512 



NATURE 



[March 29, i960 



But to return to Dr. von Rohr's book. It is divided 

 into two parts, theoretical and historical respectively ; 

 the first, occupying about 80 pages of the whole 400 in 

 the book, is clear and interesting so far as it goes, but it 

 is hardly satisfying. The author has abstained, no doubt 

 wisely from some points of view, from attempting to give 

 the mathematical proof of most of his propositions ; the 

 result is that the reader is often brought to a standstill 

 with the question — But how does this follow ? Without 

 a much greater acquaintance with optics than can be 

 gained from the book, he would find much of it difficult 

 to read with profit. It is all very well to be told, to take 

 at random a very simple example, that the correction for 

 chromatic aberration for two colours depends, for a 

 " thin " lens, only on the focal lengths and refractive 

 indices of the two lenses concerned, and not on the 

 curvature of their faces — so long as the focal length is 

 not altered by change of curvature — but an intelligent 

 reader would like a proof of this. 



The author starts from Gauss' theory of lenses, which 

 is only applicable to small pencils centrically incident 

 and inclined at a small angle to the axis ; he extends 

 this practically by the assumption that lenses can be 

 constructed for which Gauss' theory, freed from the 

 restriction of nearly direct incidence, would hold strictly ; 

 and then he examines the points in which actual lenses 

 diflfer from this ideal system. 



Each error is discussed in turn, the method of correct- 

 ing it is described, and the possibility of combining the 

 corrections for various errors is considered. Admitting 

 the difficulty of inserting the mathematical proofs, and 

 the probability that if they had been inserted the book 

 would have been useless to many for whom it was 

 intended, it may be said that all this part is well done. 

 The result of the discussion is summed up on page 56, 

 in the section on Seidel's five spherical errors, and the 

 impossibility of completely removing them from a photo- 

 graphic object-glass. A complete freedom from spherical 

 aberration cannot be combined with absence of distortion 

 for all positions of his object. 



A further section of this part deals with chromatic 

 aberration ; the relations between the conditions for 

 freedom from both spherical and chromatic aberration 

 in a thick lens are specially well treated. 



The second, and by far the larger, part of the book is 

 an historical account of the development of a photo- 

 graphic objective, and this is written with great insight 

 and judgment. 



It may be felt by some that a too marked prominence 

 is given, in the account of recent years, to the work of 

 the Jena school ; such prominence is only natural con- 

 sidering the circumstances of the author, and it is certainly 

 true that he shows a high appreciation of the work of 

 the distinguished English opticians, to whom so much of 

 the advance in photographic lenses is due. 



The work is very complete ; it begins with the first 

 camera obscura made by Giambattista della Porta in 

 1589, and carries us down to a lens patented by E. von 

 Hoegh in 1899. The cuts illustratmg the various lenses 

 are carefully drawn — as far as possible to scale — reduced 

 to a common focal length of 100 mm., and the nature of 

 the glass used is indicated by special shading. Altogether 

 the book deserves careful study. 

 NO. 1587. VOL. 61] 



EXPERIMENTS ON ANIMALS. 

 Experime/its on Animals. By Stephen Paget, with an 

 Introduction by Lord Lister. Pp. 269 ; 3 illustrations. 

 (London : Fisher Unwin, 1900.) 

 T^HERE are many people who write about experi- 

 -*- ments upon animals, but only very few who have 

 under their constant notice the actual facts relevant to 

 the subject. In this connection, not merely is a knowledge 

 of fact required, but an intellectuality capable of ap- 

 preciating the significance of fact. The person most 

 competent from this standpoint is one of the Inspectors 

 under the Act. These Inspectors are most carefully 

 chosen by the Home Office on account of special qualifi- 

 cations which they possess. It must not, however, be 

 assumed that because they are the only officers paid by 

 the Crown they are the only men of science who serve 

 it. Most zealous and somewhat thankless help is afforded 

 to them by those authorities who, by virtue of their 

 position and attainments, are regarded as competent to 

 support the candidate in his application for a license or 

 certificate. It would not, however, be comely for a person ; 

 holding an official appointment to write a book upon the 

 subject-matter of his office. Every vivisector, a terrible 

 term by which to designate any one who merely pricks a 

 guinea-pig, knows full well that no one, with the above 

 exceptions, is more entitled to write upon the subject of 

 animal experiments than Mr. Stephen Paget, who for 

 twelve years was the active and long-suffering secretary 

 of the Society for the Advancement of Medicine by 

 Research. During this time most licensees under the 

 .'\ct were brought into contact with the author of the 

 book before us. 



The volume does not simply concern itself with the 

 working of the Act, but must be regarded as a weighty 

 contribution to the polemical literature of the subject. 

 It shows not only that the Act is vigorously worked by 

 the authorities, but also enters largely into the question 

 of the justification of animal experiment. 



The part of the book devoted to this subject will be of 

 the greatest interest to the general reader, and it is 

 sincerely to be hoped that he will take advantage of it, 

 for, while the diatribes of those who oppose all animal 

 experiment are thrust almost weekly into the hands of 

 the public, the altera pars says but little. Mr. Paget 

 classifies the experiments that have up to the present 

 been performed upon animals according to the in- 

 dividual field of medical science enriched by their 

 results. The most casual reader must gather from his 

 pages how in almost every domain, medicine, using this 

 term collectively, has learnt from animal experiment, 

 and how the treatment of disease has by its means ad- 

 vanced from mediaeval empiricism to its present con- 

 dition. To twit workers in the medical sciences with 

 the fact that certain discoveries in physiology, estab- 

 lished by means of vivisection, have not so far led to the 

 curability of apparently cognate diseases, shows a want 

 of that intellectuality which is capable of appreciating 

 the significance of scientific fact. We might as well 

 deny the value of the discovery of Africa because some 

 parts of it are uninhabitable. As hygienic science ad- 

 vances, and our knowledge concerning both the methods 

 of extinction of pathogenic micro-organisms, and the 



