Abstract: A structural arrangement for mounting a mirror in a reflex optical system. The mirror is mounted on an apertured rear wall of an opaque plastic structural member which defines an exposure chamber of the system. The aperture formed in the rear wall has edges lying substantially in the mirror plane. These edges define the limits of the reflecting surface of the mirror. The orientation of the plane of the mirror is defined by three mounting bosses extending from an exterior surface of the rear wall. Surrounding the aperture are outwardly extending flanges designed to cradle the mirror when it is placed over the aperture. A capping retainer clips onto the rear wall to secure the mirror in place and cooperates with the flanges to form a light seal.