. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, etc.. is of shortest radius of curva-ture. 3. Symmetric Astigmatism.—When the combinedvalues, in degrees, of the meridians of shortest or longestradii of curvature in both ej-es equal 180 degrees (no moreand no less), then the astigmatism in the two eyes is spokenof as symmetric. For example, if the cylinder in the righteye is at a.xis 75 degrees, and in the left eye at 105 de-grees ; 75 degrees and 105 degrees added together will 128 REFRACTION AND HOW TO REFRACT. make i8o d
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. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, etc.. is of shortest radius of curva-ture. 3. Symmetric Astigmatism.—When the combinedvalues, in degrees, of the meridians of shortest or longestradii of curvature in both ej-es equal 180 degrees (no moreand no less), then the astigmatism in the two eyes is spokenof as symmetric. For example, if the cylinder in the righteye is at a.xis 75 degrees, and in the left eye at 105 de-grees ; 75 degrees and 105 degrees added together will 128 REFRACTION AND HOW TO REFRACT. make i8o degrees. (See Fig. loo.) Or if each eye takesa cylinder axis at 90 degrees, they are also symmetric, 90degrees and 90 degrees making 180 degrees. If both e)eshav^e axes 180 degrees, they are symmetric also, onemeridian being considered as zero (o). 4. Asymmetric astigmatism is the reverse of sym-metric, and is, therefore, the condition where the combinedvalues, in degress, of the cylinder axes do not make 180degrees. For instance, if the right eye has a cylinder ataxis 75 degrees and the left at 120 degrees, then these. 180 0 75 90 105 Fig. 100. — Illustrating SymmetricAstigmatism.