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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    lens. A lens with a curvature in only one direction—a so-called cylindrical lens—is requircd. A lens with no curvature in the vertical, and a corresponding positive curvature in the horizontal meridian will, in the above example (see Fig. E), correct the abnormally weak hori- zontal curvature of the refracting surface. Sorae ophthal- moscopes permit of the use of such cylindrical lenses, but they are not absolutely necessary for the measure- ment of astigmatism. With a little practice it is usually quite possible to measure the refraction for every indi- vidual meridian. [Dr. B. Alex. Randall has designed an ophthalmoscope which carries a disk, by means of which cylindrical lenses can be rotated behind the sight-hole, which is of great practical value.—Ed.] The student should remember that the vertical meridian of a spherical or sphericocylindrical (astigmatic) lens corrects the horizontal and not the vertical lines of the object, while the horizontal meridian corrects the vertical and not the horizontal lines. If, therefore, the horizontal vessels running from the optic disk toward the macula are seen distinctly, it shows that the refraction of the vertical meridian of the refractive System has beeil cor- rectly determined. Another glass will be requircd to obtain a clear image of the vertical lines (those, for in- stance, that run into the macula from above or below), or of the temporal margin of the optic disk which at one point is approximately vertical. This glass corresponds to the refraction of the horizontal meridian of the refrac- tive System. It happens sometimes that the principal meridians intersect cach other obliqucly instead of at a right angle. In that case the use of a certain lens will enable the observer to see the vessels running outward and upward near tlie optic nerve, while those which run outward and downward appear quite indistinct. To see the latter another glass will be required, with which again the vessels running outward and upward will appear in- distinct. In measuring the astigmatism it is of the