THE Focal Press is to be congratulated in having secured Mr. Cox as the author of this volume on camera lenses and allied optical equipment. The book gives a non-mathematical account of lens defects and describes the various lens combinations employed in photographic objectives to improve the definition and increase the aperture. Very useful information about the focus, angular field, aperture and type of lens has been tabulated and illustrated for a wide range of lenses produced by many different firms. Optics The Technique of Definition. By Arthur Cox. Pp. 331. (London and New York: The Focal Press, Ltd., 1943.) 15s.