. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, 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,an
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, 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. ¥iG. 107. the lines and interspaces \\ill form an anq;le of 5 minutes inwidth consistent w ith the distance at which the test is to bemade : if at six meters, mm. ; if at four meters, charts have the lines subtending an angle much greaterthan 5 minutes for the distance at which they are andin tliis way llic true delicacy of the test for errors or *A black card wiili wliite lines is also used. (See Fig. 108.) ASTIGMATISM. 137 amounts of astigmatism is sacrificed. The purpose of thechart is to detect, by the patients answer, whether astigma-tism is present, and, if so, in which meridian. The chart, illuminated by reflection from a steady artificiallight, is placed on a horizontal line perpendicular to thepatients eyes ; it should never be hung at an angle, andmust always be perfectly flat. Each eye is to be testedseparately. Looking at such a chart, if all the lines appear
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