. A treatise on diseases of the eye . hat it apj)ears to be behind theexamined eye and in the erect or uprigiit position. J^iie magnidcalioMby this method is about sixteen diameters.^ ^^^len the .examined eye is hyperopic, the emergent ravs will bedivergent, so that in (jrder to bring them to a focus on the retina of the 1 That is, the image on the examiners retina is about \G limes as great (in pa<h dimension) asif the same portion of fimdus were seen at a distance of 25 cm. (1(1 inclics), and willioul llic mag-nifying action of the examined eye. 106 GENERAL PRINCIPLES OF OPTICS emmetropic
. A treatise on diseases of the eye . hat it apj)ears to be behind theexamined eye and in the erect or uprigiit position. J^iie magnidcalioMby this method is about sixteen diameters.^ ^^^len the .examined eye is hyperopic, the emergent ravs will bedivergent, so that in (jrder to bring them to a focus on the retina of the 1 That is, the image on the examiners retina is about \G limes as great (in pa<h dimension) asif the same portion of fimdus were seen at a distance of 25 cm. (1(1 inclics), and willioul llic mag-nifying action of the examined eye. 106 GENERAL PRINCIPLES OF OPTICS emmetropic observer without exercise of his accommodation a convexlens, which is attached to the ophthahnoscope, must be brought to thesight-hole of the mirror. On the other hand, if the examined eye is myopic, the emergent rayswill be convergent and a concave lens must be used to bring them to afocus on the retina of the emmetropic observer. If the observer is hyperopic or myoj)ic, a suitable lens must be usedto correct his ametropia. Fig. 77. Illustrating the path of the refracted rays in direct ophthalmoscopy. Indirect Method.—^The indirect method differs from the direct inthat in the former a convex lens (about 13 D.) is held near the eye underexamination (at a distance about etjual to the focal length of the lens),by means of which a real inverted aerial image of the illuminated por-tion of the fundus is formed. The examiner then having his eye about25 cm. ( 10 inches) from diis image, sees it either by aid of his accommo-dation or with a convex lens (4 D.). In this case the apparent positionof the image is between the observer and the examined eye. The mag-nification is considerably less than by the direct method—about 4 or5 diameters—but the field of view is correspondingly larger. Skiascopy (Shadow Test, Retinoscopy).—This consists in reflectinglight into an eye from a mirror held at a distance of about one meter,rotating the mirror, and observing the characteristic appea
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Keywords: ., bookcentury1900, bookdecade1910, booksubjecteyediseases, bookyear
