. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, at axis 165 he reads ^, thenhe is astigmatic in the 165 meridian. The amount of theastigmatism can be calculated by placing spheric lensesback of the slit and finding the difference in strength ofthe spheres which bring the vision up to the normal. Forexample, when the slit is at axis 75 and the patient reads]^, if a -f S. is used, and the vision becomes y|, corrects axis 75. Turning the slit to axis 165, pro-ceeding in the .same way, and + S. brings t
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, at axis 165 he reads ^, thenhe is astigmatic in the 165 meridian. The amount of theastigmatism can be calculated by placing spheric lensesback of the slit and finding the difference in strength ofthe spheres which bring the vision up to the normal. Forexample, when the slit is at axis 75 and the patient reads]^, if a -f S. is used, and the vision becomes y|, corrects axis 75. Turning the slit to axis 165, pro-ceeding in the .same way, and + S. brings the visionfrom ^ to ^; then + corrects axis 165, the differ-ence between the and + being i D., and theformula would be -fi-SO sph. Q -f cyl. axis 75degrees. This test is not often used, and when resortedto, the eyes should be under the influence of a cyclo-plegic. This test is of special service in some ofmixed astigmatism, irregular astigmatism, presbyopia, andaphakia. ASTIGMATISM. 135 5. Astigmatic Chart.—There is an infinite variety ofthese cards (see Fig. 106), and the student is puzzled. Fig. 106.—Astigmatic Cliarts of Dr. John Green. which one to select. Ordinarily, the clock-dial willanswer every purpose. (Fig- 107.) This is a white 136 REFRACTION AND HOW TO REFRACT. card * with peripheral Roman characters corresponding tothe characters on the clock-face, hence its name. Fromthese figures a series of three parallel and uniformly blacklines, with interspaces of the same width as the lines, crossfrom XII to VI, III to IX, IIII to X, V to XI, VII to I,and VIII to II. This chart should be so calculated that
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