There are certain prominent lines in the spectra of the galactic nebulæ— in particular the “nebulium” green lines, N
1
(λ = 5007) and N
2
(λ = 4959), which dominate most nebular spectra—whose identification remained till recently a mystery. In 1927, however, Bowen proposed his now well-known hypothesis that such lines are due to transitions from metastable states in certain ions. The spectra involved are those of O III (which includes the lines N
1
, N
2
), N II, O II, S II, and O I. To explain the fact that such lines occur, although they are “forbidden” transitions, Bowen postulated that there is a non-zero, though relatively small, probability of transition between the states in question. The strength of such lines is then accounted for if we suppose there is a large concentration of ions in the initial states in question due to transitions from higher levels, and that, due to the very low density in the nebulæ, such atoms remain undisturbed by collisions long enough for the radiation to become effective. The non-occurrence of such lines in the laboratory (except the O I lines observed by Hopfield,
loc. cit.
) is thus explained by our inability to reproduce such conditions.