An apparatus for inserting a measurement light beam which can be projected onto a target object, comprising a light-refracting component having an optical axis, and at least two mirrors for deflecting the measurement light beam, and an optical overall body consisting of at least one optically effective material, wherein the optical overall body is configured in such a way that the at least two mirrors are fitted inside or at the edge, in particular as a part, of the optical overall body, in such a way that the measurement light beam entering the optical overall body is offset parallel to the incident measurement light beam when emerging from the optical overall body, so that the emerging measurement light beam lies on the optical axis of the light-refracting component of the apparatus and is projected in this form onto the target object.