the Hm, or if necessary, both Hm and Hl as well as the As should be neutralised, and the glasses will do equally well for distance or near work; but if the patient has become presbyopic, the spherical curvature will have to be increased for reading, (see p. 80). When cases of astigmatism have been worked out under atropine, and when the power of accommodation returns concave glasses may not be quite strong enough whilst convex are rather too strong; a little perseverance will usually overcome the difficulty, if not, the concave curvature may be slightly increased or the convex slightly reduced.

CHAPTER V.

DISEASES AND INJURIES OF THE EYEBALL AND ITS APPENDAGES.

MANY of the more important affections of the eyeball and parts adjacent can only be remedied by operation; the present section includes those forms of disease and injury which with a few exceptions (e.g., glaucoma), require only medical treatment.

AFFECTIONS OF THE OCULAR MUSCLES, EXTERNAL AND

INTERNAL.

AFFECTIONS of the ocular muscles may be divided into those of the external muscles, recti and obliqui, and those of the internal muscles, muscles of the iris and the ciliary muscle.

Muscular asthenopia signifies weakness of some of the external ocular muscles. For all practical purposes it is sufficient to consider the disease as affecting the internal recti, but at the same time we may remember that it is possible to get symptoms of asthenopia from imperfect action of the external, or even of the superior or inferior recti or the oblique muscles.

The symptoms of muscular asthenopia always manifest themselves when reading or doing near work; they are, pain in the eyes, headache, indistinct vision with a tendency to diplopia. We are told that the page of the book seems to widen out, the print to become

misty, and the lines mixed up; or inability to do near work for any length of time, without these symptoms appearing, is complained of.

If we tell the patient to fix some small object-the end of a pen or pencil-and then move it nearer the eyes, they become unsteady, and soon one or other diverges, double images resulting. The images are crossed, that of the right eye being to the left of that of the left eye, and vice versa. If we cover one eye and direct the patient to fix an object situated at about ten inches with the other, we shall find that the covered eye diverges, and on removing the hand he will see double; the diplopia may be only momentary, by an effort the deviated eye can be made to fix the object, and we shall see it move inwards; but after looking steadfastly for a short time one or other eye will diverge.

Muscular asthenopia occurs in all conditions of refraction, but is perhaps most frequently met with in myopia. The treatment of muscular asthenopia depends on the condition of refraction. In myopia the difficulty arises from the far point (r) being very near the eyes, and much convergence being necessary to make the visual lines meet at the required distance. All that is necessary is to give a pair of glasses which remover to a distance at which convergence is easy. For example, if myopia 8 D is present, r lies at five inches from the eye, at which distance it may be impossible to maintain convergence. If we order - 6 D to be used for reading, we reduce myopia to 2 D (8 D — 6 D = 2 D), and remove r to about twenty inches from the eyes, at which distance convergence is easy (see also p. 17).

In emmetropia and hypermetropia the symptoms are

* In myopia the furthest point of distinct vision lies at the negative focal length of the lens which corrects it. The negative focal length of a lens of 8 D is five inches, of a lens of 2 D twenty inches.

due-not as in myopia-to the necessity for excessive convergence, but to real weakness or insufficiency of the internal recti, rendering them incapable of maintaining convergence to the distance at which ordinary print can be read. Such cases are remedied by optical means. Our aim is to make rays of light appear to come from a point situated at a greater distance from the eyes than the object looked at, and so lessen the amount of convergence required. This can be done by the use of prisms, or lenses arranged so as to have a prismatic action.

The action of a prism is to displace objects seen through it towards its summit (see p. 3). Consequently if we place in front of the eyes a pair of prisms with their summits or edges outwards, any object looked at through them is displaced outwards and can be seen distinctly with convergence to a point further from the eyes than that at which the object is situated; the distance is greater as the angle of the prism is larger. A very convenient way of finding the strength of the prism required is to place before the patient's eyes at a distance of 10 inches a straight vertical line with a round black dot in its centre. He will probably see the line and the dot single. Then place before one eye a prism of 5°, edge upwards, two dots at once appear; if no asthenopia is present they will both appear to be situated on the same line, if muscular asthenopia is present, the upper dot-that belonging to the eye covered by the prism-will be seen situated on a separate line, to the further side of the lower dot, crossed diplopia is produced. If we take a weak prism and place it before the eye already covered by the first prism, but with its edge outwards instead of upwards, we shall find the upper dots come more nearly over the lower; we try stronger prisms until we arrive at one

which makes the dots stand directly over each other. This prism will quite correct the muscular asthenopia and greatly assist the weak internal recti. It is not necessary to test both eyes; the prism found as above gives all the correction necessary for the two, and we divide it between them. Thus suppose a prism of 6° held edge outwards causes the dots to stand directly one over the other on a continuous line, we order a prism of 3° edge outwards for each eye; these are placed in an ordinary spectacle frame and used for reading, etc. In a case of emmetropia this is all that is needed; if hypermetropia is present, or if the patient has become presbyopic, the required convex curvature must be ground on one surface of the prisms as found above; or the convex lenses which correct the hypermetropia or presbyopia may be "decentrated" inwards, ¿.e., the optical centre of each lens instead of being in the middle of the ring of the spectacle frame is put close to its inner edge-only half a lens is used-by which means in addition to its action as a convex lens, it has that of a diverging prism.

PARALYSIS OF EXTERNAL OCULAR MUSCLES.

The external rectus (sixth nerve), the superior oblique (fourth nerve), may be paralysed separately; the internal inferior and superior recti, and the inferior oblique (third nerve) as a group, or the whole of the muscles may be palsied together (ophthalmoplegia externa).

The symptoms of paralysis of a single external ocular muscle, or group of muscles, are double vision, with more or less marked deviation of the eye in some directions and want of mobility in others.