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N.
OPERATIONS IN DETAIL
10. Use of Field Diaphragm
(Field diaphragm control ring)
The field diaphragm is used for determining the illuminated area on the
specimen. The size of the diaphragm is adjusted by manipulating the
field diaphragm control ring. In general stop down the diaphragm to
such an extent that the circumference of the illuminated area
circumscribes or inscribes that of the eyepiece field of view. If a wider
area than required IS illuminated, extraneous light will enter the field of
View, causing flare In the Image and lowering the contrast. Therefore.
especially
In
photomicrography. the proper adjustment of the field
diaphragm is very Important. Generally, good results will be achieved
when the diaphragm IS stopped down to such an extent that the
diameter of the illuminated area is slightly larger than the diagonal of the
film format.
Note
eThickness of the glass slide must be 1 . 7mm or less,
otherwise. the field diaphragm image may not be focused
on the specimen surface.
eThis diaphragm does not work as the field diaphragm when
the condenser top lens IS swung out of the optical path. In
this case. fully open the diaphragm as the numerical
aperture of the illuminator will be cut off when this
diaphragm is excessively stopped down.
11 . Optical Path Change-Over
(Optical path change-over knob of the trinocular eyepiece tube)
As shown in Fig. 17. when the change-over knob is pushed until It
reaches the limit
ell
100% of the light enters the observation tube.
When the change-over knob reaches the Intermediate click
1Zl.
the
proportion of light entering the observation tube and photo tube will be
14: 86. When the change-over knob is pulled to the
limit~.
100% of
the light enters the photo tube.
Fig.17
24
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