. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, t seenwhen looking into a myopic eye. (Fig. 83.) necessary to turn a plus lens in front of the sight-hole ofthe ophthalmoscope ; the strength of the convex lens thusemployed, other things being normal, is the amount of therefractive error of the eye being examined. Figure 83 shows rays of light passing out of an ej^econvergently, and to have them parallel, so as to focusupon his own retina (emmetrojjic), it is necessarx^ to turn aconcave lens in front of the sight-hole
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, t seenwhen looking into a myopic eye. (Fig. 83.) necessary to turn a plus lens in front of the sight-hole ofthe ophthalmoscope ; the strength of the convex lens thusemployed, other things being normal, is the amount of therefractive error of the eye being examined. Figure 83 shows rays of light passing out of an ej^econvergently, and to have them parallel, so as to focusupon his own retina (emmetrojjic), it is necessarx^ to turn aconcave lens in front of the sight-hole of the ophthalmo- 96 REFRACTION AND HOW TO REFRACT. scope ; the strength of the concave lens thus employed,other things being normal, is the amount of the refractiveerror of the eye under examination. The Observers Accommodation.—It has already beenstated that, when using the ophthalmoscope, the observershould wear any necessary correcting lenses. If the ob-server has a refractive error and does not wear his glasses,he must deduct this amount from the lens used in theophthalmoscope. If he has two diopters of hyperopia. Fig. 83.—T B indicate points at the edge of the disc from which rays passout of the eye convergently in the direction T B, and, being received bythe observers eye, are projected backward, forming an erect magnifiedimage at T B. This image is much larger than that seen when look-ing into tlie hyj)eropic eye. (Fig. 82.) himself, and the lens used in the ophthalmoscope is plusfour diopters, then the eye under examination has onl) twodiopters. It is not unusual for beginners to see the e)e-ground (disc) in hyperoj)ic eyes with a strong concavelens ; this is due to the fact that the} accommodate. Prac-tice will overcome this habit, and it should be mastered assoon as possible. There are .several wa\S of doing this :one is to begin the examination at a distance of 30 or 40 OPHTHALMOSCOPE. 97 cm. from the eye, with both eyes open, and to gradual 1)approach tlie eye
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