REVIEWS 565 



at greater length in the treatment of the fundamental equations of strain : its 

 properties are demonstrated in a very lucid manner. The section on vortex motion 

 affords an excellent example of the advantages of vector methods in dealing with 

 problems of this kind. 



The author has evidently spared himself no pains to make the book clear and 

 consequent, and the care with which the difficulties likely to present themselves to 

 the student have been foreseen bears witness to his discrimination. The abstract 

 dynamical principles are illustrated by simple and direct examples, which make 

 their scope and meaning clearer than could be done in a discussion occupying 

 many times their space. One of the most striking features of the book is the 

 brevity which has been achieved without sacrifice of either clearness or 

 accuracy. 



The student will find here an excellent introduction to the whole field of 



vector mathematics, and especially, although electrical problems are not directly 



treated, a very good preliminary to the study of modern electrodynamics. The 



collection of examples, with hints for the solution of the harder ones, is likely to 



prove exceedingly useful. 



E. N. da C. A. 



Researches in Magneto-Optics. By P. Zeeman. [Pp. xi -f 219.] (Macmillan 

 & Co., 191 3. Price 6s. net.) 



In this book, which forms one of Messrs. Macmillan's excellent series of Science 

 Monographs, the famous author gives an account of the researches carried out on 

 the phenomenon associated with his name — the modification of the nature of the 

 emitted light which takes place when the source of light is placed in a magnetic 

 field — and the closely allied magnetic rotation and magnetic double refraction of 

 light. While dealing largely with the author's own experiments, as is necessarily 

 and desiredly the case, the book gives an account of all work done on the subject 

 from the fundamental discovery in 1896 down to the middle of the year 1913. 

 Starting with a chapter on modern spectroscopes — in which he discusses Ray- 

 leigh's theory of resolving power and the performances of the Rowland grating, the 

 echelon, e'talon, and other recent devices for the finer analysis of light — the author 

 passes on to the fundamental experiment, the magnetic resolution of emission 

 lines, giving his original paper and Lorentz's simple and brilliant theory of the 

 effect. It is interesting to note that this supplied the first proof that the centres of 

 light emission are negative electrons, and that the value ofe/m deduced was one of 

 the very earliest determinations. In other chapters he treats of the inverse effect 

 — i.e. the multiplication of the absorption line when the absorbing body, such as a 

 salted flame, is placed in a magnetic field, the types of resolution more compli- 

 cated than that indicated by Lorentz's simple theory and found in the first experi- 

 ments, and the magnetic rotation of the plane of polarisation. This phenomenon 

 follows theoretically from the unequal velocities of propagation of the right-handed 

 and left-handed circularly polarised components, demonstrated by Zeeman to 

 exist for rays propagated paralled to the force when the absorbing body is placed 

 in a magnetic field. The importance of the Zeeman effect for astrophysics is 

 brought out in the chapter on Hale's researches, which revealed the effect in the 

 light coming from the sun in the neighbourhood of spots. This is in striking 

 accord with the theory that the sun-spots are solar vortices, for the electrons 

 whirled round in these vortices would produce the magnetic field required for the 

 phenomenon. 



