Abstract: The apparatus has a first and a second member (5,6) and a device (7) for the fixing of a guide rail (1) with an optical apparatus to the members. The device (7) contains a blocking member (8) and a lever (9) with a holding device (10) standing under spring action, the lever being pivotally mounted on the blocking member and contacting the second member (6) in such a way that the lever fixes the optical apparatus in a first position, frees the optical apparatus for displacement in a second position and releases the optical apparatus for removal in a third position.