One of the strangest and most useful devices from the golden age of tubes was the magic eye. They were used from the late 1930s through the sixties, and have all but disappeared. The most common use was as an indicator for how well a station was tuned in on a radio. They were also quite popular with scientific and test equipment.
The principle is simple; a plate is bombarded with electrons, causing it to glow (much like a picture tube). As the control voltage is increased or decrease, the area lit up changes. They were originally seen as a less expensive replacement for analog meters, with the advantage of no mechanical lag time. It was later recognized that they created a better user experience, and could be displayed in an artistic manner.
For more information and a gallery of beautiful pictures, see All About Magic Eye Tubes. Read more »
RIP 
