Results of observations with the 82-inch telescope of the McDonald Observatory in February and March, 1953, are compared with spectral classification by W.W. Morgan, with colour indices (B-V) on Harold Johnson's system, and with absolute magnitudes from trigonometric parallaxes and spectroscopic determinations.
The photometric probable errors (one observation) of a (B-V) value predicted from the two classification indices is +-0.006, corresponding to a probable error +-0.02 of a spectral class. The photometric probable error (one observation) of a predicted value of the visual absolute magnitude Mv is +-0.18. The comparison with the data mentioned above indicates that the influence of cosmical scatter on the predicted (B-V) and Mv values is small, or negligible.
Applications in the study of galactic structure is briefly discussed.