Atlas and epitome of ophthalmoscopy and ophthalmoscopic diagnosis.

  • Haab, O.
Date:
[1901], ©1901
Catalogue details

Licence: In copyright

Credit: Atlas and epitome of ophthalmoscopy and ophthalmoscopic diagnosis. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

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    be produced, it follows that an astigmatic surface can produce only a distorted image. Hence the image on the retina of an astigmatic eye will be distorted whether the abnormal curvature is in the lens or in the cornea; and, conversely, when such an eye is examined with the ophthalmoscope, the markings on the eye-ground (optic disk, vessels of the retina, etc.) appear blurred and can- not be brought out clearly with any additional spherical lens. A lens with a curvature in only one direction—a so-called cylindrical lens—-is required. 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. Some 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 ot a spherical or spherocylindrical (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 been cor- rectly determined. Another glass will be required 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 refmc tive system. ldL It happens sometimes that the principal meridians