divide the light equally, sending half to the left eye and half to the right eye. The coating
on the mirrors is enhanced by aluminum to increase the reflectivity. The binocular body
is protected by cover glass seals to keep dust from entering. The binocular compound
microscope is the preferred microscope for routine hematology.
(2) Phase. Phase microscopy is becoming increasingly prevalent in platelet
counting. In bright-field illumination, a completely transparent specimen is difficult to
see in any detail. By using phase contrast, transparent living objects can be studied.
Phase microscopy operates on the principle that if a portion of light is treated differently
from the rest, and caused to interfere with the rest, it produces a visible image of an
otherwise invisible transparent specimen. Phase contrast accessories are available
from the standard optical companies.
2-9.
CENTRIFUGES
These are laboratory devices or units that apply a relatively high centrifugal force
(up to 25,000 g) to a specimen, causing its separation into different fractions according
to their specific gravities. Centrifugation is the process of separating components of a
mixture (away from a center as in centrifugal force) on the basis of differences in
densities of the different components using a centrifuge.
a. Table Top Models. These units are mounted on rubber feet that absorb
vibration. The speed is controlled by means of a rheostat on the front panel. Top
speeds of centrifuges will vary and the top speed of a particular instrument should be
known in order to use the speed control device. Those centrifuges have adapters to
hold 6 tubes and adapters for 12 tubes.
b. Floor-Mounted Models. The heavier floor-mounted models accommodate a
large number of tubes at one time. The top speed of these instruments is higher than
that of table models. Because of their increased inertia, they are equipped with a brake
to facilitate stopping. In these units, the tubes are placed in balanced receptacles that
are mounted on spokes emanating from a central hub.
c. Microhematocrit Centrifuge. This centrifuge is a special type of high-speed
centrifuge employed to spin capillary tubes (see figure 2-6). The circular tube holder on
this centrifuge is flat, surrounded by a rubber ring. It has a capacity of 24 capillary
tubes. After a capillary tube is filled with blood, it is closed with a commercial plastic
sealing material. During centrifugation the sealed end is always placed in position
facing toward the outside of the holder plate. Most centrifuges of this type spin the tubes
at 10,000 rpm.
d. Precautions. In all instances where centrifugation is required, careful
attention must be given to balancing the units. This means that tubes must be placed
exactly opposite each other, they must be of identical weight, and they must contain the
same amount of fluid. If at all possible, centrifuges should be equipped with
tachometers so that speed nay be checked and controlled. Certain procedures, such as
hematocrits, require a critical relative centrifugal force (RCF or g). Relative centrifugal
MD0853
2-14
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