. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, FiG. 8i. vergently from the concave mirror, and focusing in the vitre-ous, they cross and form an area of illumination on theretina at IF. The retina, situated at the principal focus ofthe dioptric media, naturally projects out from its indi-vidual points rays of light which are parallel as they leavethe eye ; some of these pass through the sight-hole of themirror and meet upon the retina of the observers emme-tropic eye. There are two very important points which nuist
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, FiG. 8i. vergently from the concave mirror, and focusing in the vitre-ous, they cross and form an area of illumination on theretina at IF. The retina, situated at the principal focus ofthe dioptric media, naturally projects out from its indi-vidual points rays of light which are parallel as they leavethe eye ; some of these pass through the sight-hole of themirror and meet upon the retina of the observers emme-tropic eye. There are two very important points which nuist bo OIIIIIIALMOSCOIE. 95 considered when usinL; the ophthahiioscope in the directmethod : one is the direction which the rays of hght takeas they leave the eye under examination, and the other isfor the observer to keep his own eye emmetropic ; in otherwords, the observer wearing his correcting glasses shouldnot accommodate. Figure 82 shows that rays of light passing out of an eyedivergently must be made parallel, so as to focus upon thesurgeons own retina (emmetropic), and to do this it is. Fig. 82.—T B indicate points at the edge of the disc from which rays passout of the eye divergently in the direction T B^, T W, T^ W, and beingreceived by the observers eye, are projected backward, forming an erectmagnified when image at T^ B^^. This image is not so large as that 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 o
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