September 22, 192 1] 



NATURE 



109 



ally attractive to chemists concerned with the 

 application and manufacture of dyes. 



Praise is due also for the general index, and, 

 excepting- a few lapses into laboratory slang and 

 the "of course" habit, the literary form is com- 

 mendable. In the second edition, however, i* is 

 to be hoped that the author will establish uni- 

 :ormity of construction in the compound words; 

 ro find "'octa-chlor anthracene" in one line, and 

 " tetrachloranthraquinone " in the next, is ex- 

 asperating to a methodical reader, and a bad 

 example to students. This fault is persisted in 

 throughout, and is the only blemish in an other- 

 wise admirable treatise. M. O. Forster. 



The Future of Geometrical Optics. 



,\) Geometrical Investigation of the Formation of 

 Images in Optical Instruments, embodying the 

 Results of Scientific Researches conducted in 

 German Optical Workshops. Edited bv M. 

 von Rohr. (Forming vol. i of ''The Theory 

 of Optical Instruments.") Translated by R. 

 Kanthack. Pp. xxiii4-6i2. Printed and pub- 

 lished for the Department of Scientific and In- 

 dustrial Research by H.M. Stationery Office, 

 1920. (From any bookseller or through 

 H.M.S.O. at Imperial House, Kingsway, 

 \V.C.2, and 28 Abingdon Street, S.W.i; 37 

 Peter Street, Manchester; i St. Andrew's 

 Crescent. Cardiff; 22 Forth Street, Edinburgh; 

 or from E. Ponsonby, Ltd., 116 Grafton Street, 

 Dublin.) 2/. 5s. net. 

 2) Die Binokularen Instrumente : Xach Ouellen 

 und bis zum Ausgang von 1910 Bearbeitet. By 

 Prof. Moritz von Rohr. Zweite, \'ermehrte, 

 und \'erbesserte Auflage. (Xaturwissenschaft- 

 liche Monographien und Lehrbiicher. Zweiter 

 Band.) Pp. xvii-303. (Berlin: Julius 

 Springer, 1920.) 40 marks. 



AT a meeting of the Optical Societ>' 

 J^x. held in Cambridge on May 21 last, the 

 luture of geometrical optics formed the subject 

 >f an interesting discussion, in which the points of 

 view of mathematicians and practical designers 

 respectively were expressed. The subject has re- 

 gained actuality in recent years in view of the 

 undoubted superiority in optical design possessed 

 )y the German manufacturers in 1914. a superi- 

 ority which proved a serious handicap to us in 

 the manufacture of optical instruments such as 

 range-finders, etc., required for military and naval 

 nurposes. The importance of this branch of 

 knowledge was then realised ; unfortunately, 

 before the war the subject had been gradually 

 dropping out of university curricula, the labori- 

 NO. 2708, VOL. 108] 



ous algebra involved and the stereotyped methods 

 of treatment combining to render it distasteful to 

 mathematical teachers and students. Relegated 

 to a corner of the mathematical syllabus, geo- 

 metrical optics was too often reduced to a few 

 formulae crammed in a hurry, and it lacked the 

 vitalising influence of really interesting and prac- 

 tical illustrations. The manufacturing optical de- 

 signer, on the other hand, tired of waiting for 

 mathematical developments adequate to his needs, 

 became increasingly empirical in his methods, and 

 even now depends almost exclusively upon tri- 

 gonometrical tracing of a few rays, which is, in 

 fact, nothing else but trial and error. Probably 

 this almost complete divorce between theorj' and 

 practice accounts largely for the unprogressive 

 character of pre-war British optical design as 

 compared with the German. 



(i) It is a natural inference that in some way the 

 British type of text-book on this branch of science 

 fails to stimulate the reader, and, bearing in mind 

 the pioneer work of Abbe, Seidel, Steinheil, 

 Koenig, and von Rohr in Germany, it is obvi- 

 ously desirable that the work of these masters of 

 the subject should be made readily accessible to 

 English-speaking students. The Department of 

 Scientific and Industrial Research is therefore to 

 be congratulated upon bringing out a translation 

 of the classical treatise on -' The Theory of Optical 

 Instruments," edited by Moritz von Rohr. The 

 translation has been carried out by Mr. R. 

 Kanthack, and the work has evidently been done 

 with great conscientiousness and accuracy. The 

 translator acknowledges the valuable help of 

 Messrs. J. W. French and E. B. Knobel and of 

 Prof. J. W. Nicholson. In various respects the 

 translation is an improvement on the original, 

 the numbering of the paragraphs and equations 

 greatly facilitating reference. Additions and 

 modifications have also been made to the biblio- 

 graphy, and the figures have been improved and 

 various errors corrected. The book is well got 

 up, and altogether a very creditable production, 

 which meets an undoubted need. 



When all this has been said, however, it may 

 be doubted whether, after all, von Rohr's treatise 

 is really likely to stimulate the student, at any 

 rate in this country. For one thing, it is not the 

 work of a single mind. Chap, i, on the funda- 

 mental principles, is by H. Siedentopf; chap. 3, 

 on .\bbe's theory of optical images, by E. 

 W'andersleb ; chap. 4, which treats of optical 

 images from a different point of view, by P. 

 Culmann ; chap. 8, on prisms, by F. Loewe ; Dr. 

 Koenig has written chaps. 6 (on chromatic aberra- 

 tions) and 7 (on the computation of optical 



