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8.8 Aperture diaphragm
The Numerical Aperture (N.A.) value of the aperture
diaphragm affects the image contrast. Increasing or
reducing this value one can vary resolution, contrast
and depth of focus of the image.
Move the diaphragm ring
①
(Fig. 20) on the value
corresponding to the objective in use. In this case the
optimal setting of the condenser is achieved.
I
t is possible, however, move the ring to lower or hi
-
gher values to adapt the observation to personal pre
-
ferences.
• With low contrast specimens set the numerical
aperture to about 70%-80% of the objective’s
N.A. If necessary, remove on eyepiece and, lo
-
oking into empty sleeve, adjust the condenser’s
diaphragm in order to obtain an image like the
one in Fig. 21.
8.9 Use of the polarizer (optional)
1.
Remove the specimen from the stage.
2.
Looking inside the eyepieces, rotate the polarizer
until the darkest position is achieved.
3.
Once the dark is achieved (“extinction” or “Cros
-
sed Nicol” position) it is possible to begin the
observation.
Fig. 21
IRIS DIAPHRAGM
FIELD OF VIEW
30-20%
70-80%
Fig. 20
①
