A cubical crystal of the NaC1 type should, according to the lattice theory, | possess only one frequency which would respond to infra-red light, this vibration being caused by the motion of the rigid lattice of positive ions against the rigid lattice of negative ions. We should, therefore, expect a single maximum in the absorption and in the reflexion ; this presupposes an harmonic motion of the particles of the lattice. This main maximum is a well-known feature of the properties of ionic crystals in the infra-red, but the work of CZERNY and his collaborators has shown that, besides this, there are subsidiary maxima both in absorption and in reflexion ; furthermore, the representation of the optical constants, in particular of the absorption coefficient (K) by means of a dispersion formula with a damping constant as suggested by DRUDE, broke down completely on the short wave-length side of the maximum.