As known as THE MAGIC EYE. It is in principle a cathode ray tube used to indicate whether the receiver is correctly tuned. Basically the magic eye is a thermionic valve consisting of three electrodes.

In this way, it has the cathode, a metal anode that is coated with a fluorescent material like a screen, which under the influence of the electron current will light up. The third electrode, in a special shape is mounted between the cathode and the anode, in order to focus the electron beam according to the voltage applied to it.
In the receiver, the magic eye is connected to the automatic gain control stage (AGC). It supplies the suitable control voltage that is applied directly to the focussing electrode in such away the light spot area changes proportinally as the frequency signal is stronger or weaker. (a)
When compared with the first types of electro-mechanical tuning devices, the magic eye as a kind of thermionic valve had many advantages. The most important one was its inertia-free operation. It is interesting to note that the company RCA originally introduced in 1930 the term: MAGIC EYE, as a kind of trade name.

(a) Several types of tuning valves or Magic Eye.

(b) Schematic showing the tuning valve assembling in a receiver chassis.