. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, ing purposes. To use this disc, the patient isplaced with his back to a strong light from a window, oran artificial light may be placed o\er his head. The sur-geon holds the disc with the sight-hole close in front ofhis own eye, and with the light illuminating the disc, thepatient is instructed to look into the perforation. The sur- ASTIGMATISM. 133 geon then approaches the eye until the corneal image ofthe outer edge of the instrument corresponds to the outeredge of th
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, ing purposes. To use this disc, the patient isplaced with his back to a strong light from a window, oran artificial light may be placed o\er his head. The sur-geon holds the disc with the sight-hole close in front ofhis own eye, and with the light illuminating the disc, thepatient is instructed to look into the perforation. The sur- ASTIGMATISM. 133 geon then approaches the eye until the corneal image ofthe outer edge of the instrument corresponds to the outeredge of the patients cornea. When this distance isreached, a convex 2, 3, or 4 D. sphere may be placed inthe slot of the disc so as to magnify the corneal the cornea is not astigmatic, then the black and white circles will appear uni-form throughout; but ifthere is astigmatism, thecircles will appear moreor less oval. If irregu-lar astigmatism or coniccornea is present, the cir-cles will appear brokenor distorted in certainparts. This test has be-come almost
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