In the course of this solar work, Wild became interested in the radio spectrum of hydrogen and wrote up an internal report related to the potential for spectral lines in the solar bursts. When Ewen and Purcell in the US first observed the 1420 MHz transition in 1951, he went back to his report, generalised it to include the interstellar medium, and six months later published the first detailed theoretical paper on the hydrogen lines – a classic in the field.