During the latter years of the nineteenth century, it was observed that when some gases, or mixtures of gases, were excited by an electrodeless discharge, a luminescence or afterglow with a continuous spectrum was produced. In 1900, Lewis reported that nitrogen subjected to a condensed discharge gave an afterglow different from those previously obtained from other gases. He described it as a “rich chamois yellow fog”. It lasted for several seconds after the discharge had been turned off, and its spectrum consisted of narrow bands. Several years later, Strutt, who later became Lord Rayleigh, observed that the spectra of elements and compounds excited in the presence of the yellow nitrogen were the same as those produced by other methods.