Quantitative formulae for the intensities of lines from a pair of interacting states to a common lower state are applied to perturbations in band spectra. It appears that rotational and vibrational perturbations show quite a different behavior. In the former there is a direct superposition of intensities of the two interacting states whereas in the latter interference effects give an entirely different picture. The general formulae are applied to a few cases in the ${\mathrm{H}}_{2}$ spectrum.