. Spectacles and eyeglasses, their forms, mounting, and proper adjustment . placed opposite the center of one pupil, the center of theother may be marked by the physicians thumb nail, asrepresented in Fig. 28, and the distance between them readoff the scale. This distance seldom varies more than 5 60 mm., or 2J in. It will be observed that as thephysicians eyes are less than the length of his arm awayfrom the patients face when this measurement is taken,in fact, about two feet away, the marks upon the rule, thoughapparently opposite the pupils, will in reality be a littlewithin the cen
. Spectacles and eyeglasses, their forms, mounting, and proper adjustment . placed opposite the center of one pupil, the center of theother may be marked by the physicians thumb nail, asrepresented in Fig. 28, and the distance between them readoff the scale. This distance seldom varies more than 5 60 mm., or 2J in. It will be observed that as thephysicians eyes are less than the length of his arm awayfrom the patients face when this measurement is taken,in fact, about two feet away, the marks upon the rule, thoughapparently opposite the pupils, will in reality be a littlewithin the centers; so that the distance obtained will be alittle less than it should be. When the physicians eyesare two feet away from those of the patient, and the ruleis one inch away from them, the error in measuring aninterpupillary distance of 60 mm. by this method is almostexactly 2 mm. This amount should, therefore, be added PRESCRIPTION OF FRAMES. 55 to the apparent interpupillary distance to obtain the trueone. The measurement obtained in this way is sufficiently Fig. 2. accurate for most purposes, but if a greater degree of accu-racy be desired in any case it may be attained by means Fig. 30. Fig. 92.
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Keywords: ., bookcentury1900, bookdecade1900, booksubjecteyeglasses, bookyear1
