410 



SCIENCE 



[N. S. Vol. XXXIV. No. 874 



enhanced by an exposition of A. B. Porter's 

 ■work on the nature of optical images, by the 

 author's clever vectorial treatment of second- 

 ary maxima in grating spectra, by an account 

 of the Echelette grating, and by a neve view- 

 point from which to regard Talbot's fringes 

 — all of which have recently appeared in the 

 Philosophical Magazine. The discussions of 

 interference and polarization remain almost 

 intact. A brief but entirely new chapter is 

 here given to " Meteorological Optics." 



The beautiful method of focal isolation by 

 which Rubens and Wood have recently been 

 enabled to measure heat waves which are 

 more than one tenth of a millimeter in length, 

 has been inserted in the chapter on the theory 

 of dispersion. But this well illustrates the 

 impossibility of keeping a treatise up to date; 

 for notwithstanding the preface is dated May, 

 1911, the infra-red spectrum has since then 

 been extended more than an octave and a 

 half; so that now the gap between the elec- 

 trical and optical spectra is something less 

 than three octaves — the difference between a 

 third of a millimeter and two millimeters. 



Interesting additions, dealing with the ab- 

 sorption of gases, have been made to Chapter 

 XV., including Wood's extension of Balmer's 

 series for sodium vapor absorption lines to 

 the 48th member. 



The discussion of magneto-optics has been 

 greatly and properly enlarged, and is followed 

 by an entirely new chapter on electro-optics. 



Chapter XX., dealing with fluorescence and 

 related phenomena, has a long title of nine 

 words which, in the opinion of your reviewer, 

 might well be replaced with the single term 

 " Photo-luminescence." The Laws of Radi- 

 ation (Ch. 21) have been made to include the 

 recent achievement of Lebedew in demonstra- 

 ting experimentally that a beam of light ex- 

 erts a definite pressure upon an absorbing gas. 



The remainder of the volume differs little 

 from the former edition except that a final 

 chapter on the principle of relativity has been 

 added. But to attempt an elementary ex- 

 position of the ideas of Einstein and Min- 

 kowski in twelve small pages is well nigh at- 

 tempting the impossible. Lack of consecutive- 



ness makes this last chapter the only unsatis- 

 factory one in the entire volume. 



Of the book as a whole it ought to be re- 

 marked that it teems with practical hints of 

 great value, clever bits of experimental ex- 

 perience from the large fund for which the 

 professor of experimental physics in Johns 

 Hopkins University is justly celebrated. The 

 policy of the author in maintaining a quanti- 

 tative discussion throughout, and yet refrain- 

 ing from the use of very advanced or severe 

 or complicated mathematical methods, is to be 

 highly commended. It is a matter of con- 

 stant surprise to find what a large proportion 

 of all really important phenomena can be 

 described by very simple differential equations 

 and can be discussed by the ordinary analysis. 

 This remark is, of course, only intended to 

 apply to the mathematical method of a treatise 

 for students of physics, and not to an original 

 paper describing the results of a mathematical 

 investigation. 



A work so altogether admirable as this 

 should not be marred by so many typograph- 

 ical errors and incomplete references. Some- 

 times one is referred merely to "Phil. Mag." 

 at other times, as for instance in Eunge's 

 exquisite treatment of the concave gi-ating, 

 p. 233, one is merely told that the discussion 

 is " due to Runge." Economy of energy on 

 the part of the reader will be greatly served 

 by the correction of many of these slight mat- 

 ters in a future revision. 



However views may diverge as regards the 

 treatment of various topics in this volume, 

 there can surely be no difference of opinion 

 among English-speaking students as regards 

 the generosity which Professor Wood has 

 shown in taking time and energy from a 

 strenuous life of research to prepare this 

 clear, scholarly and thoroughly modern treat- 

 ise. Appreciation of this can hardly fail to 

 show itself in a long and wide-spread use of 

 the book. 



Henry Crew 

 \ 

 Atlas of Zoogeography. By J. G. Bartholo- 

 mew, W. Eagle Clark and Percy H. Grim- 

 SHAW. Series title: Bartholomew's Physical 



