Ocular refraction and the shadow test . rinciples, and used in the same manner, up to a certainpoint, viz:—the illumination of the retina. In chapter I, figures 8 and 9, illustrate two conditions underwhich light is reflected. Figure 8, shows that lignt rays that strike asurface perpendicularly are reflected back along the path by whichthey approached; the paths of the incident and reflected rays beingthe same. Figure 9, shows that when the incident ray forms an OCULAR R K F K A C T angle wilh the perpendicular, the reflected ray forms an equal anglewith the perpendicular. In the use of the op
Ocular refraction and the shadow test . rinciples, and used in the same manner, up to a certainpoint, viz:—the illumination of the retina. In chapter I, figures 8 and 9, illustrate two conditions underwhich light is reflected. Figure 8, shows that lignt rays that strike asurface perpendicularly are reflected back along the path by whichthey approached; the paths of the incident and reflected rays beingthe same. Figure 9, shows that when the incident ray forms an OCULAR R K F K A C T angle wilh the perpendicular, the reflected ray forms an equal anglewith the perpendicular. In the use of the ophthalmoscope, the conditions shown by figure8 are followed. Light is reflected through the pupil to the retina,which absorbs a portion of it, but a portion is reflected from the retinaoutward through the pupil. The operators eye is so placed behindthe ophthalmoscope, that it is in the path of these emergent rays, andreceives some of them. The pupil of the observed eye no longerappears black under these conditions, but is Incorporated in the are lenses that arebring the emergent rays to a focus, so that the observer maystudy the pathologic conditions of the retina. The retinoscope is simply an instrument to project light uretina; it carries no lenses back of tne peep hole, such as arethe ophthalmoscope. used tosee and pon thetised in ^ Figure i i6. Dr. Thoringtons Retinoscope. It consists of a mirror which is capable of refit cting the lightfrom some definite source, through the refracting system and pupil of R E T I N O S C O P Y. 151 the observed eye, so that a portion of the retina becomes an illumin-ated body. To simplify the subject as much as possible, only the plane mir-ror rctinoscope will be considered, and the method of its use explain-ed. The concave mirror will be taken up later. Figures J15 and 116, will illustrate two forms, and it is simply amatter of personal preference to select either the large or the smallmirror; either will do.
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