guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Microscope
1.7 Trouble-shooting
1.7.1 Optimising the image of the specimen
It is assumed that the microscope has been aligned for Köhler illumination (Ch. 1.5).
(1) If the thin section can be perfectly focused with low magnification objectives (M O = 2.5 to 10),
but remains out of focus when objectives of higher magnification (M O = 20, 40, 63 etc.) are used, it
should be checked whether the thin section is lying on its wrong side, i.e. with the cover glass
down (see free working distance of objectives, Table 1).
(2) If the correctly placed thin section cannot be focused with high-magnification objectives, the
cover glass is too thick and must be replaced by a cover glass of the standard thickness of 0.17
mm.
(3) Proper focusing may also be hampered by dusty surfaces of the optical components. The dust
is best removed with a soft and completely grease-free brush.
(4) Indistinct and blurred images result when the front lenses of objectives and oculars are
smudged with fingerprints resp. the grease of the eyelashes. In such cases it is advised to breathe
on the lenses and then carefully clean them with a lens tissue or a lint-free paper tissue. Oily stains
(e.g. by immersion liquids) are best removed with a cotton bud dipped in ether or ethyl alcohol.
Solvents, however, should be applied with caution as the lenses are mounted in synthetic resins.
To avoid fingerprints on the front lenses, objectives should always be changed with the fingers on
the revolving turret, not on the objectives.
1.7.2 Eliminating poor illumination
Raith, Raase & Reinhardt – February 2012
The microscope alignment for Köhler illumination ensures a perfect, evenly illuminated field of
view (Ch. 1.5). However, problems may still be encountered with the quality of illumination.
If the field of view is unevenly or not at all illuminated, although the light source is working and
the field diaphragm illuminated, potential causes of such problems in the substage assembly are:
(a) The front lens of the condenser or an extra lens below the condenser have not been properly
put into or out of the optical pathway. As a result, the lens mounts may partially or completely
block the light beam.
(b) Filters are not correctly positioned.
(c) When using low-magnification objectives for observation, the field of view will be poorly
illuminated if either the field diaphragm is closed down, the condenser is in a high position, or the
front lens of the condenser is in the light path.
The sub-stage illumination is perfectly adjusted if the thin section, when viewed sideways from
the top, shows an evenly and brightly illuminated round field which can be enlarged or reduced in
size by opening or closing the field diaphragm.
If the image of the thin section remains imperfect when viewed through the oculars, the causes for
poor illumination must be sought in the tube part of the microscope. The light path may be
partially or completely blocked if:
(a) The revolving nosepiece was not properly mounted on the microscope stand or an objective
has not clicked into place after a change of magnification.
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