Abstract The curious optical emission spectrum of the solar corona puzzled spectroscopists for decades until it was suggested by Grotrian ( l 931, 1933, 1939) that the temperature is of the order of 106K. With this possibility in mind Edlen (1942a,b) was able to identify nineteen of the coronal lines (Lyot, 193:J) as arising from forbidden transitions of highly ionized atoms (e.g., Fex, f-exIv, NixIII, CaxII), clearly establishing an electron temperature of the general order of l06 K. Radio observations corroborated this result.