REVIEWS 



The Theory of Light. By the late Thomas Preston. Fourth edition. Edited 

 by W. E. Thrift, M.A. [Pp. xxiii + 618.] (London: Macmillan, 1912. 

 Price 1 5-y. net.) 



In this fourth edition of Preston's Theory of Light the unique character of the 

 original work has been jealously preserved. The additions made to the text 

 include a fuller treatment of dispersion, an account of radiation phenomena in a 

 magnetic field and a more complete presentation of the electromagnetic theory. 

 The additions made to the text in these respects and by the description of modern 

 experimental work amount to some thirty pages but the additions have been 

 enclosed in brackets in order that they may be distinguished readily from the 

 original text. The brevity of the description given of recent experiments would 

 be regrettable but for the fact that they are described in detail in Prof. Wood's 

 Physical Optics, published in the same series of volumes. Under these conditions 

 there is every justification for retaining the historical and mathematical form of 

 Prof. Preston's work, the value and vigour of which are undiminished after twenty- 

 two years of active service. 



T. M. L. 



The Age of the Earth. By Arthur Holmes, B.Sc, A.R.C.S. [Pp. 189, 

 illustrated.] (Harper's Library of Living Thought. Price 2s. 6d.) 



Two years ago, Mr. Holmes published a research on the association of lead with 

 uranium minerals and its application to geologic time. 1 On the assumption (not 

 yet directly proved) that lead is the final product of the uranium series, and on 

 several other assumptions, the quantity of lead contained in a mineral affords 

 some clue to the date when it was laid down. Mr. Holmes's results were 

 unusually concordant, and, emboldened by his success, he has essayed to treat the 

 whole subject of geologic time. 



Needless to say, the chapters (in all comprising nearly half the book) dealing 

 with radioactivity and cognate subjects are the most valuable. A somewhat fuller 

 account of experiments such as those he has himself carried out would have been 

 welcome, but this part of his work is clear and carefully written. Nor is he unduly 

 dogmatic concerning the validity of his own method compared with those of other 

 workers. There is a danger of our repeating the error of the last generation and 

 laying too much stress on the validity of physical methods of investigation. In 

 place of the dogmatism of Lord Kelvin and Prof. Tait, we are liable to substitute 

 that of modern exponents of radioactivity. But such an attitude, if it occurs, 

 will not be favoured either by Prof. Strutt or by his pupil Mr. Holmes. 



Nevertheless, Mr. Holmes, having reached the conclusion that many minerals 

 were laid down 1,500 million years ago, is bound to try to correlate other lines of 

 evidence, and to attempt to show that, if rightly understood, they support his 

 view. He has against him the fact that the greatest modern authorities, arguing 

 from many diverse lines of thought, have repeatedly stated that 100 millions of 



1 Proceedings of the Royal Society, Series A, April 11, 191 1. 



16S " 



