LENS. A lens is a transparent body constructed for the purpose either of concentrating or scattering rays of light transmitted through it.
Lenses are in general made of glass or rock crystal, and ground with spherical surfaces ; the axis of the lens being the line joining the centres of the spheres, and therefore a line with respect to which the lens is symmetrical.
In the above figure six different forms of lenses are exhibited ; and light is supposed to pass through them in the direction shewn by the arrow.
No. 1 is called " bi-convex ; No. 2, piano-convex ; Nos. 3 and 4, concavo-convex, or more frequently " meniscus ;" No. 5, concavo plane ; No. 6, double concave. The nature of the surface upon which light is incident deternaines the first word of the compound appellative.
The first three lenses are thicker in the middle than at the edge, and are called " converging lenses," because they cause pencils that are refracted through them to converge more than they did before. Nos. 4, 5, 6, are thicker at the edge than in the middle, and are called " diverging lenses," because they cau.se penciLs that are refracted through them to diverge more than before.
It has been shewn in the artide on the prism that when a ray of light is refracted through a prism it is turned, or caused to deviate, from the edge of the prism. Now, we may suppose that the small portion of the spherical surface of a lens, on which a ray of light is incident, coincides with the tangent plane to the sphere at that point ; and similarly with respect to that small portion of the opposite spherical surface from which the ray emerges.
If then QRS T be a ray of light refracted through a lens, the small parts of the lens at R and S may be supposed to coincide with the tangent planes at those points, and these planes will in general intersect, if produced, in a straight line, thereby forming a prism whose refracting angle is S A R. Now, if we draw Sd parallel to QR, the emergent ray ST will have its angle of deviation, dS T, from the edge of the prism, and will follow the course indicated in the figures. The mode in which a convex lens causes a pencil to converge, and a concave lens to diverge, will therefore be easily understood from the following fig-ures, bearing in mind that the greater the angle of a prism may be, the greater is the deviation of a ray refracted through it.
No single lens is free either from spherical or chromatic aberra tion (q. v.). These errors are corrected by combining two or more lenses together. In a single convex lens, the surfaces of which have radii of different lengths, there is the least spherical aberration when the most convex side is presented to the incident pencil. When the refractive index of the glass is 1.5, the radii of the surfaces should be as 1 : 6, and the most convex side presented to the origin of light. This lens has the least spherical aberration of all single lenses, and is called the " crossed lens." If we call the aberration of the crossed lens unity, the following table, calculated by Sir John Herschel, shows the aberration of some other forms of lens :— If two plano-convex lenses are placed with their convex sides in contact, and the focal length of the first be to that of the second as 1 : the aberration will be only one fourth that of the single crossed lens.
Aberration may be entirely destroyed by placing a meniscus and convex lens with their centres in contact, according to the following table,—the convex lens being turned to parallel rays.
Focal length of convex lens . . —10.000 Radius of first surface . . . — 5.833 second surface . . +35-000 Focal length of meniscus . — 5.497 Radius of first surface . . — 2.054 f second surface . . — 8.128 Focal length of compound lens . . — 3.474 In correcting for spherical aberration the same kind of glass may be used for both lenses, but when chromatic aberration is cured by combining lenses in contact, they must be made of different kinds of glass. The principle on which lenses are achromatized is ex plained in the article " Prism." It is only necessary to remark in this place that the formula by which the central pencil refracted through a compound lens is rendered achromatic, does not also include the case of oblique and excentrical pencils, otherwise than as an approximation to the desired result.
Having thus briefly introduced the subject of lenses, we will pro ceed to discuss more minutely the construction of those with which the photographer is principally concerned. They are the achro matized meniscus of Dr. Wollaston, intended principally for views, and the portrait and view combinations of Professor Petzval, the latter of which has been called by M. Voigtlander the " Orthoscopic lens."