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THE subject of physical optics has undergone a strange vicissitude. Not long ago it ranked as one of the great divisions of mathematical science; now it has become almost wholly absorbed by a sister science. The phenomena of ioptics, by their variety and ever-increasing practical importance, attract and deserve specialised study; but the underlying theory can no longer be studied apart from electricity, and the long succession of theories of the aether in the nineteenth centuiy form a closed chapter in the history of science. There seems little likelihood that the chapter will be reopened. In these circumstances the best approach to the subject may be a matter of doubt, depending a great deal on the temperament of the student. Those who are historically minded will urge that the present position is best apprehended by following the steps which have led to it; others will consider that adherence to the traditional mode of approach tends to root in the mind an obsolete mode of thought, and it is better not to trifle with the freedom which is now offered. Dr. Mallik's book offers a compromise which should be acceptable to both sides. A survey of the more essential properties and differences of the mechanical aethers that were once proposed, subordinated to the purpose of illustrating the difficulties which the electromagnetic theory strides over so easily that they pass almost unnoticed; afterwards, an account of the modern theory and results, contrasted where necessary with the mechanical aethers-these seem to bring out the essential aspects of our present knowledge, without undue neglect of the lessons of the past.Optical Theories, Based on Lectures delivered before the Calcutta University.By Dr. D. N. Mallik. Pp. 181. (Cambridge: At the University Press, 1917.) Price 7s. 6d. net.