A Treatise on Light.

DR. Houstoun has now remedied one rather serious defect in his well-known “Treatise on Light,” namely, the out-of-date character of the section dealing with spectral series. That some modification was desirable will be apparent from the remark still to be found in the third edition (1923) that “all attempts to explain the origin of spectral series are regarded as unsuccessful.” In the present edition a brief non-mathematical account of Bohr's theory is introduced, and some of its simpler applications, as to the spectra of hydrogen and ionised helium, are considered. Only a very few pages are devoted to this, for in the author's view the quantum theory of spectra belongs rather to mathematics and the theory of atomic structure than to light. It is perhaps debatable ground, like so much else in modern physics, but those who do not share his opinion will doubtless temper their criticism with appreciation of the many merits of the book in other respects.A Treatise on Light.Dr.R. A.HoustounBy. Fourth edition. Pp. xi + 486 + 2 plates. (London: Longmans, Green and Co., 1925.) 12s. 6d. net.