The mechanical (movable) stage holds the
specimen in place and is the means by which the
specimen may be moved about on the stage.
The base is the structure on which the microscope
Ideal illumination of a specimen viewed under the
microscope requires even light distribution.
objectives must also be entirely filled with light from
To fulfill these requirements, the
illumination system of the compound microscope
consists of three parts:
an internal light source, a
condenser, and an iris diaphragm. See figure 7-5.
Internal Light Source
The internal light source is built into the base of the
microscope. It provides a precise and steady source of
light into the microscope.
The condenser is composed of a compact lens
system and is located between the light source and
stage. The condenser concentrates and focuses light
from the light source directly through the specimen.
An iris diaphragm located on the condenser
controls the diameter of the light sources beam. To
improve resolution, the operator should adjust the
opening of the iris diaphragm to approximately the
same size as the face of the objective lens. In addition
to the diaphragm on the condenser, an iris diaphragm
may be located on the internal light source. This iris
diaphragm controls the amount of light sent to the
condenser from the internal light source.
The magnification system of the compound
microscope contains at least two lens systems. The
two lens systems are mounted on either end of a tube
called the body tube. The lens nearest the object is
called the objective lens, and the lens nearest the eye is
the ocular lens or eye piece. See figure 7-5.
On a compound microscope, there is usually a set
of three objective lenses (or objectives). This set of
objectives is the component most responsible for the
magnification and resolution of detail in a specimen.
Each objective lens has a different focus distance and
magnification power. A set of objectives normally
consists of a low-power lens (approximate focus 16 mm,
magnification 10X), a high-power lens (approximate
f o c u s 4 m m , m a g n i f i c a t i o n 4 5 X ) , a n d a n
oil-immersion lens (approximate focus 1.8 mm,
Objective lenses are color
coded for easy recognition:
16 mm-10X (green),
4 mm-45X (yellow), and 1.8 mm-100X (red).
The revolving nosepiece contains openings into
which objective lenses are fitted, and revolves
objectives into desired position.
The body tube is a tube that permits light to travel
from the objective to the ocular lens.
Ocular lenses, or eyepieces, are located on top of
the body tube and usually have a magnification power
To calculate the total magnification of a
specimen, you multiply the magnification power of the
objective by the magnification power of the ocular
lens. Examples of total magnifications are provided in
Focusing is accomplished by moving the stage up
or down with the coarse and fine control knob
(fig. 7-5). Whether the stage needs to be raised or
lowered depends on the focal length of the objective
Table 7-1.Examples of Total Magnifications