. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, ment.—TJic IntcrpnpUlary Distance.—Toaccurately measure the distance from the center of onepupil to the center of the other is not always an easy thingto do, especially if the pupils are dilated ; hence, it is goodpractice to measure this distance from the inner side or edgeof one pupil to the outer edge of the other. This measure- FIRST MEASUREMENT. 289 ment can be made with an ordinary rule divided to six-teenths of an inch or in millimeters, or with a special instru-
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, ment.—TJic IntcrpnpUlary Distance.—Toaccurately measure the distance from the center of onepupil to the center of the other is not always an easy thingto do, especially if the pupils are dilated ; hence, it is goodpractice to measure this distance from the inner side or edgeof one pupil to the outer edge of the other. This measure- FIRST MEASUREMENT. 289 ment can be made with an ordinary rule divided to six-teenths of an inch or in millimeters, or with a special instru-ment for the purpose, called a pupilometer. The patientis told to look directly to the front, at an object across theroom, and the surgeon, in front, with his head nearly in theline of sight, holds the rule across the patients face, asclose as the bridge of the nose or eyelashes will his thumb-nail as a marker, the surgeon gages thedistance as indicated (see Fig. 192), which illustrates theconditions. In taking this measurement the surgeon shouldbe at an arms length from the eyes, for the reason that his. Fig. 192. own eye forms the apex of a triangle of which the eyes ofthe patient form the base, and the measurement is apt tobe two or three or four millimeters short if he gets tooclose. If the glasses are to be worn for distance onl), then themeasurement must be for the full interpupillary distance, asthe patient looks into infinity ; but if the glasses are for near-work onl}% then the distance between the pupils must becorresponding!}- diminished, and the measurement takenas the patient looks at a near point. If the glasses areto be worn for both near atid far vision, for constant use,25 290 REFRACTION AND HOW TO REFRACT. then the center of the lenses must be placed intermediatebetween the distance and near measurements. Second Measurement.—The Bridge—The regulationspectacle bridge is known as the saddle-bridge, and shouldconform to the exact shape of the
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