New truths in ophthalmology / as developed by G. C. Savage.

  • Savage, G. C. (Giles Christopher), 1854-1930.
Date:
1896
Catalogue details

Licence: Public Domain Mark

Credit: New truths in ophthalmology / as developed by G. C. Savage. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

    17/294 (page 3)
    An astigmatic eye is one whose cornea has not the same radius of curvature for all its parts. That part of the cor- nea having the shortest radius is the most rapidly curved; while the part with the longest radius is the least rapidly curved. These two parts are always at right angles the one to the other.* As a rule the meridian of greatest curvature is in the vertical or nearly so; while the meridian of least cur- vature is nearly or quite horizontal. A^s we go from the meridian of greatest toward the meridian of least curvature we find every meridian less rapidly curved, hence each having a longer radius than the one preceding it. The law of refraction by curved surfaces I will illustrate by supposing that we have before us two spheres of crown glass, one having a radius of two inches, and the other a ra- dius of one inch. Parallel rays of light, on entering either of these spheres, will undergo such a bend at the points of entrance as will bring them to a focus at the opposite ex- tremity of the diameter of the sphere. The larger sphere, having a diameter of four inches, has, therefore, a focal power of four inches; the smaller sphere, having a diameter of two inches, has a focal power of two inches. The den- sity of the one sphere being the same as the density of the other, the difference in focal power is due solely to the want of similarity in curvature. The more rapidly curved *The Ophthalmometer sometimes shows a slight variation from this rule.