On the theory of the ophthalmoscope / by George Rainy.

  • Rainy, George
Date:
[1860]
Catalogue details

Licence: Public Domain Mark

Credit: On the theory of the ophthalmoscope / by George Rainy. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    rather embaiTass the observer in examining hyperpresbjopic eyes in the direct manner, viz., that in extreme cases the apparent magnitude of the image of an object, such as the papilla, may not exceed a third or a fifth part of what it appears in a normal eye. A much larger area is then seen, and this circumstance may add to the difficulty of illuminating hyperpresbyopic eyes, referred to in Part II. The illumination may or may not be fainter than it is in a normal eye, according to the mode in which it is accom- ])lishcd, and according to the cause of the hyperpresbyopia, whether dependent on absence of the lens, &c., or on shortening of the axis of the eye; but the area illuminated will always appear smaller than one of equal dimensions in a normal eye, and it will seldom occupy the whole of the observer's field of vision. Apparent motions of objects situated in different planes within the eye, are often very difficult to distinguish from real ones. They may depend on motions of the eye obsei-ved, or on motion? of the observer, or the biconvex lens held in his hand. The apparent changes in the position of objects caused in this way may be regarded as an exaggeration of those observed in looking at objects from, different points of view under ordinary circum- stances ; except that in the direct mode of examination, it is the most distant objects which appear to move most rapidly, instead of the nearer ones, as is always observed in nature, and generally in the inverted image. ERRATA, &c. Page 6, et passim, for Helmholz read Helmholtz. Page 21, formula (V.), for — (F -[- r), in the denominator, read f—(F + r). Page 29, line 33, for lost by transmission or absorption, read lost by transmis- sion, absorption, or irregular reflection. Page 32, line 32-37. The limit in this case is the angle subtended at the optical centre of the eye by the image of the lens produced by the reflector. Page 53, line 37, and page 5-t, line 2. In the examination of hyperpresbyopic eyes by the direct method, we may employ a convex lens, as a concave one is employed ' examining myopic eyes. If this convex lens were held close before the patient's ey it would be theoretically possible to see a virtual erect image of the fundus as mu magnified as a real inverted one could appear with equal brilliancy, provided the patient's pupil were not larger than the observer's. Printed by AVilliam Mackenzie, 45 & 47 Howard Street, GlasgoTr.