172 



NATURE 



{Dec. 22, 1887 



by the best makers in London, with a well-chosen water- 

 immersion of ten years ago ; and both these with a recent 

 homogeneous glass of the same power with a numerical 

 aperture of i'5. Or still better, a dry g'^-inch objective, of 

 the same date and the same makers, of numerical aperture 

 0*98, with a water-immersion lens of the same power of 

 say ten years ago, having an aperture of 1*04, and a recent 

 homogeneous T^J^-inch, with a numerical aperture of i "38. 

 Still more strikingly, let the same observations be made 

 with a dry yVinch objective of twenty years ago, with a 

 numerical aperture of 0*99, and a homogeneous lens of the 

 same power, with numerical aperture i"5 ; and, finally, both 

 these with an apochromatic objective of the same power by 

 the same London makers and an aperture of i "40. We 

 venture to say, to histologist, bacteriologist, diatomist, and 

 all other serious workers with the microscope, that there 

 can be no proper comparison of the results ; or, rather, the 

 comparison is odious indeed for the oldest, and even the 

 elder, lenses. 



But, as we have stated, it is to Germany we are 

 indebted for the kttowledj^e out of which, alone, these 

 improvements could have arisen. In spite of the length 

 and abundance of English treatises on the microscope, 

 it has never been part of the scope of the respective 

 authors to do other than make the scantiest reference 

 to the principles of the microscope ; and nothing is 

 found that will elucidate the theory of the construction 

 of objectives, and eye-pieces, and the possible and real 

 relations of each to the other. There is nothing to be 

 found indeed in our language, except in the invalu- 

 able translations published in the successive Journals 

 of the Royal Microscopical Society, which discusses the 

 phenomena of diffraction, of polarization, of the principles 

 of the true interpretation of microscopical images, and the 

 theory of work with the microscope. English workers 

 with high powers have discovered painfully where their 

 lenses during many years were at fault ; they could show 

 our opticians what they wanted ; but it has been only as 

 the result of the laborious mastery of the theory of 

 lens-construction by German investigators, with Abbe 

 at their head, that the English worker has been 

 able to get his wants, in object-glasses and eye-pieces, 

 supplied. 



But like all advances in insight and analytical power, 

 ihese very improvements, so welcome and so helpful to 

 isearchers in many important branches of science, only 

 open up the horizon of the unknown more fully ; and the 

 very knowledge we get, through the inestimable improve- 

 jnents, only reveals new difficulties ; and again creates 

 optical wants. It is, then, with pleasure indeed that we hail 

 this excellent translation of Naegeli's work on the theory 

 and practice of the microscope The book has long been 

 announced, and many have looked, year after year, 

 eagerly for its coming. But a series of untoward cir- 

 cumstances have combined to make the delay inevitable. 

 The translation was begun some ten years since by Mr. 

 Frank Crisp, the Secretary of the Royal Microscopical 

 Society, purely in the interests of microscopy in England. 

 He wished to fill the blank in the microscopical literature 

 of the country, which had, in fact, become almost a dis- 

 honour to us. This book of Naegeli and Schwendener is 

 a thorough treatise on the theory of the microscope, giving 

 a detailed theoretical exposition of the construction of 



objectives, eye-pieces, &c., with analytical determination of 

 the path of the rays in refracting systems ; discussing 

 exhaustively chromatic and spherical aberration ; the 

 influence of the cover-glass ; the flatness of the field of 

 view ; the centering of systems of lenses ; the importance 

 of aperture, with a discussion of the diffractional action 

 of the aperture of the lenses ; and the question of 

 illumination. 



With equal care and thoroughness there is a discussion 

 of the testing of the microscope, in all its branches, which 

 cannot but make the student conversant with every 

 essential point in the construction of the instrument ; and 

 an absolutely invaluable monograph on the theory of 

 microscopic observation, which no one attempting to pub- 

 lish results of any importance dare leave unread or even 

 unstudied. The phenomena of polarization receive equal 

 care in treatment and must prove of the utmost value. 



To put such a book within the reach of English readers, 

 Mr. Crisp rightly felt, would be to give the needed stimulus 

 to English microscopical observation : it would put them 

 on the same horizon with German specialists. But the 

 first impediment to its appearing in print was, that Mr. 

 Crisp was compelled, by the weight of other claims upon 

 his time, to relinquish the task of preparing the translation 

 for the press when only eighty pages were in type ; and 

 a large lapse of time ensued before the labour was at 

 length resumed by Mr. John Mayall, Jun., one of the 

 editors of the Journal of the Royal Microscopical Society. 

 But, beyond this, when Mr. Mayall had done his work and 

 the printing of the work was complete, a fire destroyed 

 the premises of the printer and all but a small portion of 

 the type was wholly lost. The present issue is therefore 

 an entire reprint. 



There is but one point that we can see in the book, as it 

 now stands, that need call for the slightest reflection : it is 

 that the authors adopt, and discuss at considerable length, 

 a method of testing the resolving power of objectives 

 which has had — in another connection — the honour of a 

 mathematical refutation by the highest living authority 

 on microscopical optics. Prof. E. Abbe, of Jena. This 

 method consists of viewing, with the objective to be tested, 

 what were assumed to be " miniatured images " of a net- 

 work of wire gauze produced by minute globules of oil 

 and other matters, which images were supposed to be re- 

 duced to the " limit of discrimination " by simply distanc- 

 ing the wire gauze from the oil globule. Prof. Abbe's de- 

 monstration {vide Journ. Royal Micros. Soc. 1882, pp. 

 693-96) of the fallacy of this method proves that the 

 combination of a microscope with a minute oil globule, or 

 its equivalent, for viewing a distant object — whether wire 

 gauze, or a so called " double star" arrangement as ad- 

 vocated by Dr. Royston Piggott — serves no purpose what- 

 ever in determining the limit of the resolving power of 

 the objective ; but merely produces a very low-power tele- 

 scope ; the power of which may easily be so low, indeed, 

 that the eye fails to differentiate, or even to perceive, the 

 image ! 



The adoption of this fallacious mode of reasoning, 

 however, amounts only to a blemish in an otherwise most 

 excellent work ; and with the publication and accessi- 

 bility of Abbe's correction can do but little harm. 



It would have given a still higher value to the book if 

 the chapters devoted to an exposition of Prof. Abbe's 



