164 ON THE CHOICE OF A MICROSCOPE. 
compound achromatic microscope, which exalts it so far above 
the simple, is its capacity for exhibiting opaque objects with bril- 
liancy and sharpness. And thisis the special province of low 
and medium object-glasses—opaque objects, too, being (as old 
Dr. Goring truly said) severer tests than transparent ones of 
the penetration and definition of these powers of a microscope. 
When the achromatic object-glass was in its infancy, the 
best test object now, as then, of the goodness of a deep ob- 
jective, the lines on the Podura scale, could be resolved by 
transmitted light under a fine triplet, with a sharpness at 
least approaching the modern one eighth; but what glass, 
save the achromatic, was then capable of showing the com- 
monest opaque objects satisfactorily? The old compound 
microscope was incapable of showing them with any distinct- 
ness, owing to the aberrations of its uncorrected object- 
glass, which, together with the eye-piece, broke up the light 
into red and blue colours, and if these were corrected by 
limiting the aperture of the object-glass, the hight was insuf- 
ficient for any satisfactory result. If, on the other hand, 
the simple microscope was employed even in its best form of 
doublet or triplet, though its aberrations were nearly got rid 
of, and sufficient angular aperture was obtained to resolve 
the ordinary test objects, yet, as they were not so totally 
destroyed as to allow the employment of an eye-piece to 
increase the magnifying power of the object-glass, the ampli- 
fication of the objects submitted to it was too trifling for any 
practical purposes. And if, again, a deeper triplet lens was 
used to raise the magnifying power to the proper point, the 
focus between the object and object-glass was so short, that 
no reflected light could be interposed between them. So 
that now, in fact, the lowest achromatic object-glass, com- 
bined with the lowest eye-piece, gives more excellent 
results in the examination of opaque objects than the 
deepest single lens that could be effectually employed 
upon them. And if this be so with the shallowest powers, 
it may readily be anticipated how splendid are the achieve- 
ments on such objects of the less shallow and medium 
object-glasses. Thus, the recently adopted 2rd in. is a 
charming power for many opaque objects, and -4,th im. even 
for such as are extremely minute, while the 4 in., especially 
that of 90° aperture,* will, probably, always maintain its 
* So much difference of opinion exists as to the advantage of small and 
large angular apertures, especially for the purposes of scientific investiga- 
tion, that it is with diffidence that I avow my decided preference for large 
apertures, presupposing, of course, the most perfect corrections on the object- 
glasses possessing them. Presupposing, then, these essential conditions, 
