ILLUMINATION SYSTEM
Ideal illumination of a specimen viewed under the
microscope requires even light distribution. The
objectives must also be entirely filled with
light from the condenser. 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.
Condenser
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.
Iris Diaphragm
An iris diaphragm located on the condenser
controls the diameter of the light source's 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.
MAGNIFICATION SYSTEM
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.
Objective Lenses
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 focus 4 mm,
magnification 45X), and an
oil-immersion lens (approximate focus 1.8 mm,
magnification 100X). Objective lenses are color
coded for easy recognition: 16 mm-10X
(green), 4 mm-45X (yellow), and 1.8
mm-100X (red).
Revolving Nosepiece
The revolving nosepiece contains openings into
which objective lenses are fitted, and revolves
objectives into desired position.
Body Tube
The body tube is a tube that permits light to travel
from the objective to the ocular lens.
Ocular Lenses
Ocular lenses, or eyepieces, are located on top of
the body tube and usually have a
magnification power of 10X. 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 table
7-1.
FOCUSING SYSTEM
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
being used. For example, the high-powered objective
of short focal length (4 mm) will need the stage raised
so the objective is very close to the
specimen, while the low-powered objective
of a longer focal length (16 mm) will
need the stage lowered so the objective is farther
from the specimen.
The coarse control knob is used initially to bring
the specimen's image into approximate focus. Once
this is accomplished, the fine control knob
sharpens the image.
Coarse Control Knob
The coarse control knob is the larger and inner
knob. Rotating the coarse control knob allows the
image to appear in approximate focus.
Fine Control Knob
The fine control knob is the smaller and outer
knob. Rotating this control knob renders the image
clear and well-defined.
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