25
8 Terminology
Aperture, Numerical (N.A.)
The numerical aperture is an important
factor determining the efficiency of the
condenser and objective. It is represented
by the formula:
(N.A. =
η
sinα
), where
η
is the refractive
index of a medium (air, water, immersion
oil etc.) between the objective and the
specimen or condenser, and
α
is half of
the maximum angle at which light enters
or leaves the lens from or to a focused
object point on the optical axis.
Anisotropic
Quality of a transparent material having
different refractive indices depending on
the vibration direction of the transmitted
light, hence any material that affects
polarized light differently according to its
direction through the material.
Axis - Optical
A straight line joining the centers of
curvature of lens surfaces.
Bertrand Lens
An intermediate lens, which transfers an
image of the back focal plane of the
objective into the primary image plane.
Mostly used for conscopic observation in
polarized light microscopy.
Calcite
A doubly refracting mineral used in the
manufacture of polarizing prisms.
Cover Glass Thickness
Transmitted light objectives are designed
to image specimens that are covered by a
thin cover glass (cover slip). The
thickness of these small glass pieces is
now standardized at 0.17 mm for most
applications.
Conoscopic Observation
The study of the back focal plane of
the objective is called conoscopic as
the observations are associated with
the cone of light furnished by the
condenser and viewed by the
objective. This study is accomplished
by inserting a Bertrand lens, by
examining the image with a Pinhole
stop or by using a phase telescope.
Diaphragm, Condenser
A diaphragm, which controls the effective
size of the condenser aperture. A
synonym for the condenser illuminating
aperture diaphragm.
Diopter Adjustment
The adjustment of the eyepiece of an
instrument to provide accommodation for
the eyesight differences of individual
observers.
Epi-Illumination
Illumination is accomplished by means of
a vertical illuminator placed above the
objective. The axis of illuminating ray
bundle falling on the object may or may
not coincide with the microscope
objective.
Focusing Eyepiece
An eyepiece with a device for focusing a
graticule mounted within it and coinciding
with the primary image plane.
Michel Levy Scale Of Retardation
Colours
Colour chart-plotting thickness of the
anisotropic specimen, its birefringence
(n1-n2) and its retardation in
nanometers. Any one of the three
variables can be determined if the other
two are known.
Micrometer:
um
A metric unit of length measurement=
1x10
-6
meters or 0.000001 meters
Nanometer:
nm
A metric unit of length measurement=
1x10
-9
meters or 0.000000001 meter.
Orthoscopic Observation
This is the normal way of viewing an
object through a microscope. With
Kohler illumination, the field
diaphragm and the ocular front focal
plane as well as the specimen will be in
simultaneous focus.
Stage Micrometer
The graduated scale used as a standard
on the stage of a light microscope for
calibrating an eyepiece micrometer.
