Uniaxial stress measurements have been made on the H3 (2.463 eV) zero phonon line in diamond by means of absorption and luminescence spectroscopy. The data are shown to be consistent with the line's being produced by a $\langle 110\rangle $ electric dipole at a rhombic I centre; this conclusion is compared critically with the results of previous studies of the H3 band. Stresses along $\langle 111\rangle $ and $\langle 110\rangle $ are shown to induce an absorption line at 2.479 eV. This is interpreted quantitatively as evidence for an optically inactive vibronic state of the H3 centre 2.479 eV above the ground state which becomes optically active through stress induced mixing with the excited state of the H3 zero phonon line. The vibronic energy eigenvalues and eigenstates are calculated for the case of two near-degenerate electronic states which interact through one mode of vibration. Transitions are assumed to be allowed to one state and forbidden to the other. It is shown that the observed differences between the H3 luminescence and absorption spectra would result when the coupling of the electronic states is equal, within the uncertainties of the model, to the value calculated from their stress-induced interaction.