REAL VALUE OF ANGULAR APERTURE. 191 



correctness of which he has been since assured, by the approval 

 of many of those who have most successfully employed the Mi- 

 croscope in Physiological investigations. " The superiority in 

 resolving power possessed by object-glasses of large angular aper- 

 ture, is obtained at the expense of other advantages. For even 

 granting that there is no sacrifice of that most important ele- 

 ment defining power (which can only be secured, with a very 

 wide angle, by the utmost perfection in all the corrections), yet 

 the adequate performance of such a lens can only be secured by 

 the greatest exactness in the adjustments. Only that portion of 

 the object which is precisely in focus, can be seen with an ap- 

 proach to distinctness, everything that is in the least degree 

 out of it being imbedded (so to speak) in a thick fog ; it is re- 

 quisite, too, that the adjustment for the thickness of the glass 

 that covers the object, should exactly neutralize the effect of its 

 refraction ; and the arrangement of the mirror and condenser 

 must be such as to give to the object the best possible illumina- 

 tion. If there be any failure in these conditions, the perfor- 

 mance of a lens of very wide angular aperture is very much in- 

 ferior to that of a lens of moderate aperture ; and except in very 

 experienced hands, this is likely to be generally the case. Now 

 to the working Microscopist, unless he be studying the particu- 

 lar classes of objects which expressly require this condition, it is 

 a source of great inconvenience and loss of time to be obliged 

 to be continually making these adjustments ; and a lens, which, 

 when adjusted for a thickness of glass of 1-1 00", will perform 

 without much sensible deterioration with a thickness either of 

 1-80" or of 1-120", is practically the best for all ordinary pur- 

 poses. Moreover, a lens of moderate aperture has this very great 

 advantage, that the parts of the object which are less perfectly in 

 focus, can be much better seen ; and therefore that the relation 

 of that which is most distinctly discerned, to all the rest of the 

 object, is rendered far more apparent. Let me remind you, fur- 

 ther, that almost all the great achievements of Microscopic re- 

 search have been made by the instrumentality of such objectives 

 as I am recommending. There can be no question about the large 

 proportion of the results which Continental microscopists may 

 claim, in nearly all departments of minute anatomical, physiolo- 

 gical, botanical, or zoological investigation, since the introduc- 

 tion of this invaluable auxiliary ; and it is well known that the 

 great majority of their instruments are of extremely simple con- 

 struction, and that their objectives are generally of very mode- 

 rate angular aperture. Moreover, if we look at the date of some 

 of the principal contributions which this country has furnished 

 to the common stock, such as the < Odontography' of Professor 

 Owen, the ' Eesearches into the Structure of Shell' carried out 

 by Mr. Bowerbank and myself, the < Physiological Anatomy' of 

 Messrs. Todd and Bowman, the first volume of the Histologi- 

 cal Catalogue,' by Professor Quekett, and the ' British Desmi- 



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