Optika Italy B-510 Serie Bedienungsanleitung Seite 15

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 under-
side of the microscope slide containing the
specimen should be also be immersed in oil.
An ideal system is schematically diagramed
in Fig. 21, where immersion oil has been pla-
ced 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 Homoge-
neous Immersion System and it is the ide-
al situation to achieve maximum numerical
aperture and resolution in an optical micro-
scope.
In this case, the refractive index and disper-
sion of the objective front lens, immersion
oil, substage 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 com-
pletely through the microscope slide, immer-
sion oil, and mounting medium undeviated by refraction at oil-glass or mounting medium-glass interfaces.
When using high-power achromat oil immersion 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 under 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
Cover Slip
Specimen
Condenser
Front Lens
Homegeneous Immersion
Page 15
Immersion Oil
Mounting
Medium
Immersion Oil
system
Fig. 21