Geometrical optics is no longer fashionable. Research workers do not expect significant new discoveries to be made in this field of classical physics. Teachers avoid the subject because its use for many generations in arid mathematical exercises has robbed it of all freshness and stimulus, with the result that it no longer seems relevant to a modern physics course. There remains - and perhaps this has grown in recent year- the technical significance of geometrical optics. It provides the basis for the design of optical instruments for use in everyday life as well as for scientific and industrial purposes. This small book is intended to treat two aspects of the subject: the laws of geometrical optics and their application to the design of optical instruments. The theory is not based on Snell’s law of refraction but on a conservation law for the radiated energy. The subject can then be treated in a manner appropriate to contemporary physics: auxiliary geometrical parameters become unnecessary and the singularities resulting from their use vanish. The laws of geometrical optics can be formulated much more simply and their physical significance is revealed more clearly. I have tried to present the material in a form satisfactory both to teachers and to workers in the technical applications of optics. The content has thus been deliberately kept within the limits of a pocket book.