A method and an apparatus for measuring the density variations, static and dynamic, in substances that are at least partially transparent to electromagnetic waves is disclosed. A special birefringement crystal phase shifting assembly encodes the angle of incidence resulting from the refractive effects of the electromagnetic waves having passed through a density variation. The angle of incidence is encoded as a poralization phase shift. Specifically, the poralization phase shift is between the ordinary and the extraordinary rays. That poralization phase shift has a known and definable relationship to the gas density experienced by the electromagnetic wave during its path.