doublet. Further, practical difficulty in duplicating the classifications has been caused by the fact that the
and
lines are so far into the violet that they do not appear at all on many slit spectra, such as those used at Mount Wilson, and when other criteria are chosen for classification, it is likely that the results will deviate somewhat from those of the Henry Draper Catalogue.
By analogy with what is observed in other types, it has been suggested that the range in line-sharpness that is found within a given class among the
stars is an effect of absolute magnitude, and the sharpness of the hydrogen lines has indeed been used at Mount Wilson[459] as a quantitative measure of luminosity. From an analysis of the widths of hydrogen lines made by Miss Fairfield,[460] it appears that the line sharpness may be used to distinguish