. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, heametropia 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 com
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, heametropia 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. Forpurposes of testing ametropia, a dark shade of blue shouldbe selected, or two or three pieces of a light shade may becemented together so as to give the desired dark glass is cut round and fitted into a trial-cell. (SeeFigs. 115 and 116.) The power of cobalt-blue glass to exclude all but blue. Fig. 117. and red rays gives this test its principle. Blue rays beingmore refrangible than red, naturally focus sooner than rays will focus back of the blue. (See Fig. 117.) There are several important details in the use of thistest which must be carefully executed if definite results areto be obtained : 1. The eye should be under the iiijlueuee of a eyeloplei^ie. 2. l^y means of a light-screen, a small rountl area ofsteady white light should be looked at from a tlistance offour or six meters. 3. Each eye is to be tested separately. ASTIGMATISM, 145
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