. The Medical and surgical reporter . le ofMr. Jessop. You see it is a strip of metal,along the edges of which are cut out a seriesof semicircles of different diameters. But for measuring the pupil, in the obser-vation of cases, I think the best instrumen tis the so-called keratometer, devised byPriestley Smith, of Birmingham, consists of a scale placed betweentwo plano-convex lenses. The observerseye is placed about the principal focusof the combined lenses (ten inches fromthem), and holding the scale before theobserved eye the cornea or pupil subtends onthe scale exactly its pr
. The Medical and surgical reporter . le ofMr. Jessop. You see it is a strip of metal,along the edges of which are cut out a seriesof semicircles of different diameters. But for measuring the pupil, in the obser-vation of cases, I think the best instrumen tis the so-called keratometer, devised byPriestley Smith, of Birmingham, consists of a scale placed betweentwo plano-convex lenses. The observerseye is placed about the principal focusof the combined lenses (ten inches fromthem), and holding the scale before theobserved eye the cornea or pupil subtends onthe scale exactly its proper length, whateverits distance from the scale, because rays con-verging to the principal focus of a lens sys-tem must have entered that system parallel. *For full descriptions of these and other forms of appa-ratus, see Trait6 Coniplet dOphthalmologie, of De Weckerand Landolt, t. premier, pp. 942-953. f See Ophthalmic Review, November, 1886. This is illustrated by the solid lines of fig-ure 1, A, where the observers eye is placed,. Fig. 1. being a principal focus of the lens B C, andB C must be parallel; hence B B exactlyequals C C no matter how far they are equality or inequality in the size ofthe two pupils and abnormalities of shape
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