NATURE 



lOl 



THURSDAY, APRIL lo, 1919. 



MODERN OPTICAL INSTRUMENTS. 



The Theory of Modern Optical Instruments: 

 A Reference Book for Physicists, Manufac- 

 turers of Optical Instruments, and for Officers 

 m the Army and Navy. By Dr. Alexander 

 Gleichen. Translated from' the German by 

 H. H. Emsley and W. Swaine. With an ap- 

 pendix on " Rang^efinders. " Pp. xii + 376. 

 (London : His Majesty's Stationery Office 

 1918.) Price 125. 6d. net. 



T^HE book before us is of special interest, 

 -■- because it is the first of several German 

 treatises on optical subjects which were selected 

 for translation into English by the Standing- Com- 

 mittee on Glass and Optical Instruments, ap- 

 pointed in December, 1916, by the Advisory 

 Council for Scientific and Industrial Research. 



There can be but little doubt that Dr. Gleichen 's 

 book was included by the Committee primarily on 

 account of the unusually complete descriptions and 

 illustrations of modern optical instruments which 

 fill the greater part of the volume. Its title is, 

 in fact, somewhat misleading, for the theoretical 

 part does not go beyond elementary principles, 

 and is not by any means exhaustive even with 

 regard to these. To be really useful, a theory 

 of modern optical instruments must deal chiefly 

 with the aberrations, and especially with those of 

 higher order. For with regard to telescopes a 

 dictum of the elder Steinheil is still true, that " all 

 improvements of these instruments have aimed, 

 consciously or unconsciously, at making them 

 shorter"; with regard to microscopic lenses the 

 ■chief aim is to increase the numerical aperture 

 and to extend the useful field without loss of 

 ■definition, and with photographic objectives fine 

 definition, freedom from distortion, rapidity, and 

 a flat field of wide angle are the principal 

 desiderata and subjects of competition. In every 

 case success depends almost entirely on skilful 

 distribution of the higher aberrations and on the 

 •discovery of types in which these are of sufficiently 

 small magnitude. On this important subject the 

 book before us is practically dumb. 



Following the eighty-four pages devoted to the 

 elementary theory, we find chap, vi., in which the 

 human eye is very fully and clearly dealt with. 

 The author returns to this important subject in 

 chap. XV., in which the more elaborate aids to 

 defective vision are described, and ophthalmo- 

 scopes are also reviewed in a very lucid manner. 



The description of telescopes is found in 

 chap. ix. Prismatic instruments naturally receive 

 most attention. Designers will welcome the large 

 number of different types of erecting prisms, of 

 which clear illustrations are given, but will look 

 in vain for details of the essential constructional 

 ^ata and for a discussion of the conditions- on 

 \vhich perfect results depend. 



Rangefinders are dealt with in chap. xi. It 



NO. 2580, VOL. 103] 



will be taken for granted that German ' instru- 

 ments monopolise the author's attention. This 

 deficiency is very fully rectified by an ap- 

 pendix to the book, in which the translators (both 

 of whom are members of Messrs. Barr and 

 Stroud's staff) give a full and excellent account of 

 British rangefinders. This appendix concludes 

 vvith a description and illustrations of a captured 

 German rangefinder made by Goerz. 



Chap. xii. will be welcomed by English 

 opticians, because it gives useful information on 

 cystoscopes, which up to the time of the outbreak 

 of war were practically a German monopoly. 



The chapter on the microscope is too short to 

 do justice to this instrument. 



Photographic objectives are the subject of 

 chap. xiv. This chapter contains a large amount 

 of useful information and numerous detailed 

 formulae of actual lenses — mostly modern — which 

 will provide interesting material for study by 

 designers of such instruments. 



A curious and possibly significant omission in 

 the book is that there is no definite mention at 

 all of submarine periscopes; there is only a 

 vague hint on p. 160 that they "may have a 

 length of several metres." 



It will have been gathered that the work, whilst 

 not quite answering to its principal title, contains 

 a large amount of information not otherwise 

 readily accessible, and that it should therefore 

 prove of value as a reference-book. Its utility in 

 this respect is enhanced by a bibliography and 

 a very complete alphabetical index at the end. 



The translation is very well done, and, in addi- 

 tion to the appendix already mentioned, the trans- 

 lators have inserted numerous notes at the ends 

 of various chapters and at the foot of pages, all 

 of which are to the point and of decided value. 



A. E. C. 



THE USEFULNESS OF PSYCHOLOGY. 

 (i) Present-day Applications of Psychology, with 



Special Reference 'to Industry, Education, and 



Nervous Breakdown. By Lt.-Col. Charles S. 



Myers. Pp. 47. (London : Methuen and Co. , 



Ltd., 1918.) Price is. net. 

 (2) War Neuroses. Bv Dr. J. T. MacCurdv. 



With a Preface by Dr. W. H. R. Rivers. 



Pp. xi + 132. (Cambridge: At the University 



Press, 1918.) Price 75. 6d. net. 



IF evidence be required as to the threadbare 

 condition of the old gibe at psychology as 

 a statement of obvious facts in unintelligible 

 language, we have it in abundance in these books 

 by Col. Myers and Dr. MacCurdy. For, while the 

 clarity of the authors' expression is obviously the 

 natural outcome of a firm grasp of their subjects, 

 the facts which they present are probably far from 

 obvious to the mind which is not conversant vvith 

 the rapid progress of present-day psychology. 

 Moreover, these unoJjvious facts are not mere 

 freakish curiosities, but important strands in the 

 material of our social life. 



