. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, ¥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 ast
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, ¥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. Fig. 108. equally black, astigmatism of any considerable degree oramount may often be excluded ; but if the patient selectsone series of lines as darker than others, then the presenceof astigmatism may be diagnosed. If the astigmatism isof a very high degree, the patient may see the three linesas one solid black line without interspaces. Rule i.—The meridian of the eye which corresponds tothe dark lines selected is the meridian of astigmatism. Example.—If the horizontal lines (from III to IX) appear 138 REFRACTION AND HOW TO REFRACT. darker than all the others, then it is the horizontal meridian(o or 180 degrees) of the eye which is astigmatic. Or ifthe lines from VI to XII are darkest, then the vertical mer-idian of the eye is astigmatic. In other words, the seriesof darkest lines indicates the meridian of greatest ametropia. Rule 2.—The axis of the cylinder in the prescriptionwill be opposite to the meridian of the dark lines. Example.
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