The effect of rotation on quantities used in classifying B -type
stars is discussed. The tendency for stars with high rotational velocities
to have weaker Balmer lines for the same (U - B)₀
has been investigated
using published data. Abt's explanation of the affect in terms of the
change in effective surface gravity due to rotation is confirmed. A
correction term in Crawford's method of determining ages of clusters and
field stars from Balmer line intensities and UBV photometry is derived.
It is shown that errors in age determinations due to rotation are much
smaller when the wavelength λ₁ of the Balmer discontinuity is used
instead of Balmer line intensities. A colour index sensitive to λ₁ is
discussed. Methods of determining the inclinations of the axes of rotation
are suggested. It is shown that the "cosmic dispersion" in the relation
between Balmer line intensities and absolute magnitudes is largely caused , by rotation. A scheme of two -dimensional spectral classification using
measurements of the position and size of the Balmer discontinuity on low
dispersion objective prism plates is outlined. Rotational velocities for
18 stars have been derived from slit spectrograms; several lines including five Balmer lines were measured for each star
