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MYOPIA, TREATMENT. 51 tomed always to wear their neutralizing glasses feel very uncom- fortable so soon as they take them off, even for near vision.

If from any cause the use of neutralizing glasses cannot be allowed, we still often have occasion to remove the far point to a given distance, in order to allow the pursuit of some particular oc- cupation, piano-playing, for instance. The suitable lenses can be easily calculated. If we wish with myopia ^ to remove the far point to 18 inches, that is to reduce this M^ so that only M^ re- mains, then ^ — -i=-jlg-, consequently i=^. Concave 9 is there- fore the requisite lens. Of course the calculation and direct ex- periment should always verify each other. Here, too, the weak- est lenses which will fulfil the requirement are the ones to be chosen.

Insufficient movement of the eyes in consequence of their change of form exists generally only in high degrees of myopia. It appears best that the relative divergence in near vision which results from it should not be interfered with. Under these cir- cumstances little is to be accomplished with the prismatic spectacles which are so often employed. The optical effects of prisms will be more particularly explained hereafter.

If, on the contrary, the difficulty of binocular vision does not depend upon a limitation of the movement of the eyes but upon an elastic preponderance of the external recti muscles, and if it is evident that binocular vision can be maintained only by a strain upon the internal recti muscles, the indications are for a tenotomy of the rectus externus. This condition occurs congenitally, as does myopia; it is often a complication of slight degrees of myopia.

The symptoms of irritation which frequently appear at the period of puberty, characterized by hypertemia of the optic nerve, quick fatigue and pain in the eyes, especially when working in the evening, demand a strict hygienic regimen. There must be good light, frequent interruption of work, the head must not be bent forward, all influences which tend to congestion of the head or eyes must be avoided, the feet must be kept warm, the bowels open, the douche must be used with closed lids and sometimes blood is to be drawn with the artificial leech. In slight degrees of myopia it is advisable under these circumstances to avoid the use of spectacles ; in high degrees, if the difficulties are not