Up to the present we have only considered lenses in which the curved surfaces are spherical, and for this reason are called spherical lenses. There are, however, other forms of lenses used, especially for the correction of defects in vision, principally astigmatism, of which the important mportant has one, sometimes both, of the curved surfaces cylindrical. These are different curvature. Its form is illustrated by a section of a bicycle tire. (Fig. 7). These lenses are much in use for lighthouses, being usually built up by a series of 'concentric rings of almost prismatic form.
Type lenses are combined for economic use generally in four ways: (1) to produce an en larged image, as in the reading-glass and mi croscope; to produce a smaller image, as in the photographic camera; (3) to gather and project rays of light in a cylindrical beam, as in the lens of a lighthouse, or the condenser of a stereopticon; (4) to condense and project to a focal point, as in the burning glass. The lenses for telescopes, microscopes, cameras and stereopticons require to be ground and polished with the most exquisite accuracy. In a some what less degree accuracy of form an finish is required in spectacle lenses. The following list comprises the usual forms of lenses in commercial use Achromatic In its simplest form is a combinatitin of a converging crown glass lens and a diverging flint glass lens so .proportioned that the chromatic aberration is corrected for two colors. These lenses are generally com bined by a thin transparent cement which makes them together appear and operate as one. lens. (See Fig. 8). They are also made up of three, four or five lenses cemented together.
Aplanatic lens or a lens system which is free from spherical aberration.
Apochromatic achromatic lens in which the chromatic aberration is corrected for three colors and the spherical aberration is also very perfectly corrected.
Bifocal double focus spectacle lens first introduced by Benjamin Franklin, and now commonly made by adding to or inserting a segment in the lower half of the lens.
piano convex lens of relatively short focus used for illuminating pur poses, generally in a hand lantern.
Cataract short focus convex lens to aid vision after the removal of the crystalline of the eye for cataract.
Coddington Originally the central portion of a glass sphere but is now generally made a single lens of considerable thickness with convex surfaces, with a circular groove to cut out the marginal rays.
Collective In a microscope or tele scope eyepiece the large lens nearest the objective.
Compound Lens (a trade term).—A lens in which combinations of piano, spherical, cylin drical or toric surfaces are made, sometimes to the extent of obtaining a prismatic effect.
Condensing convex lens or a sys tem of lenses for concentrating light to a point or on a surface.
Coquille Lens (trade term).—A piece of colored glass of uniform thickness and having concentric spherical surfaces.
Crossed Either a double convex or double concave lens with the radii of curvature in the proportion of 1 to 6, and giving the mini mum amount of spherical aberration.
Crystalline The natural double convex fluid lens in the eye situated behind the iris and aiding to form the image on the retina.
Demonstration series of lenses of pronounced curvatures to illustrate the various types of lenses.
Doublet combination of two separate single lenses. These lenses may be either simple or compound.
The lens nearest the eye in eye-pieces (oculars) used for microscopes and telescopes.
Eyeglass spectacle lens used to aid vision. As a single lens it is held in position by muscular contraction. The eyeglass with lenses for both eyes is generally accepted to mean the form which is held in position by clamping upon the nose (French, or it may be attached to a handle.
Finder lens or a combination of lenses attached to a camera to locate the bound aries of the field of view, Fluid lens produced by filling the space formed by two•transparent surfaces with a suitable liquid.
Fresnel lens formed of a cen tral piano convex or tonic convex tens bounded by ring-shaped prisms and lenticular prisms, used to project the rays from a lamp, as in a lighthouse or railway signal light.
Immersion microscope objective of high power, the front lens of which is con nected by fluid to the cover glass of an object, thus giving increased angular aperture.
Kryptok spectacle lens com pounded by countersinking the ordinary lens and filling the aperture with glass of a different angle of refraction and fusing the two into one in the electric furnace.