ON THE CHOICE OF A MICROSCOPE. 163 
of penetration and definition. Now, although this may be 
quite true in many cases, the following instance may serve 
to show that the largest angle that a medium object-glass 
can be made to bear, may be obtained when truly corrected 
without any injury to the perfection of its penetrating and 
defining qualities. And if this be true with medium object- 
glasses, it is still more so with deep ones, in which not pene- 
trating, but resolving power is the principal requisite. Mr. 
T. Ross, for example, has enlarged the angles of his objectives 
altogether as much, and of his medium object-glasses beyond 
those of other opticians; and yet on carefully testing some 
recent medium object-glasses, by a very eminent maker, 
whose angular apertures were considerably less than his, I 
found their penetration and marginal definition greatly 
inferior to them, so that their performance on opaque objects 
was altogether indifferent; and this conclusion has been 
confirmed by other microscopists of far greater scientific 
knowledge than I possess. Hach of our principal makers will 
of course always have their special advocates. There will 
always be in the microscopic world Powellites, Rossites, 
Smithites, Pillischerites. To undervalue, therefore, in the 
slightest degree the undoubted excellences in the glasses 
manufactured by other opticians, is far from my thoughts; I 
only say that, as far as the preceding experiment goes, from 
the large angular aperture, and consequently great resolving 
power which Mr. Ross obtains throughout his object-glasses, 
it is only natural to argue that their spherical and chromatic 
aberrations must be exquisitely corrected to ensure along 
with it so great an amount of penetrating and defining power. 
The object-glasses manufactured by Messrs. Smith and Beck 
justly bear a high character both for the optical skill dis- 
played in them, and for the great excellence of their work- 
manship. ‘Too high praise, again, can scarcely be accorded to 
Messrs. Powell and Lealand for their untiring efforts in pro- 
ducing with such success the deepest power for resolving the 
delicate points or lines of very minute structures, namely, their 
~;th and .1,th objectives, though their sphere of usefulness, 
owing to their great expensiveness and extremely short focal 
lengths, becomes somewhat limited; especially too, as the angu- 
lar aperture of such glasses cannot be increased with advan- 
tage, beyond that which the --,th already possesses. It must be 
remembered, moreover, that the principal work of the micro- 
scope is effected with low and medium powers. How often 
do we find ourselves using, I will not say a =,th, or +),th, or 
ith object-glass, but even a 1th, compared with the powers 
~ below them? Besides, the peculiar attribute, after all, of the 
