. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, he two lights, as in hyperopia. That plus cylinderplaced before the disc which unites the two lights in theametropic meridian represents the amount of the astigma-tism. Diagnosis of Simple Myopic Astigmatism.—Thelights are red and white in one meridian, as in simple In-per-ojiic astigmatism, but the red light is seen in the directionof the red glass, and when the disc is rotated to the oppo- ASTIGMATISM. 143 site meridian, only one light appears, and of a pink
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, he two lights, as in hyperopia. That plus cylinderplaced before the disc which unites the two lights in theametropic meridian represents the amount of the astigma-tism. Diagnosis of Simple Myopic Astigmatism.—Thelights are red and white in one meridian, as in simple In-per-ojiic astigmatism, but the red light is seen in the directionof the red glass, and when the disc is rotated to the oppo- ASTIGMATISM. 143 site meridian, only one light appears, and of a pink amount of the astigmatism is represented by thestrength of minus cylinder which brings the two lightstogether in the ametropic meridian. Diagnosis of Compound Hyperopia Astigmatism.—Allmeridians show two lights, the red light being in the direc-tion of the clear opening in the disc, but one meridian willshow a greater separation of the lights than in the meridianat right angles. To find the correction and the amount ofthe astigmatism, proceed as in simple hyperopia, correctingeach meridian separately with a Fic. 115. Fig. 116. Diagnosis of Compound Myopic Astigmatism.—Thisis the same as in compound hyperopic astigmatism, with areversal of the position of the lights, and the amount of theametropia is obtained with minus spheres. Diagnosis of Mixed Astigmatism.—One meridianshows as in simple hyperopic astigmatism, and the meridianopposite to it shows as in simple myopic amount of the astigmatism is calculated as in thesetwo conditions. 10. Chromo-aberration Test.—This is also knownas the cobalt-blue glass test. Cobalt is a mineral, and is 144 REFRACTION AND HOW TO REFRACT. used as a coloring-matter by glass-blowers. Cobalt-blueglass comes in two forms : one where the glass is coloredthroughout, and the other where it is colored only on onesurface, known as flashed. To the eye, cobalt-blue glassappears dark blue, but contains a great deal of red
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