A NEW treatise on the higher branches of physical optics must be welcome to all who are interested in the subject. Mr. Basset explains in the preface the scope and aim of his book, and it is needless to say that he performs the task he has set himself with ability and success. If, nevertheless, we close the book with a feeling of disappointment, it is because we could have wished that the author had been more ambitious, and attempted to give us a little more than a compilation of the standard papers on the subject. There is one sentence in the preface which, though it evidently does not express what the author meant to say, yet may serve as a peg whereon to hang the only criticism which can fairly be raised against Mr. Basset's treatment of his subject. “I have a profound distrust,” says the author, “of vague and obscure arguments based upon general reasoning instead of upon rigorous mathematical analysis.” Now, if we are to have vague and obscure arguments, it does not seem to matter much whether they are founded upon general reasoning or upon mathematical analysis, however rigorous that may be. In a subject which is in a state of growth, it may be possible to hide, but it is impossible to avoid, all obscurity and vagueness; and original work ever consists in the attempt to overcome such obscurities. By purposely excluding everything that is vague from a physical treatise, we destroy all possibility of making the work useful in stimulating further research. There are two ways of dealing with difficulties: we may try to overcome them, or we may run away from them. Mr. Basset chooses the latter course, and though some of us might have wished him to be a little more venturesome, we gratefully accept what he has given us, and the above remarks only apply to certain parts of the book. After an introductory chapter, Mr. Basset treats of the interference of light. He follows the time-honoured custom of taking Fresnel's mirrors and the biprism as the simplest case of interference. The effects which are observed are seriously modified, however, by so-called diffraction effects, and we might perhaps have expected a book of this kind to have entered a little more fully into the subject. That the author avoids all reference to experimental details is a distinct advantage, and renders his book more lucid and valuable for reference. It is much to be wished the author's plan could be more generally followed, and that all lengthy discussion of instrumental details could be kept out of theoretical treatises, and relegated to separate books. A Treatise on Physical Optics. By A. B. Basset (Cambridge: Deighton, Bell, and Co., 1892.)