The fundus oculi with an ophthalmascopic atlas, illustrating its physiological and pathological conditions / By W. Adams Frost.

  • Frost, W. Adams (William Adams), 1853-1935.
Date:
1896
Catalogue details

Licence: Public Domain Mark

Credit: The fundus oculi with an ophthalmascopic atlas, illustrating its physiological and pathological conditions / By W. Adams Frost. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

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    these together so as to form a single picture. The practice is too obviously convenient to need defence. The drawings were made by the aid of artificial light, and appear to the best advantage when viewed under the same conditions. They are life-size, but it must be remembered that the apparent size of the ophthalmoscopic image may vary in different observers. The actual retinal image received by the observer must be of the same size as the part of the patient’s eye which produces it (if both his eye and that of the patient are emmetropic and of the same dimensions). But the apparent size of the mental picture will depend upon the distance to which the observer projects his retinal image. Fig. 1.—Diagram to illustrate the enlargement of the image in direct ophthalmoscopic examination. This will be evident from Fig. 1, in which A represents the eye of the observer, and B that of the patient. Both are emmetropic and of identical dimensions, and they have a common principal axis, x y. Let a b represent a portion of the patient’s eye seen by A. b lies on the principal axis of B, therefore its image, V, is on the principal axis of A. Let a ray, a o, from a pass through the optical centre o, then all other rays from a will, after emerging from the eye, be parallel to a o. Let one of these, a o', pass through o, the optica] centre of A, then it will be unrefracted, and will cut the retina at a. This point will give the position of the retinal image of a. It will be found that the image a b' is equal to the object a b. For since a o and a o are parallel, the angles at o and o are equal, and since the distance from o and o' to their respective retinae are equal, a b = a' V. It is evident that A may project the retinal image a V to bv b2, or b3, or to any other distance, and that the size of the virtual image will vary accordingly. Since the apparent size of the ophthalmoscopic image may not be the same to different observers, it is necessary in describing