. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. chap, xxviii.] THE OPHTHALMOSCOPE. 353 near enough, an image is seen of the retina, optic nerve entrance, etc. The image is virtual, erect, and magnified, as represented in Fig. 159, ed. This image can be obtained only if the observing and the observed eyes are both focussed for an infinite distance. This is practically secured by making the person look to a distant point, say at the other end of the room, and by the observer looking
. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. chap, xxviii.] THE OPHTHALMOSCOPE. 353 near enough, an image is seen of the retina, optic nerve entrance, etc. The image is virtual, erect, and magnified, as represented in Fig. 159, ed. This image can be obtained only if the observing and the observed eyes are both focussed for an infinite distance. This is practically secured by making the person look to a distant point, say at the other end of the room, and by the observer looking as if to a distance. This condition, however, it is often not possible to obtain. The result is that the reflected rays from the eye are not accurately focussed on the. Fig. 159.—The Ophthalmoscope with Erect Image. retina of the observing eye, and circles of diffusion are formed. Under such circumstances the use of a diverging lens will render the image distinct. The action of such a lens is shown in Fig. 159. The observed eye is A, and the small arrow a represents a part of the retina 011 which light is thrown by the concave mirror. Rays from a passing outwards would be converged by the media of the eye, and would come to a focus at 6. An image would thus be formed, 6c, magnified, and inverted, a real image moreover. But by the action of the concave lens B (whose focal distance is pB) the rays are made to diverge, and thus a virtual image is formed behind the eye, an image larger than the object, but erect. That is, the rays from a, which reach the observer's eye, appear by the action of the lens to proceed from the point d. x—8. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original M'Gregor-Robertson, J. (Joseph), 1858-1925. Philadelphia, H. C. Lea's son & co.
Size: 2450px × 1020px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1880, bookpublisherphila, bookyear1884
