=371. Construction of Real Images.=--There are five positions at which an object may be situated in front of a concave mirror, namely: (1) _beyond C_; (2) at _C_; (3) _between C and F_; (4) _at F_ and (5) _between F_ and _V_. There are two ways by means of which the image formed at each of these positions may be located, namely; (1) _experimentally_, by allowing the rays of light from a luminous body to focus on a screen and (2) _diagrammatically_. By the latter method the two rays of light are considered the course of each of which may easily be determined; first, the ray which strikes the mirror parallel to its principal axis and which after reflection passes through the principal focus; second, the ray which passing through the center of curvature strikes the mirror at right angles and therefore after reflection must pass directly back along its incident path. Where these two reflected rays intersect is located the real image of the object. Whenever these two rays of light do actually intersect, as in Fig. 360, a real image (_ab_) is formed of the object _AB_. The points _A_ and _a_, _B_ and _b_ and others similarly situated on an axis extending through the center of curvature _C_ are called _conjugate foci_, for they are so related that an object being at either one, its image will be found at the other. [Illustration: FIG. 362.--Action of a convex mirror upon parallel rays of light.] =372. The Convex Mirror.=--There are few practical uses to which convex mirrors can be put. They are sometimes used to give the chauffeur of an automobile a view of the road behind him. It is then attached to the wind shield by a short rod. The reflected rays coming from a Convex mirror are always divergent (see Fig. 362), hence the image is always virtual and located behind the reflecting surface. The method of construction for images formed by a convex mirror is similar to that for concave mirrors. (See Fig. 363.) The center of curvature and principal focus are behind the mirror and consequently the reflected rays have to be produced backward until they meet. The images are always _virtual_, _erect_ and _smaller_ than the object. [Illustration: FIG. 363.--Construction of an image by a convex mirror.] [Illustration: FIG 364.--Illustrations of Spherical Aberration.]