A manual of diseases of the eye / by Charles H. May and Claud Worth.

  • May, Charles Henry, 1861-1943.
Date:
1924
Catalogue details

Licence: Public Domain Mark

Credit: A manual of diseases of the eye / by Charles H. May and Claud Worth. Source: Wellcome Collection.

    378/524 (page 322)
    divergent, forming a circle of diffusion and consequently a blurred image (Fig. 277, P P F). Certain divergent rays, coming from the myopic far point, are focussed on the retina (Fig. 277, D X) without accommodation. The greatest distance at which the patient can read fine print is the far point. This is always at a definite distance corresponding to the amount of M. ; the higher the M., the closer to the eye is the far point ; the distance of the latter is the measure of the M. For example, if the far point is at 20 inches (o‘5 metre) the M = 2 D. (|§ or -d^0- = 2) ; if at 10 inches (0*2^ metre) the M. =4 D. In these two instances concave lenses of 2 and 4 D. respectively would render parallel rays as divergent as if they came from a distance of 20 and 10 inches (0*5 and 0*25 metre) ; and with these lenses the myope would be able to see distant objects distinctly (Fig. 278). Etiology. — Myopia almost always depends upon a length¬ ening of the antero-posterior diameter of the eyeball (axial myopia) ; in M. of 3 D,, for example, the eyeball measures 24 mm. in its antero-posterior diameter, and in M. of 10 D., 27 mm. from before backward, instead of 23 mm., the normal diameter. Much less frequently M. is due to increased curvature of the cornea (anterior staphyloma and kerato- conus), increase in the refraction of the lens from swelling in incipient cataract (often referred to by the laity as ‘ second sight/ because it sometimes enables an old person again to read print for a time without glasses), and spasm of accom¬ modation. The determining causes are associated with the demands which civilization and education make upon near vision. It is rarely congenital, though there is often an hereditary tendency for its development. It is an acquired Fig. 277.—The Focussing of Parallel and Divergent Rays in Myopia. Fig. 278.—The Correction of Myopia by Means of a Concave Lens.