## Abstract

The echelon data tabulated and described in the preceding paper referred to here as Paper I are used to investigate the fine structure of the H$_{2}$ band lines discovered by Richardson & Williams in 1931. Practically all the lines with resolved fine structure have 2p$^{3}\Pi $ as their lower states and the number of such resolved lines is now very greatly increased. The new information is applied in the first instance to the critical examination and improvement of the existing tables of the (composite) 2p$^{3}\Pi $ level differences. These are a basic tool in the elucidation of this spectrum. One result of this part of the investigation is that the pairs of lines from which each composite level difference is derived divide themselves into two groups which have slightly different values. The fine structure shows itself visually as a resolution of the lines hitherto regarded as single into two components which we denote by a and b, a having the higher frequency and intensity. The differences for a or b are considered separately and shown to be different from each other and from the differences of the composite lines. A number of regularities among these new differences are pointed out. The largest doublet separation $\Delta \nu $ observed is about 0$\cdot $22 cm.$^{-1}$ and the smallest 0$\cdot $06 cm.$^{-1}$. The lines (which go down from 3s$^{3}\Sigma $ and 3d$^{3}\Sigma $, $\Pi $, $\Delta $ to 2p$^{3}\Pi _{c^{d}}$) are divided into a 'regular' and an 'irregular' group. Lines of the regular group have a larger $\Delta \nu $ and the intensity ratio of a to b is nearly constant and close to 2$\cdot $4, whereas for the irregular group this ratio falls from about 5 to about 1 as K$^{\prime \prime}$ increases from 1 to 4. The regular group consists of all the lines which go down to 2p$^{3}\Pi _{c}$, and the irregular group of all which go down to 2p$^{3}\Pi _{d}$. The greater part (about 90%) of the doublet separations arises from fine structure splitting of the lower 2p$^{3}\Pi $ levels. When effects arising from the upper levels are eliminated it is found that the splitting, both of 2p$^{3}\Pi _{c}$ and 2p$^{3}\Pi _{d}$, diminishes as K$^{\prime \prime}$ increases and at an increasing rate at higher K$^{\prime \prime}$. In general these fine structures are closely parallel with those of He$_{2}$ discovered by Monk & Mulliken in 1929. The H$_{2}$ data cover a wider range of states and fill in the gaps left by the 'missing lines' in the He$_{2}$ spectrum. There are, however, some differences outstanding which seem to call for further investigation.