302 



NATURE 



[June 19, 1919 



families of dicotyledons and monocotyledons. The 

 fifth section, " Plants in Relation to their Environ- 

 ment, " contains chapters on "fitness" or adapta- 

 tion to environment, a very useful one on trees, 

 one each on climbing- plants and water-plants, and 

 a brief introduction to the study of plant associa- 

 tions. 



{3) The new edition of "Lowson's Text-book 

 of Botany," adapted for the use of Indian 

 students, represents a widely diiferent type of 

 text-book. It contains a great deal of in- 

 formation clearly expressed in numbered and 

 headed paragraphs, which are illustrated by plain, 

 carefully indexed diagrammatic sketches such as 

 a lecturer would use for a class of elementary 

 students. It suggests the lecture notes made by 

 an accurate and conscientious student, and if re- 

 garded as such may serve a useful purpose pro- 

 vided the student can clothe the skeleton with the 

 living and working tissue. But it is not a book to 

 put into the unaided hands of a beginner, or to 

 excite a love of botany in the heart of the amateur. 

 Like the original, which is well known among a 

 certain class of students, it is obviously written 

 for examination purposes. In the new edition 

 Mr. Sahni has introduced additional matter into 

 the chapters on the natural orders dealing specific- 

 ally with the Indian flora, and also a number of 

 vernacular plant-names, which are separately 

 indexed at the end of the volume. These are both 

 useful additions, but when one recalls the richness 

 of the Indian flora and its remarkable diversity, 

 ranging from tropical to high alpine, and includ- 

 ing biological groups of great variety and in- 

 terest, one could wish for a more attractive and 

 living introduction to its study. 



OPTICS AND MECHANICS. 



(i) Mirrors, Prisms, and Lenses. A Text-hook 

 of Geometrical Optics. By Prof. James P. C. 

 Southall. Pp. xix + 579. (New York : The 

 Macmillan Co.; London: Macmillan and Co., 

 Ltd., 1918.) Price 175. net. 



(2) Notes, Problems, and Laboratory Exercises 

 in Mechanics, Sound, Light, Thermo-mechanics, 

 and Hydraulics. Prepared for Use in Connec- 

 tion with the Course in Natural and Experi- 

 mental Philosophy at the United States Mili- 

 tary Academy. By Prof. Halsey Dunwoody. 

 Pp. V + 369. (New York : John Wiley and 

 Sons, Inc. ; London : Chapman and Hall, Ltd., 

 1917.) Price 135. 6d. net. 



SCIENTIFIC writers in the United States 

 appear to have laboured under smaller diflfi- 

 culties due to war conditions than British authors. 

 Here are two new American text-books, of which 

 the first is sure to be welcomed by a wide circle 

 of readers, whilst the second is adapted for a 

 special class of students. 



(i) Prof. Southall is known as the author of a 

 treatise on "The Principles and Methods of Geo- 

 metrical Optics." The present volume, although 

 in some sense an abridgment of the larger work, 

 contains a considerable mass of new and orig-inal 

 NO. 2590, VOL. 103] 



material. It is intended to serve as an introduc- 

 tion to the theory of modem optical instruments, 

 but only the simplest mathematical processes have 

 been employed. In the earlier and more ele- 

 mentary portions of the subject the author has 

 purposely entered into much detail, and he has 

 been very successful in imparting fresh interest 

 to an old and well-worn subject. The need for a 

 text-book dealing with ophthalmology and applied 

 optics on modern lines has long been felt, and 

 there is no doubt that certain portions of this 

 volume will be helpful to the modern oculist and 

 optometrist. Thus, for example, the author has 

 been at some pains to explain the fundamental 

 principles of ophthalmic lenses and prisms. It 

 is unfortunate that lack of space prevented the 

 detailed description of any single optical instru- 

 ment, and it may be suggested that an account 

 of the microscope should be included in a future 

 edition even at the expense of some other sections. 

 The book is provided with a number of problems 

 appended to each chapter, and with clearly 

 executed diagrams. The photographic illustra- 

 tions of reflection from plane mirrors are of 

 special interest. 



" Unfortunately, at present geometrical optics 

 would seem to be a kind of Cinderella in the 

 curriculum of physics, regarded perhaps with a 

 certain friendly tolerance as a mathematical dis- 

 cipline not without value, but hardly permitted to 

 take rank on equal terms with her sister branches 

 of physics. On the contrary, it might be inferred 

 that any system of knowledge which had already 

 placed at the disposal of scientific investigators 

 such incomparable means of research as are pro- 

 vided by modern optical instruments, and which 

 has found so many useful applications in the arts 

 of both peace and war, would be deserving of the 

 highest recognition, and would be fostered and 

 encouraged in all possible ways." 



Prof. Southall 's book should serve to stimulate 

 the study of optics in our collegfes and universities. 



(2) It is difficult to understand the principles 

 on which Prof. Dunwoody has arranged the mis- 

 cellaneous contents of this volume. The title on 

 the cover of the book is even more misleading- 

 than that on the title-page. It is "Laboratory 

 Exercises, Notes, and Problems in Physics." Yet 

 only about twenty pages are concerned with 

 "Sound, Light, Thermo-mechanics," and about 

 ten with "Hydraulics." The greater part of the . 

 contents is concerned with "Mechanics," about #| 

 150 pag-es being devoted to "N6tes on Mechanics,'^ n' 

 including graphical statics and the mathe- 

 matical treatment of translation and rotation in 

 the case of a rigid body. About an equal ^^ 

 number of pages is given to an extensive list of ^ 

 problems ranging in difficulty from those suitable 

 for schoolboys beginning the subject to those* 

 requiring a knowledge of diff'erential equations. 

 In some cases hints for the solution of the problem 5,^. 

 are added. 



Of the laboratory exercises, fifteen in all, 

 thirteen are on mechanics, including one on the 



