Optika B-380 Serie Manual De Instrucciones página 16

Microscope objectives designed for use with immersion oil have a number of advantages over those that are
used dry. Immersion objectives are typically of higher correction (either fluorite or apochromatic) and can have
working numerical apertures up to 1.40 when used with immersion oil having the proper dispersion and viscosity.
These objectives allow the substage condenser diaphragm to be opened to a greater degree, thus extending the
illumination of the specimen and taking advantage of the increased numerical aperture.
A factor that is commonly overlooked when using oil immersion objectives of increased numerical aperture is
limitations placed on the system by the substage condenser.
In a situation where an oil objective of NA = 1.40 is being used to image a specimen with a substage condenser
of smaller numerical aperture (1.0 for example), the lower numerical aperture of the condenser overrides that of
the objective and the total NA of the system is limited to 1.0, the numerical aperture of the condenser.
Modern substage condensers often have a high degree of correction with numerical aperture values ranging
between 1.0 and 1.40. In order to effectively utilize all the benefits of oil immersion, the interface between the
substage condenser front lens and the underside of the microscope slide containing the specimen should be
also be immersed in oil.
An ideal system is schematically diagramed in Fig. 22, where immersion oil has been placed at the interfaces
between the objective front lens and the specimen slide and also between the front lens of the condenser and
the underside of the specimen slide.
This system has been termed a Homogeneous Immersion System and it is the ideal situation to achieve maxi-
mum numerical aperture and resolution in an
optical microscope.
In this case, the refractive index and dispersion
of the objective front lens, immersion oil, sub-
stage condenser front lens, and the mounting
medium are equal or very near equal.
In this ideal system, an oblique light ray can
pass through the condenser lens and comple-
tely through the microscope slide, immersion
oil, and mounting medium undeviated by re-
fraction at oil-glass or mounting medium-glass
interfaces.
When using high-power achromat oil immer-
sion objectives, it is sometimes permissible to
omit the step of oiling the condenser top lens.
This is because the condenser aperture
diaphragm must often be reduced with lesser-
corrected objectives to eliminate artifacts and
provide optimum imaging.
The reduction in diaphragm size reduces the
potential increase in numerical aperture (provided by oiling the condenser lens) so the loss in image quality un-
der these conditions is usually negligible.
Darkfield microscopy is a specialized lighting technique that uses oblique illumination to improve contrast in
samples that cannot be well observed under normal brightfield illumination conditions.
We are all quite familiar with the appearance and visibility of stars on a dark night, despite their huge distances
from the Earth. Stars can be seen because of the sharp contrast between their weak light and the black sky.
Objective Front lens
Coverslide
Specimen
Condenser
front lens
Homogeneous Immersion System
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Immersion oil
Mounting
media
Immersion oil
Fig. 22