

Notices respecting New Books. 705 



the measurements of the radium content of these materials. 

 This result generally confirms the statistics of solvent 

 denudation. But in a subsequent paper I hope to show that 

 a full consideration of the data now at our disposal respecting 

 the sub-oceanic sediments leads to the conclusion that the 

 less radioactive deposits have been subject to considerable 

 changes in chemical composition during recent geological 

 times, and do not possess throughout their entire mass the 

 chemical composition we observe in samples dredged from 

 their surface. The subject is of sufficient interest to merit 

 separate treatment. 



I desire to thank Mr. Louis B. Smyth and Mr. A. L. Fletcher 

 for some experiments made in connexion with this paper, 



Iveagk Geological Laboratory, 

 Trinity College, Dublin. 

 'Sept. 1912. 



LXIX. Notices respecting New Books. 



Physical Optics. By E. W. Wood, LL.D., Professor of Experi- 

 mental Physics in the Johns Hopkins University. Xew and 

 Eevised Edition. Pp. xvi + 705. The MacMillan Company, 1911, 

 Price 22s. net. 



^TUTIS is a new edition of a notable book. It is not an ordinary 

 -*■ textbook — it is too full of original matter to serve the 

 purposes of such a manual, and yet it does in some respects 

 endeavour to meet the requirements of the routine studies which 

 every physical student must undertake in order to ground himself 

 well in bis subject. Its chief characteristic is that it brings 

 together the large amount of experimental work of a kind for 

 which Professor Wood has a very happy knack, especially in 

 connexion with the subjects of optical dispersion and absorption. 



The weakest part of the book is probably that dealing with 

 polarization, especially the chapter on double refraction. Although 

 even here the author brings his characteristics into full play, he 

 is not happy in dealing with the various ellipsoids which are made 

 use of in this part of the subject, Eor example, the account on 

 p. 310 of the derivation of the ellipsoid of elasticity is either 

 incorrect or it is so obscure that the intended information is not 

 given ; the former alternative is probably the correct one. Again, 

 no clear distinction is drawn between ray-velocity and wave- 

 velocity. In connexion with the change of dispersion of the 

 optic axes with temperature it is asserted that " a further elevation 

 of temperature causes the axes to cross one another, so to speak, 

 the crystal becoming again biuxal," ^so student would surmise 

 from this description that the new plane of the optic axes is at. 



