The original magic eye
 

The thread titled "Magic Eye" brought back memories of magic eye detectors. Essentially they were small, valve-like devices that were used as null detectors in bridge equipment. My recollection is of their use in a Mullard (I think) conductivity bridge. The user would see a small, circular window. In this there was was what looked like 2 glowing green segments of circles, apex to apex. As the bridge was brought into balance by twiddling the large Bakelite knob the segments would pass through a minimum size.