772 



NA TURE 



[June 17, 1922 



and achieved fame. It was accuracy and honesty of 

 thought which carried him through. 



Prof. Becquerel gives us a text-book of the whole 

 matter for those who are prepared to go into the complete 

 mathematical presentation. There is nothing original ; 

 it is a plain and unvarnished account of the theory as 

 it stands, including the generalisations of Eddington 

 and Weyl. We may note that the author adopts the 

 general conclusion that the gravitational field is the 

 manifestation of the non-Euclidean character of the 

 structure of the Universe, and that mechanics and 

 physics are reduced to geometry. Is it not time that 

 this statement should be examined more carefully ? 

 It suggests that the cart is pulling the horse and that 

 the concrete arises out of the abstract, the known out 

 of the unknown. When the physicist lapses into 

 metaphysics he is apt to leave his terms undefined. 



Of the remaining books before us we may briefly 

 say that Prof. Mie's is a French translation of a German 

 pamphlet recently noticed in these columns, that 

 M. Lemeray traces the history of the rise and fall 

 and ultimate extinction (as he seems to consider it) of 

 the idea of the " ether," and that Dr. von Horvath 

 has thought it well to try to give a new presentation 

 of the restricted principle of relativity. 



E. Cunningham. 



Principles of Spectacle Design. 



Handbuch der gesamten Augenheilkunde. Begriindet 

 von A. Graefe und Th. Saemisch, fortgefiihrt von 

 C. Hess. Herausgegeben von Th. Axenfeld und A. 

 Elschnig. Dritte, neubearbeitete Aufiage. Die Brille 

 als optisches Instrument. Von. Prof. Dr. M. von 

 Rohr. Dritte Aufiage. Pp. xiv + 254. (Berlin: J. 

 Springer, 1921.) In Germany, 66 marks; in 

 England, 132 marks. 



SINCE the first edition of this work appeared ten 

 years ago, the design of spectacles has under- 

 gone important developments which are attributed 

 by the author to the increased competition of large 

 specialist manufacturers, to the interest of ophthal- 

 mologists and technical scientific workers, and to recent 

 war experience. 



The work is worthily dedicated to Allvar Gullstrand, 

 upon whose optical treatment of the subject the 

 theoretical portions are based, and more particularly 

 those sections concerned with the eye in motion. It 

 deals in a thorough and comprehensive manner with 

 the comparatively simple geometrical principles in- 

 volved in the design of spectacles for the correction 

 of abnormal vision, but, as is clearly stated in the 

 preface, it does not enter the sphere of the optical 

 computer to whom the practical design is entrusted. 

 NO. 2746, VOL. 109] 



Those who are interested in the theoretical problems 

 of spectacle design will find in this work a clear exposi- 

 tion of most aspects of the subject. The formulae 

 as well as the diagrams can be relied upon, as they are 

 particularly free from errors. An introductory section 

 deals with spectacles of various materials for special 

 purposes. Section I. is devoted to anastigmatic lenses 

 for both fixed and moving eyes. Section II. deals 

 with astigmatic lenses, under which are included the 

 various toric forms and combinations. In Section III. 

 chromatic aberrations are considered, and apparent 

 aberrations in the image space resulting from vision 

 through spectacles are discussed in the concluding 

 section. Although excellent source, name, and subject 

 indexes are provided, there is unfortunately no index 

 of the symbols employed. The systematic historical 

 index of the first edition has been dispensed with, as 

 the information is included under the various subject 

 headings. 



Nearly one-half of the text is devoted to historical 

 references which increase the general interest of the 

 work. As the author states in his preface, the desire 

 to recognise the true inventor lies close to his heart. 

 The task is a difficult one, and, if the author has failed 

 in some respects, it may be attributed to a too ready 

 use of German sources of information. The historical 

 notes do not extend much beyond 191 7, although, 

 according to the title-page, the edition dated 1921 

 is stated to have been newly revised. Indeed, there is 

 only one reference as recent as 1920. An English 

 author is pilloried (p. 114) for the absurd accuracy of 

 certain numerical data, but the author himself, in many 

 instances records to within i/iooo mm. ocular dimen- 

 sions that are comparatively indefinite. 



There is a tendency throughout the work to sacrifice 

 clearness to generality. Spectacles, for example, are 

 inadequately defined as " optical instruments that 

 can be carried continuously before the eyes," the 

 object being not to exclude extreme cases such as 

 protective glasses for stone-hewers, spectacles for 

 drivers, and those in which use is made of mica, horn, 

 and other substances. But the reader will find more 

 in this book by Dr. M. von Rohr for warm approval 

 rather than for criticism. James Weir French. 



Our Bookshelf. 



A Manual of Determinative Mineralogy. By Prof. 

 J. Volney Lewis. Third, revised and enlarged 

 edition. Pp. V-F298. (New York: J. Wiley and 

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

 1921.) i6.y. 6d. net. 



We have here an excellent guide to the recognition 

 of mineral species. It is not intended to supersede 

 the use of a standard work on mineralogy, but to train 



