A compend of the diseases of the eye and refraction : including treatment and surgery / by George M. Gould and Walter L. Pyle.

  • Gould, George M. (George Milbry), 1848-1922.
Date:
1899
Catalogue details

Licence: Public Domain Mark

Credit: A compend of the diseases of the eye and refraction : including treatment and surgery / by George M. Gould and Walter L. Pyle. Source: Wellcome Collection.

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    Numbers 3 and 6, having opposite sides at different surfaces, are called periscopic or tneniscus lenses, and are used to avoid spheric aberration and to gain a greater field of clear vision. The principal axis of a lens is a line passing through the optic center, at right angles to the surfaces of the lens. Rays passing through this axis are not refracted. Rays passing through the optic center of a lens, but not passing through the principal axis, are slightly deviated, although in practical optics they may be considered as straight lines. The focal length of a lens is the distance from the lens to the point at which parallel rays, refracted by the lens, focus. A cylindric lens is a lens with a plane surface in one axis, and a convex or a concave surface in the axis at right angles to it. This form of lens is really a segment of a cylinder. Examination of any cylinder—as, for instance, a bottle — will show there is curvature only in one direction, from side to side, and not in the axis. As the axis of a cyhnder has a plane surface, the rays are only refracted at right angles to the axis, and the strength of the cylinder depends on the curvature possessed by the surface at right angles to the axis. The axes of the cylinders in a test case are usually shown by grinding and making partially opaque portions of each side of the lens in the direction of the axis. It must be remembered that a cylinder refracts rays of light only in the meridian at right angles to its axis, while a spheric lens refracts rays of light in every meridian. Classification and Numbering of Lenses.—Lenses are numbered according to their focal distance, and the strength of the lens varies inversely as its focal distance. Formerly lenses were numbered according to the inch system, the unit of which was a lens whose focal distance equaled one inch,—a very strong lens. This system necessitated the use of large numbers or fractions ; and, moreover, the denomination inch had different significations in different coun- tries, the French inch, for example, varying from the English inch. The metric or dioptric system of numbering lenses is now in use, and the unit is a lens having a focal distance of one meter, and is called a lens of one diopter strength—a comparatively weak lens; a two- diopter lens is one having half the focal length of the one-diopter lens, or >^ of a meter. Decimals, of course, are used instead of fractions; a lens of a focal length of four meters is called an 0.25- diopter lens.