In optics a lens is considered to be a portion of a refracting medium bounded by two curved surfaces, or by one plane surface and one curved surface, and is usually made of glass or other transparent substance.

The variety of lenses made for different purposes is very great, and a few of the general terms with definitions are as follows :-

Achromatic

A lens tha t is corrected for chromatic aberration, as compared with an apochromatic lens; it is corrected for only two colours.

Aplanatic

A lens that is free from spherical aberration.

Apochromatic-A lens corrected for chromatic aberration for the secondary spectrum.

Coddington

A cylindrical lens with a deep groove cut in midway between the ends, and acting as a diaphragm giving a good definition, and wide field, generally used as a pocket magnifier.

Concave or Diverging. A lens that is thicker at the edge than in the centre.

Convex Or Converging

A lens that is thinner at the edge than in the centre.

Fluid

A lens in which a liquid is imprisoned between circular glass, or transparent discs of the required curvature.

Fresnel

A compound lens formed by placing around a central convex lens rings of prisms so shaped as to have all the same focus ; used in lighthouses.

Meniscus

A lens that is convex on one side, and concave on the other, but of different curvatures, so that the lens is thicker at the centre than at the edges, used for the purpose of minimising spherical aberration.

Photographic

A lens specially designed for taking photographs.

Portrait. Hectilinear lenses of special construction designed to give, when used with a very large aperture, a brilliant image, very sharply defined over a limited area, of an object lying approximately in one plane.

Rapid Rectilinear

These lenses consist usually of two single meniscus lenses placed at a suitable distance apart with their concave surfaces inward and an adjustable diaphragm between them.

Stanhope

A lens with its focus at one of its surfaces.

Telephoto

A compound photographic lens giving large pictures at short camera extensions for distant objects.

Wide Angle

A photographic lens of short focal length giving a wide angle of view.

Lenses are expensive articles, and therefore are well worth treating with care, The chief points are : Keep in a dry place, dust with a camel-hair brush, never unless absolutely necessary take the lens or parts of a compound lens arrangement out of its mount, never rub the surface with any old cloth that is handy ; if a cloth of some sort is necessary then use a perfectly clean, damp wash leather. It is more or less easy to scratch a lens surface by careless rubbing, and so ruin it for fine work. It is worth remembering that no foreign made lens can put up a better performance than the best British lenses, and that in the event of any accidental damage to the lens, or its mount, or if any alteration be wanted, that (inside the United United Kingdom) the works of the British Makers are on the spot for any work required, a great point if time is valuable.