Vacuum tubes ruled electronics for several decades and while you might think of them as simple devices analogous to a transistor or FET, there were many special types. We’re all familiar with nixie tubes that act as numeric displays, and there are other specialty tubes that work as a photomultiplier, to detect radiation, or even generate microwaves. But one of the most peculiar and distinctive specialty tubes has an intriguing name: a magic eye tube. When viewed from the top, you see a visual indication that rotates around a central point, the out ring glowing while the inner is dark, like an iris and pupil.
These tubes date back to the RCA 6E5 in 1935. At the time, test equipment that used needles was expensive to make, so there was always a push to replace them with something cheaper. They were something like a stunted cathode ray tube. In fact, the inventor, Allen DuMont, was well known for innovations in television. An anode held a coating that would glow when hit with electrons — usually green, but sometimes other colors. Later tubes would show a stripe going up and down the tube instead of a circle, but you still call them magic eyes.
The indicator part of this virtual meter took the form of a shadow. Based on the applied signal, the shadow would be larger or smaller. Many tubes also contained a triode which would drive the tube from a signal.
There’s a great web site full of information on these venerable tubes and it has examples of these tubes appearing in plenty of things. They frequently appeared in service equipment, radios, and tape recorders. They even appeared in pro audio equipment like the Binson Echorec echo-delay unit.