This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1908 Excerpt: ...of the shortest or longest radii of curvature in both eyes equals 180. For example, if the cylinder correcting the astigmatism of the right eye is placed in axis 80 and the cylinder of the left eye is placed in axis 100, these added together equal 180. The same is true if each cylinder is placed at 90. If both cylinders are placed in axis 180, the astigmatism is also symmetric, one being called o. Asymmetric astigmatism is when the combined value in degrees of the cylinder axes of both eyes does not make 180. The combined value may be more or less. If, for example, the cylinder correcting the right eye is placed in axis 105, and the cylinder of the left eye is 120, the combined value is 225. Or, again, the cylinder correcting the right eye may be placed in axis 45, and that correcting the left eye in axis 75. The combined value will be less than 180, or 120. A convex cylinder which is placed in any necessary axis within 45 degrees of 90 is said to correct an astigmatism with the rule; a concave cylinder placed in any axis within 45 degrees of 180 is said to correct an astigmatism with the rule. In other words, the cylinder correcting hypermetropic astigmatism with the rule is placed more or less vertically; the cylinder correcting myopic astigmatism with the rule is placed more or less horizontally. Etiology.--Congenital malformation of the refracting surfaces or media of the eye is the most common form. The amount of this irregularity varies somewhat with time, and such eyes need reexamination every three or five years, or as symptoms suggest possible change. Acquired astigmatism is due to operations on the cornea or sclera, such as cataract extraction, iridectomy, etc., and to cicatricial contraction from wounds, ulcers, and other inflammatory changes in ...
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