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 specimens image into approximate focus. Once
this is accomplished, the fine control knob sharpens
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.
FOCUSING THE MICROSCOPE
The process of focusing consists of adjusting the
relationship between the optical system of the
microscope and the object to be examined so that a
clear image of the object is obtained. The distance
between the upper surface of the glass slide on the
microscope stage and the faces of the objective lens
varies depending upon which of the three objectives is
in the focusing position. It is a good practice to obtain a
focus with the low-power objective first, then change
to the higher objective required to avoid accidentally
damaging the objective lens, the specimen, or both.
Most modern microscopes are equipped with parfocal
objectives (meaning that if one objective is in focus,
the others will be in approximate focus when the
nosepiece is revolved). With the low-power objective
in focusing position, observe the following steps in
1. Seat yourself behind the microscope, then lower
your head to one side of the microscope until
your eyes are approximately at the level of the
2. Using the coarse adjustment knob, lower the
body tube until the face of the objective is within
1/4 inch of the object. Most microscopes are
constructed in such a way that the low-power
(green) objective cannot be lowered and make
contact with the object on the stage.
3. While you are looking through the ocular, you
should use the coarse adjustment knob to
elevate the body tube until the image becomes
visible. Then use the fine adjustment knob to
obtain a clear and distinct image. Do not move
the focusing knob while changing lenses.
4. If the high-power objective (yellow) is to be
used next, bring it into position by revolving the
nosepiece (a distinct click indicates it is in
proper alignment with the body tube). Use the
fine adjustment knob only to bring the object
into exact focus.
If specimen is too dark, you can increase
lighting by opening the iris diaphragm of the
The oil-immersion objective (red) is used for
detailed study of stained blood and bacterial
smears. Remember that the distance between
objective lens and object is very short, and great
care must be employed so the specimen is not
damaged. After focusing with the high-power
objective and scanning for well-defined cells,
raise the objective, place a small drop of
immersion oil, free of bubbles, on the slide,
centering the drop in the circle of light coming
through the condenser.
Next, revolve the
nosepiece to bring the oil-immersion objective
into place, and, by means of the coarse
adjustment knob, slowly lower the body tube
until the lens just makes contact with the drop of
oil on the slide.
The instant of contact is
indicated by a flash of light illuminating the oil.
The final step in focusing is done with the fine
adjustment knob. It is with this lens in particular
that lighting is important. The final focus, clear
and well-defined, will be obtained only when
proper light adjustment is made.
CARE OF THE MICROSCOPE
The microscope is an expensive and delicate
instrument that should be given proper care.
Moving or transporting microscopes should be
accomplished by grasping the arm of the scope in one
hand and supporting the weight of the scope with the
other hand. Avoid sudden jolts and jars.
Keep the microscope clean at all times; when not in
use, microscopes should be enclosed in a dustproof
cover or stored in their case. Remove dust with a camel
hair brush. Lenses may be wiped carefully with lens