Abstract: A metal foil or plate for use in the slit mechanism of optical systems and having two jaw sections between which the slit is defined. The jaw sections are interconnected through an integral hinge portion of the plate which is adaptable to flex so that the jaw sections can move towards and away from each other and thereby vary the slit width. When incorporated in actual slit mechanism, the plate, or at least a portion thereof, is interposed between two mounting surfaces so that the jaw sections are maintained coplanar, and an actuator for causing relative movement of the jaw sections is connected to another section of the plate which is integrally connected with each jaw section through a relatively narrow plate portion. The last mentioned section of the plate and the hinge means form the only connections between the jaw sections.