. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, difficulty in appreciating these facts. The optic disc of an emmetropic eye, as seen throughthe ophthalmoscope, appears to be about 25 mm. in diam-eter, and about 250 mm. away. The retina of the emme-tropic eye is about i 5 mm. from its nodal point; then theactual size of the emmetropic disc is -J/^ of 25, or -1. mm. ; then 15 is to 250 as is to 25, or —themagnification, in other words, when the emmetropic disc isobserved, it appears about times large
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, difficulty in appreciating these facts. The optic disc of an emmetropic eye, as seen throughthe ophthalmoscope, appears to be about 25 mm. in diam-eter, and about 250 mm. away. The retina of the emme-tropic eye is about i 5 mm. from its nodal point; then theactual size of the emmetropic disc is -J/^ of 25, or -1. mm. ; then 15 is to 250 as is to 25, or —themagnification, in other words, when the emmetropic disc isobserved, it appears about times larger than it actually 98 REFRACTION AND HOW TO REFRACT. The Indirect Method (see Fig. 84).—Practising thismethod, the observer sees a larger part of the eye-ground. Fig. 84. at one time, but it is not so perfect in detail nor is it mag-nified to the same extent as in the direct method. Theobserver does not have to get so close to his patient, whichis a decided advantage in some clinical cases. Unfortun-ately, as a preliminarystep, it is often neces-sary to dilate the pu-pil. In addition tothe ophthalmoscope,there is also requireda convex lens ofknown strength andlarge aperture; theone which comes inthe case with thescope is usual!)- too small and too strong for general writer prefers his plus 13 I), with metal rim and con\-en-ient handle, shown in figure 85 (reduced one-third in size).
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