. The refraction of the eye; including a complete treatise on ophthalmometry; . at tlie size of the stepsof the mire must be exactly o mm., and the graduation on the arcenough farther apart to compensate for its longer radius. • The numberings of the graduations on the inner edge of the arc,therefore, are intended to show the number of half-centimeters dis-tance betAveen the inner edges (• X and Y,* Fig. Ill) of the whitespaces on the two mires. The two edges above mentioned are seton the slide carrying them, so that they are exactly in line withthe inner edges of the slides. And the graduatio
. The refraction of the eye; including a complete treatise on ophthalmometry; . at tlie size of the stepsof the mire must be exactly o mm., and the graduation on the arcenough farther apart to compensate for its longer radius. • The numberings of the graduations on the inner edge of the arc,therefore, are intended to show the number of half-centimeters dis-tance betAveen the inner edges (• X and Y,* Fig. Ill) of the whitespaces on the two mires. The two edges above mentioned are seton the slide carrying them, so that they are exactly in line withthe inner edges of the slides. And the graduation on the arcs nextto the inner edges of the slides, added together, give the readings ofthe instrument. For instance, in Fig. I the inner edge of the mireA stands at about 22^ on the arc, that of the mire B at 22^on its arc, making about 45 D. together; and if the corneal reflec-tions of the mires placed on the arcs as above have their inneredges just in contact; as shown in Fig. V (not separated or over-lapping, as shown in Figs. VI and VII resx^ectively), then the cur-. FiG. Y. Fig. Y1. Fig. Vn. vature of the corneal surface indicated by the positions of theslides on the arcs in that case would be such as to produce a focusof 45 D. There is no such thing as a normal curvature for the human cor-nea, but from statistics a radius of curvature of mm. has beensettled upon as that of the average cornea. Accordingly, we havestamped on the arcs the letter ^A, to indicate the position of themires for a cornea of average curvature. A smaller distance betweenthe slides than that shows a corneal curve of less than the average,and, hence, a j^resHm/^tion of hypermetropia. A greater distanceindicated the reverse of the above, or r presumjytion of myopia. ••The images of the two mires reflected from the cornea beingfarther apart on a cornea of kmger curvature, or nearer together onone of shorter curvature, it follows that, by moving the miresuntil their images appear in contac
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