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| Illumination in the transmitted light |
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A further important reason for the existence of diaphragms and filters in the microscope is that, strictly speaking, the illumination should be reset after every change of objectives for two reasons. Firstly, the size of the specimen section observed changes with every objective change. An objective with a low magnification, e.g. 4x, provides a large field of observation (with a diameter of as high as 5 mm in this case, provided that the eyepiece permits an intermediate image of dia. 20 mm).
If a switch is made to the 40x objective, the diameter of the visible field of the specimen shrinks by the factor 10 (to only 0.5 mm). The viewed area becomes as much as 100x smaller. The second reason is that the numerical aperture increases from 0.12 to 0.65 or, expressed as aperture angles, from 15° to 80°. These two cases see it in the left illustration.
However, the Köhler guidelines require that nothing more than the viewed field in the specimen is illuminated, since the “excessive” light outside the field of view is disturbing, scattered light. At the same time, however, the light cone of the illumination should always be matched to the angular aperture of the objective to allow the numerical aperture of the optics to be utilized. This is the only way to achieve maximum resolving power.
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