CEPHEID (variable stars—the name given to stars which behave like δ Cephei—have played a distinguished part in modern astronomy. Their most characteristic feature—that there is a definite relation between period and median absolute luminosity—was discovered by Miss Leavitt in 1912 when investigating the Lesser Magellanic Cloud. Owing to this relation, the measurement of period and apparent magnitude allows one to compare the absolute and apparent luminosities, and so affords a reliable estimate of distance, once the zero point of the period-luminosity curve has been fixed. In this way, Shapley determined the distances of the globular clusters, and Hubble determined the distances of the extra-galactic nebulae . The Pulsation Theory of Variable Stars By Svein Rosseland. (International Series of Monographs on Physics) Pp. viii+152. (Oxford: Clarendon Press; London: Oxford University Press, 1949). 18s net.