Page images
PDF
EPUB

YP and Y'P', coming from a very distant object, pass through the external halves of the concave glasses G H and H' G', they are deflected outwards, that is to say, toward the bases of the prisms formed by these portions of the lenses. Hence the eyes, instead of being directed parallelly, are obliged to converge in order to receive the images on their fovea centrales F and F,- that is, give their lines of fixation the directions F B and F' B, as if the object were at B.

[graphic][subsumed][subsumed][subsumed][subsumed][merged small]

If, on the contrary, an object A be looked at through the inner halves of concave glasses (Fig, 137), the latter will act like prisms having their apices turned toward the temples. The rays coming from A will undergo, at P and P', a deviation toward the bases of the prisms, and the lines of fixation will converge less, as if the object had changed its position from A to B.

When a concave lens is large enough in order that, its axis coinciding with the median line, both eyes can simultaneously look through its edges, it increases the effort of convergence, necessary for the binocular fixation of an object, by a number of metre-angles

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][merged small]

equal to the number of dioptries by which it diminishes the eye's refraction.

Let LL' (Fig. 138) be a lens of this kind, A an object situated at

1 m.

a distance of on the median line X X', which coincides with the

6

[ocr errors]

axis of the lens. If the refractive power of the latter amounts to 4 D, it increases, by 4 dioptries, the refraction necessary for the distinct vision of the object, and the convergence, required by binocular vision by 4 metre-angles. Hence the object will seem to be at B, distant from each eye, and the lines of fixation will be obliged to assume the directions F B and F' B.

1 m.

10

A lens of 5 dioptries would require, on the part of the emmetrope, in order to see at a distance, an accommodation-effort of 5 D and a convergence of 5 metre-angles. A myope of 5 D, who should use it as his correcting-glass, would not need to accommodate. It might initiate him into the art, so useful to myopes, of converging without accommodating.

But, for this purpose, we possess a much surer and simpler means -that is to say, atropine. Moreover, so powerful is the solidarity between the two functions that, in forcing convergence, we should probably only succeed in increasing the accommodation, which is already too much brought into play by young myopes. Indeed, since all attempts to increase, by exercise, the faculty of convergence-which is much less powerful than that of accommodation-offer scarcely the slightest chance of success, it is best to address ourselves to the means that we have already mentioned: a fortifying regimen in general, and ocular hygiene in particular, in the most benign cases. If the insufficiency of the internal recti is very pronounced, an operation will be indicated.

In the intermediate cases, we have recourse to a palliative treatment; instead of trying to overcome the evil by means of exercise with convergent prisms, we shall give the patient divergent ones, which enable him to see at the required distance with less convergence.

Thus it is that the prismatic effect of concave glasses may render real service. They are to be decentered, so that the patient shall look through the inner portion of the concave lens, and thus the effort of convergence necessary for the fixation of any object, seen through spectacles of this kind, will be diminished.

In virtue of the apparent displacement of objects which they may produce, it is very important to place concave glasses, as well as convex ones, at the same height in front of the eyes. Strong ones, especially, might, if this precaution were not taken, cause a very troublesome diplopia.

Just as in the case of biconvex glasses, the prismatic effect of biconcave glasses is manifested in all meridians. Only the lens may be regarded as composed of an infinite number of prismatic sections,

[ocr errors]

whose apices all converge towards its centre. It follows from this that, if one looks through the edge of the glass, so that it cuts the pupil in halves, objects are seen double, in part directly and, on the other hand, through the prism which displaces them toward the centre of the lens. If, then, the eye, wearing a convex glass, has a gap in its field of vision, the concave glass, on the contrary, favours it with a zone in which it sees the objects twice.

Among the most important secondary effects of concave lenses, we have finally to bear in mind the cylindric action which they exert when a pencil of luminous rays falls upon them otherwise than perpendicularly to their plane. We have already explained that, when a spherical lens is turned about one of its diameters as an axis, its action in the plane perpendicular to this axis is thereby sensibly increased. Thus it is that, by inclining the concave glass in the vertical, we obtain the same effect as if we had added a concave cylinder with its axis horizontal, which increases the refractive power of the vertical meridian of the lens.

The same thing is evidently accomplished when the eye looks obliquely through the glass. If, for instance, through concave spherical glasses, one direct the gaze strongly to the right or left, one is thereby placed in the same conditions as if one were wearing concave cylinders with their axes vertical: whether the line of fixation be deviated horizontally, or the glass be turned around a vertical axis, the effect is necessarily the same.

This cylindric action of concave glasses may be of service to myopes who are obliged to wear them. By inclining these glasses in front of the eyes, which is very easily done, especially if they are mounted as eye-glasses, they may correct an astigmatism of which the vertical meridian is generally the most refractive. It appears, however, according to Mauthner, that myopes often have recourse to this stratagem, not to correct an existing astigmatism, but simply to increase, at least in one meridian, the insufficient strength of their glasses. In such cases they voluntarily make themselves astigmatic; they prefer retinal images furnished by lines of diffusion to those composed of diffusion-circles (see p. 298).

Myopes whose error is of high degree, to whom their glasses permit very extended variations of action, sometimes acquire remarkable skill in the management of these glasses. The acuteness of vision which they obtain when they are allowed to hold the glasses themselves, is notably superior to that obtained when the same glasses are placed straight in a trial-frame. This is easy to understand after what we have just said on the subject.

When dealing with a strong myopia, complicated by a slight astigmatism, it is better to let the myope correct the latter by a trifling inclination of his glasses than to combine a spherical concave 18, for instance, with a number 1 cylinder. Moreover, in order to apportion to a single surface the entire action of so strong a concave lens, it must be given such an excessive curvature that the glass becomes very heavy, unless its edges be clipped away, whereby the field of its action is limited.

Under any other circumstances, it need hardly be said, we shall profit by the correcting-glass of the myopia to neutralise, at the same time, the astigmatism, according to the rules given on p. 308 and the pages following. The correcting cylinder will be added to the distance glass, as well as to that for near vision. It will be worn even when the eye does not demand the aid of a spherical glass, provided the advantage to be derived from the correction of astigmatism is greater than the inconvenience of wearing spectacles.

SURGICAL TREATMENT OF THE INSUFFICIENCY OF CONVERGENCE AND THE DIVERGENT STRABISMUS OF MYOPES.

When the insufficiency of convergence has resisted hygienic measures, general strengthening treatment, repose of the eyes, and when it is too great to be corrected by optical means, we may think of remedying it in a surgical way.

The operations which we have at our disposal for this purpose are the tenotomy of one or both of the external recti, the advancement of the interni, or even the combination of both these operations.

But surgical intervention never requires more prudence and delicacy than just in the case of muscular insufficiency. However unpleasant it may be to commit a therapeutic error in the prescription of glasses, it is but seldom followed by serious consequences, especially when it is recognised in time and corrected. But it becomes quite another matter when it is a question of surgically affecting the muscular system of the eye, and, indeed, especially in the case under consideration, where there is a simple insufficiency of the muscles. The less considerable the motor disturbance, the more difficult and the more delicate is its surgical treatment. In high degrees of strabismus, one can boldly recede and advance.

It would require a great amount of awkwardness not to be able to give the eyes a better position by an operation than they had before it; and should the result not be quite satisfactory, it is always easy to assist it.

« PreviousContinue »