. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, hed upon the catoptric image of the sta-tionary mire, then the astigmatism is calculated by the amount of this overlapping.(See Fig. 134.) Each step representing onediopter of astigmatism, one-half a step of overlappingwould represent half a diopter,etc. If, in making the changefrom the primary to the secondary position, the miresshould separate, then the surgeon would know that hissecondary position should have been his primary position,and he will have to make a corre
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, hed upon the catoptric image of the sta-tionary mire, then the astigmatism is calculated by the amount of this overlapping.(See Fig. 134.) Each step representing onediopter of astigmatism, one-half a step of overlappingwould represent half a diopter,etc. If, in making the changefrom the primary to the secondary position, the miresshould separate, then the surgeon would know that hissecondary position should have been his primary position,and he will have to make a corresponding change. As already stated, lenticular astigmatism is not a condi-tion to be ignored, as only too often it will increase,diminish, or even ncutrali/X corneal astigmatism, so that inpoint of fact the ophthalmometric findings are more oftenuseless than of real value in estimating the total refractiveerror. Cylinders should never be prescribed from theophthalmometric findings until carefully confirmed byother and much more reliable tests. As a keratometer,the instrument can not be excelled, and, therefore, it has a. Fig. 133. Fig. 134. ASTIGMATISM. I 5 I place in testing the refraction in cases of aphakia. Theophthahnometer as a means of diagnosis is suggestiverather than positive. 13. Estimation of Curvature Ametropia (Astigma-tism) with the Ophthalmoscope, Direct Method.—Thepresence of astigmatism is diagnosed by the direct methodfrom the fact that the vessels or details of the fundus arenot all seen clearly with one and the same glass in theophthalmoscope ; in other words, the vessels passing upand down on the disc are seen clearly with a different lensin the ophthalmoscope than is required to see the vesselspassing laterally or at right angles. The amount of theastigmatism is the difference in the strength of the respectivelenses used for this purpose ; for instance, if the verticalvessels are seen best with a -\-4 S., and the horizontalvessels with a 4-- S., then th
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