Page 114 - The Veterinary Laboratory and Field Manual 3rd Edition

The selection, use, maintenance and quality control of laboratory equipment and supplies 83

directs the available light onto the objective as it tance between the eyepieces is adjusted (as
is raised or lowered. The iris diaphragm located when using binoculars) until one image is seen.
in the condenser unit regulates the amount of The object is then brought into sharp focus with
light that strikes the object being viewed (much the right eye. The right eye is then closed and
like a shutter of a camera). The iris diaphragm is the left eyepiece dioptric adjustment is used to
adjusted by a movable lever. bring the object into sharp focus while viewing
Currently, long-lasting LED-light-bulbs are through the left eye.
used as a light source for microscopes. Quite The low power objective is used with the
often these LED-light-bulbs are not replaceable. coarse adjustment for initially locating speci-
mens or slides and for viewing objects.
The high power (40×) is used when greater
Coarse and fine adjustments
magnification is needed in procedures such as
The course adjustment should be used to focus cell counts and viewing urine sediments. After
the low power objective only. The fine adjust- focusing with the low power objective, the ×40
ment is required at higher power to provide a objective is rotated into position and then fine
sharper image after the object is brought into adjustment is used to bring the object into focus.
view with the coarse adjustments at low power The oil immersion objective (100×) is used to
magnification. The working distance is the dis- view stained blood smears, histological sections
tance between the objective and the slide when and stained preparations of microorganisms.
the object is in sharp focus. The higher the mag- After initial focusing at low power magnifica-
nification of the object, the shorter the working tion, a drop of immersion oil is placed on the
distance will be. slide. The oil immersion objective is then rotated
into the drop of oil, taking care not to allow any
of the other objectives to get into contact with
Stage
the oil. At the end of the day, all oil should be
The stage of the microscope is supported by the cleaned from the objective preferably using lens
arm and is located between the objectives/turret paper.
and the light source/condenser. The stage serves
as the support for the specimen being viewed Basic adjustments of the microscope
(usually a prepared microscope slide). Specimen
slides can be moved by using knobs located just The condenser and diaphragm must be adjusted
below the stage. These move the specimen slide according to the objective being used and the
left and right or backward and forward. type of specimen being observed. When view-
When the microscope is not being used, ing objects with the oil immersion lens (stained
the stage should be centred so that it does not preparations), the condenser should be raised
project from either side of the microscope. The until it is almost touching the slide. The dia-
microscope should be covered with a dust cover phragm should be completely open to give
when not in use (preferably a cotton cover, maximum light. When looking at objects with
plastic can cause condensation). low power (when viewing unstained fluids, such
as urine sediments or cell dilutions for counting)
the condenser may need to be lowered to reduce
Using the light microscope
the brightness of the light and increase the con-
The eyepieces on binocular microscopes must trast between the constituents being viewed and
be adjusted for each individual’s eyes. The dis- the background.

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