May 1 6, 1878] 



NATURE 



65 



cases by Drs. Hughes Bennett, Buzzar^!, and Urquhart. 

 Dr. Ferrier gives an analysis of an important memoir by 

 Dr. Duret, " On the Mechanism of Cerebral Concussion 

 and Compression," and Dr. Bucknill reviews severely, 

 but not beyond its deserts, a work by Dr. Bateman, in- 

 titled " Darwinism Tested by Language," in which the 

 main point sought to be established against Darwin and 

 evolution is the immateriality of the faculty of speech, 

 and its being a distinctive attribute of man. 



Several shorter notices are given of recent papers 

 and lectures relating to the brain and nervous system. 

 These might, with advantage, have been much more 

 numerous, and we hope to see this part of the programme 

 more completely carried out in subsequent numbers. 



The original articles would require each a separate 

 analysis to do them justice. We content ourselves in the 

 meantime with merely mentioning their titles. They are 

 all worthy of attentive study, and many of a high standard 

 of excellence, as might indeed be expected from the 

 names of the contributors. 



While the majority of the articles in Brain are of 

 special interest to physiologists and medical men, they 

 will, at the same time, prove a rich field of material for 

 those — a rapidly increasing army — who believe that 

 psychology is to be advanced, not merely by interrogating 

 consciousness, but by intelligent study of the relations 

 between body and mind, as indicated by physiological 

 research and the phenomena of disease. 



While philosophical speculation has interest but for 

 very few of the medical profession, the facts relating to 

 diseases of the nervous system daily observed by medical 

 men, and reported and commented on in a journal like 

 Brain, ought to prove of value to all students of the 

 problems of physio-psychology. 



We heartily wish Brain all success and prosperity in 

 its career. 



LETTERS TO THE EDITOR 



[TTie Editor does not hold himself responsible for opinions expressed 

 • bv his correspondents. Neither can he undertake to return, 



or to correspond -with the writers of, refected manuscripts. 

 No notice is taken of anonymous communications. 

 [ The Editor urgently requests correspondents to keep their letters or 

 short as possible. The pressure on his space is so great that it 

 is impossible otherwise to ensure the appearance even of com- 

 munications containing interesting and novel facts.'] 



•The New "Oil Immersion" Object-Glass Constructed 

 ■ r ■:. ■ by Carl Zeiss, of Jena 



•*■-■ -By the courtesy of its manufacturer, this remarkable l6ns was 

 sent me, a fortnight since, that I might carefully examine it. 

 The results may be of interest to those who have not seen the 

 lens : and the statement of them is due to the industry and skill 

 of- the maker. 



The lens has a focal length of onceighth of an inch : it is an 

 " immersion, " but the fluid employed is the oil of cedar wood. 

 The object of this is that the fluid placed between the lens and 

 the covering glass of the object," may have refractive and dis- 

 persive indices as nearly as possible coincident with those of 

 crown glass, the material of which the covers and the front lens 

 are composed. 01. ligni cedri is the liquid that has been found 

 to be most capable of meeting these conditions ; and by its use 

 the covering glass, thick or thin, and the oil and lens, become 

 practically one homogeneous whole ; and the need for the 

 "screw collar correction" for different thicknesses of cover, is 

 done away. At the same time, and by the same mean«, a large 

 and efficient "angle of aperture" is secured. Mr. J, W, 

 Stephenson, F,R,M.S., suggested to Prof. Abbe this method, 

 and Prof. Abbe and Carl Zeiss have together produced the glass. 



As a piece of workmanship it is extremely fine ; and it can be 

 ured with quite as much ea;:e as an ordinary inunersion ^-inch 

 objective. It works admirably with Powell and Lealand's 

 ordinary sub-stage condenser, with Wenham's reflex illuminator, 

 and with the small plano-convex lens which the maker sends 

 with it to be fastened to the under-siuface of the sKde with the 

 oil of cedar wood. But I have also secured aduiirable results 

 with the illuminating lens of Powell and Lealand's supple- 

 mentary stage, which gives entire command over the angle of 

 the illuminating ray. 



The "spherical aberration" in this lens is beautifully cor- 

 rected ; the "field " being perfectly flat. The colour corrections 

 are, so far as the lens goc^, equally perfect ; but are somewhat 

 conditioned by the dispersive power of the oil, which can be 

 modified readily, and for which Carl Zeiss provides. The 

 sharpness and brilliance of the "definition" which this lens 

 yields is absolutely unsurpassed, in my experience ; and it lias a 

 very great power of " penetration." 



I tested it with a series of " tests " with which I have proved, 

 and compared the glasses of various makers in England, the Con- 

 tinent, and America for some years. Up to the time of receiv- 

 ing this lens, the -j-inch that had done the most in my hands, 

 was one of the "new formula" lenses of Powell and Lealand. 

 It is but justice to say that all my most crucial tests were equally 

 mastered by the lens of Carl Zeiss, I have not been able to do 

 more with it, than with the English glass, but the same results 

 can be accomplished much more readily. The correction has to 

 be brought into operation, and careful adjustment made, to get 

 the finest result with the English lens ; but the German glass 

 has simply to be brought into focus, and the best result is before 

 the observer, provided that the light has been adjusted in the 

 most efficient manner. It is trae that for sharp and perfect 

 definition we must be careful to adjust the length of the draw- 

 tube ; in working this lens there is much need of attention to 

 this matter ; and speaking from a practical point of view, it 

 takes the place, in securing crisp definition, of the screw-collar 

 adjustment ; although, of course, much easier of application. But 

 it is so easy to work the lens with fine results on the more deli- 

 cate test=, that I think that those who make the resolution of 

 these their primary object in the possession of a microscope, 

 can scarcely fail in securing their utmost desire. It is a glass 

 pre-eminently suited for the resolution of difficult lined, or 

 beaded objects. 



I have in my cabinet several frustules of Navicula rhomboides 

 (" N. crasinncrvis ") which I camiot fully resolve with Powell and 

 Lealand's new formula ^\j-inch objective. But all that I can 

 resolve with the English \ I have resolved with the German 

 glass. Amphipleura pelliicida is easily resolved into delicate 

 beads when the frustules are moderately coarse ; and almost any 

 that can be met with are resolvable into lines ;-and this when 

 these diatoms are mounted in balsam. And the highest eye- 

 pieces made may be used without any practical detriment to the 

 image ; although, of course, with a reduced sharpness of the 

 definition. 



On the whole, I think it in many senses the finest lens, of its 

 power, that I have ever seen ; and in every sense, it is an admir- 

 able acquisition. 



But it is a fact that even water "immersion" lenses are of 

 very limited service in observations continuously conducted upon 

 minute living organisms in fluid. We may gladly call in their 

 aid, in the determination of a delicate change of form, or in the 

 more perfect detection and definition, of an obscure point of 

 structure ; but for steady and constant work we are bound to 

 avoid them; for the fluid under the delicate cover is in danger 

 every moment of being " flooded " by coming into contact with 

 the water en the top of the cover, and between it and the lens ; 

 because the movements of the organism have to be counteracted 

 by the movements of the mechanical stage, in order to keep any 

 form that may be studied in view constantly. But. this opens to 

 us the possibility of going to the edge of the cover at any 

 moment ; and thus, by the mingling of the fluids, rendering the 

 observation void. This, of course, will apply still more fully when, 

 as in the case of the valuable glass of Zeiss, the "immersion 

 fluid " is an essential oil. 



Happily it is only in special cases that the greater analysing 

 power, combined with larger working distance, which is pos- 

 sessed by immersion lenses, is required. It is in the earlier 

 study of an organism, and before continuous work upon it has 

 begun. And even if it be not, in the majority of cases, a first- 

 class dry English lens of a higher magnifying power, if efficiently 



