The progress of ophthalmic surgery : from the invention of the ophthalmoscope (in 1851) up to the present time : being an oration delivered before the North London Medical Society on February 11, 1863 / by John Zachariah Laurence.

  • Laurence, John Zachariah, 1828?-1870.
Date:
1863
Catalogue details

Licence: Public Domain Mark

Credit: The progress of ophthalmic surgery : from the invention of the ophthalmoscope (in 1851) up to the present time : being an oration delivered before the North London Medical Society on February 11, 1863 / by John Zachariah Laurence. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    peculiarity in his own eye: this 'in a state of relaxation collects to a focus on the retina those rays which diverge ver- tically from an object at the distance of ten inches from the cornea, and the rays which diverge horizontally from an object at seven inches distance/* Consequently, the refraction of his globe was greater in the horizontal, than in the vertical meridian. In 1827, Professor Airy published a remarkable instance of the same anomaly in his own (left) eye.f In this, the furthest point of distinct vision for vertical rays was three and a half inches; for horizontal ones, six inches; the eyeball thus being nearly double as myopic in the vertical, as in the horizontal meridian. To Airy likewise belongs the merit of first having applied cylindrical glasses to the cure of astig- matism. This has been shown by exact measurements to depend generally on an inequality of curvature of the ver- tical and horizontal meridians of the cornea. It may, how- ever, as in Young^s case, originate in an irregularity of cur- vature or position of the crystalline lens. Astigmatism is re- mediable by cylindrical lenses. These represent sections of cylinders parallel to their axes. Such lenses have the pecu- liarity of exerting a lenticular (refracting) influence on rays striking them transversely to the axis, allowing those striking them parallel to the axis to pass through no more refracted, than they would be by a piece of plane glass. Thus we may add to or subtract from, by cylindrical, convex or concave lenses, the refractive power of one meridian of the globe, leaving the other unchanged, and thus restore the equality of refraction in the two meridians—correct the astigmatism. Up to the period of Donders's recent researches, only eleven cases of this optical defect had been recorded. He has shown that astigmatism is really a very common disturbing cause of vision, and that many cases hitherto but imperfectly correctiblc by ordinary (spherical) lenses, are almost completely so by * Philosophical Transactions for 1801. t Transactions of tho Cambridge Philoso}.hical Society for 1827, vol. ii. p. 2G7.