Raman spectra excited in diatomic gases by the line $\lambda 2536$ of mercury have been photographed with a quartz spectrograph. An apparatus for using gases at pressures from ten to fifteen atmospheres has been built, with great improvement as compared with the use of gases at atmospheric pressure. Results obtained with ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, NO are reported. A particularly complete investigation has been made of the hydrogen spectrum, leading to an accurate determination of the constants of the molecule in the normal electronic state. The numerical values are (in ${\mathrm{cm}}^{-1\mathrm{}}$) $B_0=\frac{h}{8{\pi }^2{I}_{0}c}=59.40\mathrm{}$; ${\omega }_{\frac{0}{1}}=4162.1\mathrm{}$. For ${\mathrm{N}}_2$, the corresponding values resulted to be: $B_0=1.992\mathrm{}$; ${\omega }_{\frac{0}{1}}=2330.7\mathrm{}$. For ${\mathrm{O}}_2$: $B_0=1.436\mathrm{}$; ${\omega }_{\frac{0}{1}}=1554.7\mathrm{}$. All these molecules are in $\Sigma$ states, and the Raman spectra consist of $Q$-, double $R$-, and double $P$- form branches, as deduced from the Kramers-Heisenberg formula by the author and by Hill and Kemble. Nitric oxide is the only molecule so far investigated which is in a ${}_{}{}^2{}_{}{}^{}\Pi$ state. The transition between the two terms of the doublet has been observed in the scattered radiation.

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