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 glasses 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 glasses. 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 identity became established, an inequality of the retinæ; were this the case diplopia must of necessity occur on correction of the squint by tenotomy.
Commencing with the assumption of a congenital identity which led under all circumstances to the occurrence of diplopia as soon as the images of the same object fell in both eyes upon non-identical points of the retinæ, 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 circumstances; 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 identity. 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 glass 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 identity 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 homonymous; 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 identity, 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 glass 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.