Address of the President at the Annual Meeting. 41 
which causes our best makers to aim at its augmentation, as if 
it were the one thing needful. Now if we examine into the 
comparative advantages and disadvantages of such glasses, we 
shall find, I think, that for all ordinary purposes^ the latter 
decidedly predominate. The objects which can be seen de- 
cidedly better with lenses of very large, than with those of 
moderate aperture, are comparatively few in number, and of a 
very limited kind ; being such as are marked with very close 
lines, striae, spots, or other like inequalities of surface. For 
the resolution of these, it is well known that a large angular 
aperture is essential ; so that of two lenses whose performance 
may be equally good for all ordinary purposes, that which has 
the wider angle shall here surpass the other, and this in exact 
proportion to its excess. But this superiority 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 obtained, 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 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, exactly neutralize the efi*ect of its refraction; 
and the arrangement of the mirror and condenser must be such 
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 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 particular 
class 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 1-100", will perform 
without much sensible deterioration with a thickness either of 
1-80" or of 1-120", is practically the best for all ordinary 
purposes. 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. Had 
not the term ' penetration' been already applied in a different, 
and I think far less appropriate sense, I should have said that 
an objective of moderate angular aperture has far more penc- 
