A “magic eye” tube is an electronic vacuum tube that provides visual indication, usually in the form of green light, on an area called the target inside the tube. The target is partially illuminated with the exception of the shadow area, which varies in size and shape depending on the signal applied to the tube.
The “Magic Eye” tuning tube used in this video, is in and early National NC-101X. The “shaded” area will narrow up when the station is properly tuned in. You can also see the eye “open and close” as the AM signal is modulated. Hard to find these now, many of the early National sets had the Magic Eye replaced with an “S Meter”.
The phase “magic eye” become a trademark of the Radio Corporation Of America (RCA) in the mid 1930s, who introduced the tube as a visual tuning aid for radio receivers. Other names for the magic eye include “tuning eye”, “cat’s eye” and its technical name, “cathode ray indicator”.