. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, peropic eyes are seen, yetstatistics show that fully eighty per cent, of hyperopic eyeshave astigmatism. Astigmatism is located in the cornea or lens, or it may bea condition of both structures in one and the same of the lens may increase, diminish, or neutral-ize the corneal astigmatism. Astigmatism, however, ismore often a condition of the cornea than of the lens. Figure 93 shows parallel rays of light passing throughan astigmatic lens where the vertic
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, peropic eyes are seen, yetstatistics show that fully eighty per cent, of hyperopic eyeshave astigmatism. Astigmatism is located in the cornea or lens, or it may bea condition of both structures in one and the same of the lens may increase, diminish, or neutral-ize the corneal astigmatism. Astigmatism, however, ismore often a condition of the cornea than of the lens. Figure 93 shows parallel rays of light passing throughan astigmatic lens where the vertical meridian has the. Fig. 93. shortest radius of curvature, with the result that those rayswhich pass through the vertical meridian V V come to afocus before those in the horizontal meridian H H, whichhas the longest radius. Intercepting the refracted rays at I, 2, 3, 4, 5, and 6, theimage would be at i a horizontal oval, at 2 a liorizontalline, at 3 a circle, at 4 a vertical oval, at 5 a vertical line,and at 6 a vertical oval. The space between the points offoci of the two meridians (2 and 5) is known as Sturmsinterval. The importance of this space or interval is that 122 REFRACTION AND HOW TO REFRACT. it represents astigmatism. Sturms interval is the quantitywhich must be found in correcting astigmatism. Causes of Astigmatism.—Most cases of astigmatismare congenital, and some can be traced to heredity. Ac-quired astigmatism may result from conic cornea, cicatricesfollowing ulcers or wounds of the cornea, or be a tempo-rary condition from pressure of a chalazion or othergrowth ; and, in fact, astigmati
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