874 



SCIENCE. 



[N. S. Vol. IX. No. 234. 



magnifies the labor which hundreds of readers 

 are compelled to undergo. They are wholly 

 neglected in the present volume. 



There are certain topics in acoustics which 

 require the use of calculus for satisfactory treat- 

 ment, but of which the practical results are so 

 important that these cannot be omitted in an 

 elementary treatise. Such, for example, is the 

 equation expressing the relation between ve- 

 locity of propagation, elasticity and density. 

 For propagation of longitudinal waves the 

 method of deduction without higher mathemat- 

 ics, first brought out by Rankine thirty years 

 ago, is well known. In the present text-book 

 an independent method is employed in which 

 the formula is briefly deduced by discussion of 

 the displacement curve for a longitudinal dis- 

 turbance. Laplace's correction is satisfactorily 

 explained, but in the application to numerical 

 examples the student is required to apply ther- 

 modynamic principles, with which certainly the 

 elementary student cannot be assumed to be 

 familiar, but which will, doubtless, be explained 

 in the future volume on ' Heat.' 



The chapter on 'Frequency and Pitch of 

 Notes ' is particularly good. In the discussion 

 of musical quality and of concord and discord 

 prominence is justly assigned to the masterly 

 researches of Helmholtz, but very little atten- 

 tion is given to the work of Rudolph Konig. 

 In like manner the work of Mayer in America 

 fails to receive any mention. The discussion 

 of singing flames will be found better than in 

 most text-books, including an excellent expo- 

 sition of Lord Rayleigh's investigation on this 

 subject. 



On the whole the book is much to be com- 

 mended to those who are already acquainted 

 with tbe priciples of acoustics and who wish a 

 modern presentation of tbe subject by men of 

 high standing. For a class text-book, as com- 

 monly employed in America, it will scarcely be 

 found well adapted. 



W. Le Conte Stevens. 



"Washington^ and Lee University. 



Photographic Optics. By R. S. Cole, M.A. 

 New York, D. Van Nostrand Co. 1899. Pp. 

 330. 

 The aim of this handbook, which was origi- 



nally published in England by Samson Low, 

 Marston & Co., is to give an elementary 

 presentation of such of the problems of op- 

 tics as find application in practical pho- 

 tography. A careful perusal of the book leaves 

 in the reviewer's mind the impression that the 

 emphasized word in the title should be ' optics ' 

 and not 'photographic;' that is, 'photographic' 

 in the sense that most American amateur pho- 

 tographers would use the word. The book is 

 written from the standpoint of the student of 

 optics rather than that of the up-to-date prac- 

 tical photographer. 



The photographer will find given in the vari- 

 ous chapters of the book an excellent treatment 

 of the various optical conditions encountered 

 in using the camera, and this treatment is thor- 

 ough and made as simple as the nature of the 

 subject will admit ; the author going back to 

 first principles in all possible cases. 



Perhaps the most important section is the 

 one on lens testing, which contains an account 

 of the tests employed at the Kew Observatory. 

 Photographers are too apt to assign the same 

 degree of excellence to all lenses of the same 

 make ; but the fact is that no manufacturer 

 turns out two lenses just alike. 



The photographic-lens industry is assuming 

 such large proportions in this country that some 

 one of our institutions ought to establish a lens- 

 testing department which shall duplicate here 

 the work of the Kew Observatory in England, 

 so that when the practical photographer buys 

 his lens he can receive with it a certificate of 

 excellence. 



We could wish that Mr. Cole had given us 

 detailed information in regard to the construc- 

 tion and use of the various modern lens combi- 

 nations, such as the Zeiss, Goerz and Steinheil 

 lenses. These are points on which the ordinary 

 amateur photographer is utterly ignorant, and 

 even a modest amount of enlightenment would 

 be of great benefit to him. 



The author certainly dismisses too abruptly 

 the subject of calculating the brightness of the 

 image and timing exposures. It is not such a 

 wholly empirical matter as is represented. 

 Our best amateur photographers do calculate 

 as accurately as possible the time of their ex- 

 posures, and their results warrant this expendi- 



