On the iron flame spectrum and those of sun-spots and lower-type stars

In 1897 I announced to the Royal Society that the lines in the spectra of the metallic elements might be separated into two series, one seen best and sometimes alone in the hotter stars, and when higher temperature and electric energy were employed, these I termed "enhanced lines"; the other set, not visible in the hotter stars, but in stars of the solar type, and seen best with lower degrees of heat and electric energy in the laboratory, were referred to as "arc lines." In a previous paper, published in 1904, I pointed out that the similarity of spot spectra and that of Arcturus depended upon the equality in temperature of the vapours existing in these celestial light sources. At this time it was held by Prof. Hale and his colleagues, who had studied and published in detail the lines in these spectra indicated the presence on such stars of many spots like those on the sun. In a later publication, however, they state that their recent work has led them to the opinion that the comparatively low temperature of these stars offers the simplest explanation of the observations.