146 PROCEEDINGS OF THE AMERICAN ACADEMY 



successfully employed the microscope in physiological investigations : 

 " The superiority in resolving power possessed by object-glasses of 

 large angular aperture, is obtained at the expense of other advantages. 

 For even granting that there is no sacrifice of that most important 

 element, 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 exact- 

 ness in the adjustments. Only that portion of the object which is jore- 

 cisely in focus can be seen with an approach to distinctness, everything 

 that is in the least degree out of it being imbedded (so to speak) in a 

 thick fog ; it is requisite, 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 illumination. If there 

 be any failure in these conditions, the performance of a lens of very 

 wide angular aperture is very much inferior to that of a lens of mod- 

 erate 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 particular 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 i^-u", will perform with- 

 out much sensible deterioration Avith a thickness either of -gV" or of 

 y^-!s", is practically the best for all ordinary purposes. Moreover, 

 a lens of moderate aperture has this A'ery 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, further, that almost all the great achievements of 

 microscopic research 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, physiological, 

 botanical, or zoological investigations, since the introduction of this 

 invaluable auxiliary ; and it is well known that the great majority of 

 their instruments are of extremely simple construction, and that their 

 objectives are generally of very moderate angular aperture. More- 

 over, if we look at the date of some of the principal contributions which 



