Object is a metal box containing socket for a missing round flat faced vacuum tube (similar tubes are catalogued as photomultiplier tubes). The flat end of the tube faces a half-silvered mirror set at a 45-degree angle. A small glass port behind the mirror allows viewing through the mirror. One coax cable ending in a BNC connector, and one multi-conductor cable ending in a large round 15-pin screw connector are attached to the end of the box.
