Atlas and epitome of ophthalmoscopy and ophthalmoscopic diagnosis / by O. Haab.

  • Haab, O. (Otto), 1850-1931.
Date:
1910
Catalogue details

Licence: In copyright

Credit: Atlas and epitome of ophthalmoscopy and ophthalmoscopic diagnosis / by O. Haab. Source: Wellcome Collection.

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    or object glass] from the plane of tlic pupil of the ex- amined eye should be approximately equal to its focal distance. In tliis way the pupil and the iris disappear cornpletely and nothing but an image of the eye-ground is seen in the entire domain of the convex lens. The larger the glass the wider the view of the eye-ground obtained ; he nee, The Size of the Visual Field 1 in the indirect method depends chiefly on the size of the auxiliary lens, and to some extent on the refraction of the eye under examination and the focal distance of the aux- iliary lens. The higher the degree of myopia of the eye under examination, thegreater the ophthalmoscopic field ; the higher the degree of hypermetropia, the smaller the ophthalmoscopic field. The nearer the auxiliary lens is brought to the eye, that is the stronger the auxiliary lens, the greater the number of rays emerging from the eye it can receive and the greater, therefore, the visual field. Finally, as in the direct method, the size of the visual field depends on the size of the area illnminated by the ophtlialmoscope. A greater portion of the eye-ground is illnminated in the indirect than in the direct method ; the illumination is also more powerful, as a concave mirror is generally used in tliis method. The size of the pupil does not affect the size of the visual field so much in the indirect as in the direct method ; in other words, the eye- ground can be seen quite distinctly even when the pupil is very small, although, of course, a contracted pupil cuts off a great deal of the light and the image may, therefore, becorne very indistinct if the pupil is too small. To sum up, it may be said that tlie indirect method has the following advantages over tlie direct method: 1. Greater distance from the eye under examination, making the examination less irksome for both (he patient and the surgeon ; 2. A larger visual field, so that a much greater [' “ Gesichtsfeld ” is tlie Word used ; in Euglish text-books simply tlie word “field” is usually etiiployed.—Ed.]