For pt.I see ibid., vol.5, no.3, 257 of 1971. It is suggested that a zero-phonon line in CaF₂ at 677.4 nm and in SrF₂ at 769.2 nm arises from excitation of an F₃ centre composed of three nearest neighbour F centres aligned along (100). Uniaxial stress measurements on the zero-phonon line show that the transition occurs between orbital singlet states in a centre with tetragonal symmetry and that the dipole moment of the transition is parallel to the tetragonal axis of the centre. The zero-phonon lines show a quadratic Stark effect. The model proposed for the centre giving rise to the zero-phonon line is consistent with the experimental results given in the present paper and in the previous paper.