concerning them if they had remained unseen except during-eclipses, we can echo our author's opinion that success in revealing the mysterious corona in a similar manner "is certainly devoutly to be desired."
Prof. Young has so long been a conspicuous and brilliant figure in the field of spectroscopic investigation that one naturally turns to his revised chapter on The Spectroscope and the Solar Spectrum with pleasing anticipations which are not disappointed. The results of the immense work accomplished in the last decade are here ranged in order with an expert hand, and the remaining gaps in the line of acquired knowledge are made clearly apparent. The list of elements known to exist in the sun has been largely extended in the past few years, but still many that go to constitute a great portion of the crust of the earth have not been recognized in the solar spectrum. Why are they absent? Is it simply a failure to show themselves, or do they not exist there at all? Prof. Young indicates his preference for the view that the missing elements are not really absent from the sun. but only nonapparent, although he points out that the answer to the question is not easy. And then he goes on to marshal some most interesting facts and considerations relative to this subject, and discusses briefly but luminously such topics as the multiple spectra of certain elements, Lockyer's revival of the old "pantogen" speculation and his theory of "basic lines" common to the spectra of different substances, the later work on the question of solar oxygen, etc.
While this book is a record of facts and achievements rather than of theories and attempts at the interpretation of mysteries, yet the great questions still remaining to be answered are, of course, discussed, and in a masterly manner. A fine instance of the author's method of dealing with such subjects is shown in the chapter on The Sun's Light and Heat, where he succinctly reviews questions like these: "How is the sun's heat maintained?" "How long has it lasted already?" "How long will it continue?" "Are there any signs of either increase or diminution?" It is undoubtedly true, as Prof. Young remarks, that to such questions, "in the present state of science, only somewhat vague and unsatisfactory replies are possible," yet they are questions the replies to which, however incomplete, will always command deep interest. And if the facts and speculations accumulated since 1881 have not thrown much light upon these subjects, something has been gained in a clearer comprehension of both the strength and the weakness of prevailing theories. Those are pregnant sentences, for instance, in which, after pointing out the objections to the late Dr. Siemens's theory of the sun, he remarks: "And yet one almost regrets that the theory can not be accepted, for it would remove very serious difficulties that now embarrass the problem of the evolution of our planetary system. The accepted contraction theory of Helmholtz certainly appears to allow too little time for the sun's lifetime of radiant activity to be consistent with a reasonable explanation of the process by which the present state of things has come about."
In briefly summarizing the principal additions noted in the new edition the following may be particularly mentioned: The latest work on the solar parallax, including Newcomb's results and the observations on the minor planets Victoria and Sappho; accounts of all recent advances and discoveries in solar spectroscopy and spectro-photography, including the work of Hale, Deslandres, Dunér, etc.; a statement of the latest accredited theories
