Text-book of ophthalmology . atients fundus. For (Fig. 13) a certain portion of the fundus of the eye,A, is illuminated by the mirror, S S. The rays reflected from any point,as a, of this illuminated region of the retina, leave the eye in a parallel3direction, pass through the central aperture, o Oi, of the mirror, and fall intot he observi s eye, B. Here they are again united at a single point, b, uponthe retina of this eve, so here there is produced a sharp image of thepoint a. Since the same process is repeated for all the other points of the * [Parallel, because if the eye ii emmetro
Text-book of ophthalmology . atients fundus. For (Fig. 13) a certain portion of the fundus of the eye,A, is illuminated by the mirror, S S. The rays reflected from any point,as a, of this illuminated region of the retina, leave the eye in a parallel3direction, pass through the central aperture, o Oi, of the mirror, and fall intot he observi s eye, B. Here they are again united at a single point, b, uponthe retina of this eve, so here there is produced a sharp image of thepoint a. Since the same process is repeated for all the other points of the * [Parallel, because if the eye ii emmetropic) , focuses parallel rays upon its retina, then, accord-ing to the principle of conjugate foci, rays emanating from the eye will also he parallel. Being parallel, ouch rays will also be focused sharply by the observer-, eye, which is also emmetropic—D.| OBJECTIVE EXAMINATION OF THE EYES 89 illuminated region of the retina of the eye, A, a sharp image of this portionof the retina is formed in the eye of the Fig. IS.—Ophthalmoscopic Examination with the Erect Image. The eyes are drawn of the natural size of an emmetropic eye having an axial length of 24 rays that are reflected from the mirror, S S, into the patients eye are not shown, but only theraya that emerge from the latter. [If not emmetropic, both patient and observer must be rendered so, if the fundusis to be clearly seen by the direct method. This is usually effected by rotating the system of lenses behind the mirror of theophthalmoscope until the proper correction is secured. A still better way, particularly wheneither patient or observer is quite astigmatic or the patient has a high degree (12 to 20D) of myopia, is for each to wear his correcting glasses during the examination. Then thefundus is seen with great distinctness either with the simple aperture or, if either patientor observer accommodates, with a weak concave lens. As stated below, the direct method may be used not only for exam
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