A magic eye tube or tuning indicator, in technical literature called an electron-ray indicator tube,[1] is a vacuum tube which gives a visual indication of the amplitude of an electronic signal, such as an audio output, radio-frequency signal strength, or other functions.[1] The magic eye (also called a cat's eye, or tuning eye in North America) is a specific type of such a tube with a circular display similar to the EM34 illustrated. Its first broad application was as a tuning indicator in radio receivers, to give an indication of the relative strength of the received radio signal, to show when a radio station was properly tuned in.[1]