The errors of accommodation and refraction of the eye and their treatment : a handbook for students / by Ernest Clarke.

  • Clarke, Ernest, 1857-1932.
Date:
1911
Catalogue details

Licence: In copyright

Credit: The errors of accommodation and refraction of the eye and their treatment : a handbook for students / by Ernest Clarke. Source: Wellcome Collection.

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

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    grinding the surface nearer the eye more concave, and reducing the other : thus, — 8 may be, and usually is, made — 4 on each surface, or the surface nearer the eye may be made — 6 and the other - 2 ; or the lens may be ground as a diverging meniscus (Fig. 7, f) ; thus, if - 3 is required, the surface next the eye is ground as — 5, and the other surface as + 2. Cylindrical Lenses.—When a cylinder only is pre- scribed, it is ground on one surface, the other surface remaining plane. When combined with a spherical lens, it is usual to grind the sphere on one surface, and the cylinder on the other. When a convex sphere and cylinder are required, the periscopic effect can be pro- duced by grinding a concave cylinder on the surface nearer the eye, and increasing the spherical strength by the amount of the cylinder. Thus, supposing + 2 cyl. axis vert, c:^ + 3 sph. is needed, it may be ordered thus : — 2 cyl. axis horizontal 0 + 5 sph. In ordering glasses for mixed astigmatism (see page 129), the periscopic effect is produced by combining a convex cylinder with a concave sphere, and mounting the latter next the eye. As it is very important to divide the strength of the lens between the two surfaces when dealing with high powers, a cylinder, if also required in such a case, must be worked on one of the spherical surfaces. These lenses are called Toric lenses, and any combination of spherical lens (.25 to 18), with cylindrical lens (from .25 to 3) can be supplied. Thus, if — 14 sph. 0 — 2 cyl. axis horizontal is required, one surface is made - 7, and the other — 7 sph. 0 — 2 cyl. These toric lenses are very useful in high myopia, and also in aphakia. Bi-Focal Lenses (see Presbyopia, page 142).—Where different lenses are required for distance and reading, bi-focal lenses are generally prescribed. The earliest forms of these glasses were straight split