98 
PRACTICAL DIRECTIONS. 
As a general rule, it will be noticed that object-glasses, of the largest angular 
aperture, do not show parts above or below the focal plane as well as glasses of 
moderate aperture, and yet, in this respect, a great difference is perceived in the 
object-glasses of different makers. 
(a.) If two objectives, having the same magnifying power and angular aperture, 
are both perfectly corrected for chromatic and spherical aberration, the glass 
which has the greater distance between the anterior lens and the object, will have 
the greater depth of definition. 
(6.) If two objectives have the same magnifying power and the same distance 
between the anterior lens and the object, the glass having the smaller angular aper- 
ture will have the greater depth of definition, provided the angular aperture is not 
too small to admit a tolerable amount of light. 
(c.) It is possible, however, that an object-glass of small angular aperture may 
be made with its focus so near the anterior lens, that another glass of larger aper- 
ture could be constructed with a focus so far from the anterior lens that its depth of 
definition might exceed that of the objective of smaller aperture. Such a glass of 
large aperture would be, in every respect and for all purposes, greatly superior to 
the other. 
(c?.) But if an object-glass of moderate angular aperture, perfectly corrected, 
has its focus at i\\Q greatest possible distance from the anterior lens, every addition 
to the angular aperture, in another similar object-glass, necessarily (in the present 
state of science) requires the distance between the anterior lens and the object to 
be diminished. Consequently, with the enlargement of the angular aperture, the 
depth of definition is generally diminished. If opticians could procure, for the 
manufacture of object-glasses, glass of such refractive and dispersive power as 
they would prefer, it would enable them to improve very much the depth of defi . 
nition in object-glasses of large angular aperture. 
Some microscopists have had objectives of large angular aperture provided with 
a diaphragm, to be introduced behind the posterior lens, when viewing objects re- 
quiring greater extent of definition of parts above and below the focus. Some 
advantage may be gained in this manner, but object-glasses so arranged, cannot 
have as great depth of definition as if originally constructed with the greater 
distance between the anterior lens and the object, which would be possible with 
the limited aperture to which these glasses are thus temporarily reduced. 
In enlarging the angular aperture of our object-glasses, we have always sought 
to retain a reasonable working focus, or distance, between the anterior lens and 
the object, and so to combine depth of definition with the other qualities already 
enumerated, as shall give to our object-glasses the greatest degree of efficiency for 
the various uses of the practical microscopist. 
