Measurements of hydrogen lines in B-stars by a method of photographic narrow-band photometry.

A new method of photographic narrow-band photometry has been applied to the measurement of the hydrogen lines H y and H6 in the spectra of a number of B-stars. The method, which is based on the same principle as the photoelectric H O photometry of Str(5mgren and Crawford, yields indices of hydrogen line strength of high accuracy. The principal index of the mean strength Hy and H6, called I (30, 120), has a total range of approximately 0.400 magnitudes and a rms error of around + 0.007 magnitudes. An analytical expression for the rms error caused by plate graininess is derived and the principles of photographic filter photometry discussed. Key words: B-stars - spectro.photometry - hydrogen lines - narrow-band photometry