the exterior surface of a convex, i.e., gibbous and globular thing; in opposition to concavity, or the inner surface, which is hollow or depressed. See CONCAVE.

The word is of particular import in catoptries and dioptries; where it is applied to mirrors and lenses.

A convex mirror represents its images smaller than the objects; as a concave one represents them larger: a convex mirror reflects the rays from it, diverging; and therefore disperses and weakens their effect: as a concave one reflects them converging, so as they concur in a point, and have their effect increased: and by how much the mirror is a portion of a smaller sphere, by so much does it diminish the objects, and disperse the rays the more. See MIRROR.

A convex lens is either convex on both sides, called a convexo-convex; or it is plain on one side and convex on the other, called a plano-convex; or concave on one side and convex on the other, called a convexo-concave, or concavo-convex, as the one or the other surface prevails, i.e., as this or that is a portion of a smaller sphere. All convex lenses inflect the rays of light in their passage, i.e., bend them out from their convex surface converging, so that they concur in a point or focus. Hence all convex lenses magnify, i.e., represent their images larger than their objects; and this the more as they are portions of smaller spheres.