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.

    48/376 (page 44)
    when tlie lens is at a greater distance from the eye (Mauthner, loc. eit). In the indirect method the size of the image depends chiefly on the strength of the auxiliary lens ; that is, the stronger the lens, the smaller the image and the greater the visual field. According to v. Ilelmholtz (loc. cit., p. 218), if we take Lasting’s schematic eye as a basis, we obtain a magnification of 3 diameters with an auxiliary lens of 22 I) (focal distance 45 mm.), and a magnification of 4 diameters with a lens of 16.6 D (focal distance, 60 mm.). The refraction of the eye under examination also has some influence on the size of the image. In hyper- metropia the magnification is somewhat greater, and, con- versely, in myopia it is somewhat weaker. According to Mauthner (loc. cit., p. 230), the magnification is greater in hypermetropia due to diminution in the length of the axis than in hypermetropia due to diminished refractive power of the dioptric System; and, conversely, the mag- nification is less in myopia due to lcngthening of the axis than in myopia due to abnormally strong refractive power of the dioptric System. Whereas, therefore, in the direct method the fundus appears appreciably modified, that is under 12-14-20 diameters, the magnification is only from 2-4-8 diameters in the indirect method with a convex lens of 17 1), which is the most suitable for tliis method. On the other hand, the indirect method, as has beeil stated, otfers the ad van tage of a larger visual field, lience it follows that to obtain a general idea of the conditions in as short a time as possible it is best to examine first by the indirect method, and if it is desired to tuake a more accuratc study and to measure the refraction, an exami- nation may be made by the direct method which shows the eye-ground under a greater magnification. [Students in tliis country are apt to neglect indirect ophthalmoscopy. This is unfortunate, and Ilaab’s advice, to employ first the indirect and tlien the direct method, is sound and shonld be followed.—Ed.]