differs by 0 or ± 1. If, however,
in both cases, no radiation occurs. Lines corresponding to a change of
are found in strong magnetic fields, and a very few in their absence.
“The combination of two multiple terms gives rise, therefore, to a group of lines (which may number as many as eighteen). Such groups have been called multiplets by Catalan. Their discovery has afforded the key to the many-lined spectra....
“In such a group, those lines for which the changes in
and
