of the test-letters; when testing the vision of children it is often better to use figures than letters.

Further, in these cases it is much to be desired that the habit of determining the refraction and visual acuteness at the same time should be discontinued, particularly in reduced visual acuteness, as the test-tables only contain a few letters, which have to be recognised at a distance of 5 to 6 metres. If they have once been read with one eye it may easily happen that in testing the second eye they are repeated from memory, without being clearly recognised; even a child soon learns the few letters by heart. Therefore, when it has been a case of determining the visual acuteness I have always conducted the examination at a distance of one metre, as the choice of letters or figures which can be employed at this distance is much larger than for greater distances. In every case the reading of test-letters must be used as an additional means of examination. We must never forget that the test of vision is a perfectly subjective examination, and that we are obliged first of all to accept the statements of patients as they are given without knowing what they are worth. I have met with patients in the most highly educated cla.s.ses of society who, in intra-ocular troubles, for example, haemorrhage of the retinal artery in the macula lutea, could not distinguish the largest type in the first examination, and the next day (perhaps with slight difficulty) could read small print.

Such inaccuracies may continue to exist during repeated examinations and for long periods. One of my patients, for instance, who first came under treatment in the year 1873, had extreme myopia in the left eye with good visual acuity; with the right eye, which was also myopic, and had suffered for several years from choroiditis of the macula lutea he could read only No. 20 Snellen, and a year later 7-1/2 was read with difficulty, word by word. Choroiditis of the macula lutea gradually developed in the left eye, and in the same proportion the statements as to visual acuteness of the right eye improved, so that finally at the end of 1881, 05 was read with difficulty with this eye, while the left still sufficed to read 04 (at about 5 cm.). As I tried to comfort the patient, who was very anxious about his left eye, with the fact that the right eye had considerably improved in the course of the year, he replied that he might previously have seen just as well with the right eye if he had only taken the trouble, this was certainly my own opinion.

The attention and intelligence shown by patients during examination materially influences its results, and one should never hold the first trial of vision to be conclusive. We must always remember, however, that all conclusions drawn from visual acuteness become more unreliable in proportion as the latter is slight. We must attend to some peculiar difficulties in testing the vision of those who squint or we shall be liable to make great mistakes. When testing the squinting eye, particularly in children, it is not sufficient merely to cover the other or to hold the hand over it, for they know how to bring the usual eye into fixation by holding the head on one side or peeping between the fingers; we must keep it carefully closed with a bandage.

It is still more frequently the case that visual acuteness is stated to be less than it is in reality. The result of always using the better eye for fixation is, that fixation is not learnt with the weaker one. Even where there is no squint we see very frequently that in one-sided hypermetropia the accommodation is only used in that proportion which has become habitual to the emmetropic eye and does not therefore suffice to produce clear retinal images, while good visual acuteness is obtained by means of the correcting convex gla.s.ses. In the case of squinters (even without difference of refraction) it happens very frequently that the first statements as to the visual power are considerably below the truth. Patients who a.s.sert that they can only read the largest print with difficulty, frequently read smaller, and even the smallest type without more trouble, and we must be careful to ascertain this at first.

Accurate reports are usually obtained more quickly by means of convex gla.s.ses or eserine. In any case insufficient accommodation is, according to this, one of the difficulties, but not the only one, which has to be overcome before the squinting eye can be put into fixation. We can understand that the innervation necessary for distinct vision can be set aside even without loss of visual acuteness, just as we see the movement of convergence disappear without the interni losing their capacity for contraction.

In order to explain the relation between squint and defective vision, we must first consider the question hitherto neglected, or what is worse, answered with preconceived opinion, as to whether the same form of defective sight which is so common in squint also occurs without squint. No one doubts the existence of congenital amblyopia, nevertheless it has received but little attention in the handbooks on ophthalmology. Leber, for instance (in the well-known compilation, vol.

v), does not mention it at all.

A more or less considerable reduction of visual acuteness, with good field of vision, normal sense of colour and normal ophthalmoscopic condition, are characteristic of congenital amblyopia. Colour-blindness may of course be present at the same time. I also hold as probable the very rare occurrence of congenital defects of the visual field in good central vision, but I will reserve for the present the few observations I possess on the subject.

Together with congenital defective vision we must consider the depreciation in visual acuteness usually present in nystagmus, although it might be a.s.serted that it can neither be the cause nor the result of the nystagmus, for we find very considerable degrees of congenital defective sight in both eyes without nystagmus, as well as nystagmus with remarkably good visual acuteness. Not to complicate the question, however, I have excluded all cases of nystagmus from the following investigation. All cases of myopia of higher degree (_i. e._ of more than 6 D.) have also been excluded, as in such cases for various well-known reasons the full visual acuteness is never present. In the case of individual patients who remained for years under my observation I have been able to convince myself that visual acuteness decreased in accordance with the increase of myopia; on the other hand, however, it appeared to me very probable that just those cases of myopia, which from the beginning do not possess full visual acuteness, have a special tendency to increase quickly.