_Aplan´atism_ (_a_, not, πλανἁω, to wander).--The effect of spherical aberration in rendering the image of an object seen through a lens indistinct and misty may now be intelligible. In order that such image may be distinct, the rays emanating from each point of the object must converge at one spot upon the retina. But since, when spherical aberration exists, the marginal rays are more refracted than the central ones, they will meet at foci before those formed by the latter; and when the foci of one set are coincident with the retina, so that the image would otherwise be distinct, the latter is rendered confused and indistinct by the rays of the other set. In this consideration, we imply that there are only two sets of rays, the central and the marginal; but the central and marginal rays are not separate, for the rays possess every intermediate degree of obliquity, hence the foci and images are really innumerable. Now there are evidently two methods of destroying or correcting spherical aberration, viz. by excluding the marginal rays, or by altering their direction. The exclusion of the marginal rays is often adopted; and is effected by means of a diaphragm, or stop as it is called. This consists of a plate of metal, with a round aperture in the middle, and it is placed behind the lens; but it has the serious defect of diminishing considerably the amount of light transmitted. The alteration of the direction of the marginal rays is produced by refraction, a thin plano-concave lens being placed in front of the convex one (Pl. XII. fig. 25). The doubly convex lens is composed of crown glass, and the concave lens of flint glass, which has a higher refractive and dispersive power than crown glass. In this way we get a compound lens, which, if the two lenses had the same refractive power, would simply amount to a plano-concave lens with the marginal portions removed. But as the concave lens consists of more highly refracting material than the convex, if the curve and thickness of the two lenses be properly adapted, the marginal portions of the concave correct the too great convergence of the marginal rays produced by the convex lens, and so the rays are brought to nearly the same focus. An idea of this action may be obtained from fig. 25, the dotted lines indicating the direction which the rays would take, if passing through the convex lens only. A lens in which the spherical aberration is corrected is said to be aplanatic. _Achrómatism._--Supposing the spherical aberration of a lens to be corrected, there still remains the chromatic aberration (p. 173); for although the central or mean coloured rays may meet at a focus, the other coloured rays belonging to the same compound or ordinary ray will meet at different foci, so that a series of coloured images of the object will be formed at different distances from the lens; hence, at whichever focus the object is viewed, it will appear coloured.