_Analysis of over seventeen hundred ca.ses of myopia recorded in private prac tice, showing, that 7.6 per cent. exceeded 10 D. Among these it was rare to find vision above 0.4. In many instances it ‘‘as much less, w bile in four cases it reached "ho, and so continued during years. Cireuinoptical atrophy was com mon, and lesions at the macula presented themselves in varying degrees. II. D. Noyes (Archives of Ophth., July, '93).
Diagnosis.-3Iyopia is recognized and measured by: the improvement of vision by concave lenses, the weakest lens giv ing the best distant vision being the measure of ,the myopia; the blurred image in the direct method of ophthal moscopical examination, rendered clear by a concave lens which corrects the myopia; and the reversal of movement by skiascopy, the distance of the point of reversal from the eye being the focal distance of the correcting lens.
Etiology, Pathology, and Varieties.— Myopia may be due to excessive curva ture of the cornea, or crystalline lens,— myopia of currature,—or to an increase of refractive power in the lens-substance, —index myopia; but the great majority of cases are due to elongation of the eye ball,—asial myopia. Myopia is often
observed temporarily for a few weeks or months after an attack of iritis or irido cyclitis,—inflammalory myopia. This is probably due to alteration of curvature in the lens, although an alteration of in dex has been suggested. Prior to senile or diabetic cataract myopia may develop, probably by increased refractive index of the lens-nucleus. This change enables old people who have previously required convex lenses to read without them, and is therefore called "second sight." It is but a temporary benefit, and not an un mixed good.
The mass of cases of myopia develop in eyes, not myopic at birth, from ex cessive strain of near-seeing. Heredity, bad hygienic surroundings, and impaired general health are predisposing causes. But the efficient exciting cause is ex cessive near work for the eyes. During