THE MOUNT WILSON WORK ON SOLAR MAGNETISM.—Scribner's Magazine for May contains an interesting article on this subject by Prof. G. E. Hale. After a brief resume of our previous knowledge of solar physics, he describes the discovery of solar vortices by photographs with the spectroheliograph, and the discoveries of Faraday and Zeeman on the effect of a magnetic field on the spectral lines are explained. The vortices were tested in this manner and showed the Zeeman effect, indicating that the whirling electrons produced a magnetic field. The recent discovery that the law of magnetic polarities is reversed in alternate sun-spot cycles is then explained and illustrated by several diagrams (v. also NATURE, January 19, p. 105). Dealing with the northern solar hemisphere in 1903, preceding components of pairs showed S-polarity, following components N-polarity. This persisted until 1912; it was then found that the spots of the new cycle showed reversed polarity. Still another reversal has now taken place, the first spot to show it being seen on June 24, 1922, in N. lat. 31°. Several further groups have confirmed the change, so that it now appears that the full solar cycle is 22 years. It is noted that the high level vortices in the hydrogen atmosphere do not share in the reversal, which is assumed to depend on some change of circulation of matter in the sun's interior.