244 



NA TURE 



[December 31, 1908 



method constituted the chief characteristic of the book, 

 we should have but little to say in its favour. The 

 curvature method undoubtedl}- possesses many obvious 

 advantages, but the loose and in places quite in- 

 accurate manner Jn which the elementary theory of 

 mirrors and lenses is here set forth does much to 

 obscure the merits of the system. 



The definition " The curvature of a circle is the 

 angle through which a curve turns per unit length " 

 gives an early suggestion of the lack of logical clear- 

 ness which is throughout apparent. .\s an instance 

 of more serious inaccuracy the following attempt at 

 an explanation of the formation of an image bv a 

 concave mirror may be quoted (p. i8) : — 



" To explain the formation of the virtual erect 

 image we may imagine the mirror to consist of an 

 infinite number of plane surfaces (Fig.). Each of 

 these is capable of producing a virtual and erect image 

 (as previously explained) identical in position behind 

 the mirror with the object in front. .'\s the object 

 is moved from the mirror more of these supposed 

 innumerable facets take up the reflection for the eve 

 to view, and so the imag-e increases ra])idlv in 

 size. ..." 



The attempt at a formal proof, for the spherical 

 mirror, of the relation connecting conjugate foci is 

 given laler (p. 3.S). With no guidance as to the con- 

 ditions limiting the truth of the statements made, and 

 with a misleading figure, we venture to think that 

 the intelligent student would find this hopelcs.slv 

 bewildering. 



In fact, however, no real attempt is m.adc to estab- 

 lish, by the curvature method, the principk's of 

 elementary geometrical optics. Apart from the state- 

 ment in the preface one would have judged the aim 

 of the book to be quite different from this. It would 

 appear clearly to be intended for the use of those 

 actually engaged in optical workshops, and especially 

 those concerned with the practice of spectacle making 

 and of .sight testing. It would seem to aim, not at 

 providing a logical exposition of elementary principles 

 on any system, curvature or other, but at furnishing 

 a reader of the kind suggested with a sufficient 

 modicum of information about many points likely to 

 be brought under his attention in the course of his 

 practice, to enable him to deal with these not unin- 

 telligently. From this point of view it may fairly 

 be held to have achieved some measure of success. 

 Evidently written by practising opticians, it does con- 

 tain-, in small compass, a good deal of practically 

 useful and important information not to be found in 

 the ordinary elementary text-book, more especially in 

 regard to the characteristics of the eye as an optical 

 instrument and the problems involved in the correction 

 of its defects. The fact that the discussion of such 

 questions is more readily carried on in terms of 

 curvatures and focal powers than of radii of curvature 

 and focal lengths may, perhaps, be held to account in 

 some measure for the sentence quoted from the 

 preface. 



The book has some features of value to those for 

 whom quick reference is essential : a useful summary 

 at the end of each chapter; a few tables and data, in- 

 cluding a table of aberrations in lenses and their 

 NO. 2O-I4, VOL. 79] 



remedies, from a pajjcr read by Prof. Silvanus Thomp- 

 son before the Optical .Society ; a list of optical works ; 

 and, following the index, a list of articles useful in 

 spectacle work and sight testing, with approximate 

 prices. 



.\s a text-book on elementary optics the book needs 

 much careful revision. We think well enough of it to 

 express the hope that it inay receive this treatment 

 without delay. 



ECOXOMIC y.OOLOG Y. 



Economic Zoology, an Introductory Text-book in 

 Zoology, with Special Reference to its Applications 

 in Agriculture, Commerce and Medicine. By Prof. 

 Herbert Osborn. Pp. xv + 490; 269 figures. (New 

 York-: The Macmillan Company; London: Mac- 

 millan and Co., Ltd., 1908.) Price 8i-. 6(/. net. 



.1 Manual of Elementary Forest Zoology for India. 

 By E. P. .Stebbing. Pp. xxiii + 230+xxxiv ; 422 

 figures. (Calcutta : .Superintendent Governnuul 

 Printing, 1908.) Price 15*'. 



THE teacher of .roology to students whose chief 

 interest in the science depends upon its relations 

 to some branch of human industry has often to decide 

 whether he shall give the more prominent place to 

 general principles or to special and technical applica- 

 tions. The rival points of view are well illustrated in 

 these two works. Prof. Osborn, in the introduction 

 to his handy " text-book," rightly .advocates the 

 claims of sound general principles. 



" Inasmuch," he writes, " as economic zoology is 

 simply an application of our knowledge of animals 

 which affect human interests, it is easy to see that 

 almost any phase of the study will have some bearing 

 on the problems that concern us. Economic zoology, 

 if studied thoroughly, must of necessity be based on 

 accurate knowledge ... so that it involves a study 

 of the whole animal .-ind all th;it can be learned 

 regarding its activities." 



In agreement with these principles, the author has, 

 in the work before us, neglected no groups of the 

 animal kingdom, even though, like the Echinoderms 

 and the lower Chordala, they have little or no 

 " economic " importance, so far as we know at 

 present. .\ knowledge of the structure of these groups 

 is essential to any real training in zoology; and who. 

 a dozen years ago, could have foreseen the vast 

 economic importance of such groups as the Haenio- 

 sporidia, the Culicidse, and Ixodidae? The students 

 now in our colleges require training not only for the 

 known needs of the present; they are entitled to bi- 

 equipped so that they can grapple practically with tin 

 unknown problems of the future. 



But while the principles that have guided Prof. 

 Osborn are thoroughly sound, it is doubtful if he has 

 made the best possible use of the space at his disposal 

 in thus applying them. He has given clear descri|j- 

 tions of the great animal phvia from Protozoa U> 

 Vertebrata, and the leading classes and orders, illus- 

 trated by well-chosen figures culled from trustworth\ 

 sources. Such descriptions are, however, alread\ 

 available in many zoological text-books. The special 

 text-book for the student of economic zoology should 



