Physical diagnosis : a guide to methods of clinical investigation / by G.A. Gibson and William Russell.

  • Gibson, George Alexander, 1854-1913.
Date:
1890
Catalogue details

Licence: Public Domain Mark

Credit: Physical diagnosis : a guide to methods of clinical investigation / by G.A. Gibson and William Russell. Source: Wellcome Collection.

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    light were discussed under the heading Catoptrics (Kdro7TTpov = a, mirror), while those relating to refraction of light were termed Dioptrics (StoirTO/Juac = to look through). Hence the unit of dioptric (refractive) measure- ment has been called a dioptre. If a lens has only half the refractive power of the unit lens its focal point will he at twice the distance, namely, 2 metres. If it he twice as strong as the unit lens, the focal point will he at half the distance; if thrice as strong, at a third of the distance, and so on. Only convex lenses can produce positive, i.e., real foci. A concave lens causes divergence of rays, hut its refractive power can he expressed in terms of its negative focus, i.e., the point from which it makes parallel rays appear to come. Lenses are accordingly numhered thus :— Convex. — + 0*25 D. Focal distance, 4 metres, or (roughly) 160 inches. + 0-5 D. „ 2 „ „ 80 „ + 075 D. „ 1-5 „ „ 60 „ + 1D. ,, 1 metre. „ 40 „ + 2D. ,, -5 „ or 50 centim. ,, 20 ,, And so on. Concave. — -0-25 D. -0-5 D. -0-75 D. -ID. and so on. These have the same negative focal distances as the convex lenses of corresponding strength. Spherical lenses are those whose surfaces are arcs of spheres, and have the same refractive power in every meridian. Cylindrical lenses are curved only in one direction. Eays which enter such a lens in planes parallel to the axis of the cylinder pass through unchanged. Those in planes at right angles to the axis meet with curved surfaces and undergo refraction. Cylindrical lenses may he convex or concave, and are numhered like sphericals. In writing we indicate the difference thus (say), +1 D sph. or + 1 D cyl., -ID sph. or - 1 D cyl.]