CONVEX 

MIRROR Position of Object . Position of Image . Character of Image . to Between co and A A The above discussion of spherical mirrors assumes that the mirror has such a small aperture that the reflected rays from any point unite in a point . This, however, no longer holds when the mirror has a wide aperture, and in general the reflected rays envelop a caustic (q.v., see also ABERRATION) . The only mirror which can sharply reproduce an object-point as an image-point has for its section an ellipse, which is so placed that the object and image are at its foci . This follows from a property of the curve, viz. the sum of the focal distances is constant, and that the focal vectores are equally inclined to the normal at the point . More important than the elliptical mirror, however, is the parabolic, which has the property of converting rays parallel to,the axis into a pencil through its focus; or, inversely, rays from a source placed at the focus are converted into a parallel beam; hence the use of this mirror in search-lights and similar devices .