March 2, 1893] 



NATURE 



411 



made to serve as a kind of glossary ; and even more re- 

 markable is the fact that the patent lever fine adjustment 

 of Swift and Son, the only fine adjustment which, in our 

 judgment, makes the " Jackson Model " microscope 

 (which Dr. Van Heurck evidently affects) at all a practic- 

 able instrument, is treated in the same way. So indeed 

 is Campbell's differential screw ; and the highest com- 

 mendation is given to the form adopted in the author's 

 own model. No doubt in its present form it is relieved 

 of many defects incident to the form of fine adjustment 

 to which it belongs ; but it must be remembered that we 

 are told that each of the divisions of the milled head of 

 this fine adjustment corresponds to the i^Vo^h of a milli- 

 metre. Yet the screw which gives this fine result has to 

 lift the whole " body " of the instrument. In the lever 

 fine adjustments only a nose-piece is lifted, having an in- 

 considerable weight, and producing in practice no friction, 

 and to this the objective is attached ; it certainly appears 

 but reasonable, as it has proved in practice to workers who 

 have employed the several methods for continuous years, 

 that the " wear and tear " upon so fine a screw to which 

 such heavy work is given does not contribute to perma- 

 nent steadiness, or in constant work, to continued 

 accuracy. 



In fact, after careful study of the microscope special- 

 ised in this treatise, it is difficult to discover anything 

 really new or distinctive in it save the bringing of the 

 fine adjustment pinion of the sub-stage above the level 

 of the principal stage. The value of this may be variously 

 assessed, but it has the plain disadvantage of preventing 

 the complete rotation of the principal stage ; and it may 

 be doubted if it has any advantage which will compensate 

 for this. 



There is little, if anything, to enable the reader to 

 distinguish as to the practical value of one form of stand 

 as compared with another, and yet there can be no 

 greater divergence in form than that between the 

 Continental stand on the one hand with its dead 

 weight to produce steadiness, and the two English 

 models known as the Ross and Jackson models respec- 

 tively on the other. What distinguishes them, in what 

 either of them has superiority over the other, and 

 wherein in any of them what is essential to a first-rate 

 working microscope, is nowhere discussed. 



It is true that the models of many makers are presented 

 and beautifully printed; but many of these are not 

 printed in these pages as revealing essential differences 

 important for the reader to observe, but they are placed 

 amongst others simply as the productions, with slight 

 variations, of the same instruments by different makers. 

 We cannot but believe that if some plain directions had 

 been given as to the essentials of a good microscope, and 

 the principal models passed in review showing their con- 

 formity or otherwise to these requirements, the " elemen- 

 tary" object of the book Uould have been more fully 

 accomplished, and the tyro more fully aided in the 

 " choice of a microscope." 



Dr. Van Heurck has shown his practical knowledge of 

 the microscope as a manipulator in many ways, in this 

 book, but perhaps this is nowhere more fully seen than in 

 his full appreciation of the condenser as an indispensable 

 instrument in bringing out the finest optical possibilities 

 of the most perfectly constructed object glasses. His 

 NO. 1218, VOL. 47] 



book may be said to be alone amongst continental 

 treatises on the microscope in this respect. It has been 

 by very tardy steps that the continental makers, or the 

 continental microscopists, have learned to appreciate 

 the immense importance of a condenser in causing 

 optical combinations to give their highest results. It is 

 but recently that so leading a firm as Zeiss has yielded on 

 this point and produced condensers. The first was chro- 

 matic, and, as a consequence, proportionately unsatis- 

 factory. Then came the most useful achromatic form of 

 Abbe. But we are glad to observe that Van Heurck re- 

 cognises that the apochromatic immersion condenser of 

 Powell " is the most perfect condenser which exists at 

 present" (p. 85). It is inevitable that with apochromatic 

 objectives it should be. We cannot possibly see how 

 the splendid objectives on apochromatic principles can 

 give their finest results unless they are illuminated by an 

 apparatus which is not only as perfect in workmanship, 

 but of as great a numerical aperture, and with as complete 

 corrections as the objective which is collecting the light 

 and forming the image of the object the condenser is 

 illuminating. 



And for this reason, while we admit fully that the plate 

 of photo-micrographs produced in this and other volumes 

 by the very exceptional skill of Dr. Van Heurck with the 

 most remarkable object glass which the manipulative 

 skill of man has yet produced, viz. the 25 mm. with 

 N.A. I "63, is a monument to his manipulative ability, we 

 still contend that he worked under difficulties of no small 

 importance. The only condenser provided for this lens 

 by the great firm which produce it, is one which of 

 necessity has a flint front, but is as wholly uncorrected as 

 the glasses used by Hooke or Bonanni ! 



Now if it be important to use an apochromatic con- 

 denser at all, how much more important to use it on such 

 a lens, with such an aperture and such exquisitely refined 

 corrections. This objective has never yet had its best 

 power revealed, because its illumination has been always 

 a counteraction of its own refinements. 



We are surprised that in manipulation the tyro is re- 

 commended in this treatise to focus down upon the object 

 first, of course with great care, and then to find the 

 actual focal point by withdrawing the tube by either 

 coarse or fine adjustment. A far more elegant and safe 

 method is certainly adopted, and we doubt the preference 

 expressed for daylight as the best constant source of 

 illumination. It is uncertain and always variable and 

 more refractory than the edge of a good lamp-flame, 

 unless we need a monochromatic ray from a sunbeam. 



At the close of the book there is a communication 

 which had appeared before in the Journ. Roy. Micro. 

 Soc, from Dr. S. Czapski, which gives a suggestion for 

 the possible enlargement of the practical N.A. of homo- 

 geneous object-glasses, which makes an advance to 2'o 

 possible without the employment of the dense flint and 

 highly refractive media needed by the lens spoken of 

 above. In fact it is plain that true monochromatic light 

 may increase a N.A. of i'40 to 175. 



There is a chapter on photomicrography which has 

 the value that is inevitable, coming as it does from one of 

 the most practised and efficient workers ; still it can 

 hardly be expected to be exhaustive, and every practical 

 photomicrographer has, and adopts as most perfect, his 



