Lens (Lat. lens or lentis, a lentil or pea), a transparent body used for refracting light. A convex lens is usually of the form of two segments of spheres, united by their bases; such a lens is called a double convex; if the lens consist of only one segment of a sphere, that is, is spherical on one side and plane on the other, it is called a plano-convex lens. A concave lens, on the contrary, has a concavity on either side, into which part of a sphere will fit, and is called a double concave; if one side is plane and the other concave, it is called plano-concave. When one side of the lens is convex and the other concave, if the edges of the lens are thinner than the centre, it is called concavo-convex, and also a converging meniscus; if the centre of the lens is thinner than the edges, it is called convexo-concave, or diverging meniscus. Concave lenses are used in spectacles for the relief of near-sighted persons, and in the eyepiece of opera glasses and spy glasses of low power. Convex lenses are used for far-sighted persons and singly as magnifiers.
They cause the rays of light which pass through them to converge toward the central line at right angles to their surfaces; so that to an eye in the right position, rays from different parts of an object make a greater angle than if they had not come through the lens. Convex lenses are also used in combination in telescopes and microscopes, in which the image formed by one lens is looked at under the magnifying power of a second. The image is formed by a convex lens, by means of its power to make the rays of light converge, which brings all the light that emanated from each point of the object again to a point in the air on the opposite side of the lens. These points of the image have nearly the same relative position as the corresponding points in the object, and may be rendered visible by being received upon smoke or vapor, or as in the camera obscura and magic lantern upon a sheet. The image in the clear air can be seen by an eye placed in a line prolonged from the object through the image. If the image be formed by a single convex lens, it will on being magnified be found to have two principal imperfections, arising from spherical and from chromatic aberration.
The nature of these imperfections, and the means employed for overcoming them by the makers of optical instruments, are explained under Aberration, Achromatic Lens, and Aplanatic Lens. - The material employed in the construction of lenses for optical instruments is generally crown glass which contains very little lead, and flint glass which contains much lead and has a greater refractive power. The glass should be perfectly homogeneous and free from striae. The production of such glass in masses of sufficient size to make lenses for large telescopes is a work of great difficulty. The best specimens yet produced have been from the manufactory of the Messrs. Chance of Birmingham, England, made by a process invented by Guinand, a Swiss optician, the details of which have never been made public. (See Glass.)
Lens, a town of France, in the department of Pas-de-Calais, on the Souchez, 9 m. N. N. E. of Arras; pop. in 1866, 5,738. Lace and woollens are manufactured here, and in the neighborhood there are coal mines. In August, 1648, the French under Conde obtained here a great victory over the Spaniards.