AGf~ICULTURAL RESEARCH, PUSA.

52 PRACTICAL BACTERIOLOGY
correct.ed by the combination of lenses of different
dispersive power- e.g. convergent convex lenses of
crown glass having low dispersive power, and divergent
concave lenses of flint glass having high dispersive
power. By this means two of the spectrum colours
are combined, and the ordinary achromatic objective
is constructed in this manner.
APOOHROMATIC OBJECTIVES
While achromatic objectives fulfil all ordinary purposes,
they are not sufficiently corrected for critical
work, such as photography and resolution of minute
objects, for which apochromatic objectives must be
employed. These represent the highest degree of
optical perfection, and are, in consequence, very expensive.
Apochromats surpass all others in the matter
of colour correction, and the essential factor in their
construction is the use of the mineral.fluorite. Fluorite
possesses thc following valuable optical properties :-
(a) high degree of transparency;
(b) low refractive index;
(c) extremely small dispersion.
As a result of the use of fluorite at least three colours
may be united, thus eliminating the secondary spectrum.
This endows the objectives with a brilliance and
" crispness" of image not attainable with ordinary
lenses, and enables the maximum l'esolving powel' to
be obtained.
Apochromatic objectives must be used only in
conjunction with" compensating" eye-pieces, and care
must be taken to adjust the tube length carefully and
to employ a highly corrected and properly centered
condenser.
A series 'of objectives containing a certain amount
of fluorite, and which are intermediate in quality
between the apochromatic and achromatic objectives,
has been introduced by several makers. They are
known as " semi-apochromatic" or " fluorite" lenses,
and some of them have a performance approaching
that of the apochromatic objectives. The oil-immersion