Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals. J. J. Hopfield in Phys. Rev. 112:1555 (1958).  What the paper says!?

This is the paper through which John Hopfield is credited for the conceptualization of the polariton, for which he seems indeed to be the first to have coined this name, although this was in a different sense than is understood nowadays:

Namely, the polariton is (aptly) the quantization of a polarization field, as arises from a quantum theory of the classical dielectric. Excitons themselves can be the polarization field, whence Hopfield's statement that "excitons are one kind of polaritons". This differs from the common picture where the polariton is a superposition of exciton and photon. Besides, the superposition introduced by Hopfield involves four operators:

Hopfiel also finds (and states) as part of this work that «excitons are approximate bosons».

The work highlights that while propagation of light in insulating materials is well described classically by the dielectric constant in regions of no absorption, absorption itself is problematic. This problem of absorption of light by a crystal whose fundamental excitation is the exciton, «was the motivation for the investigation of the exciton-photon system.»