Page 76 - Fundamentals of Light Microscopy and Electronic Imaging

THE CARE AND CLEANING OF OPTICS 59

protected from abrasion. Strands of wood fibers in coarse paper, or worse, the stick end of
a cotton swab applicator, are strong enough to place dozens of permanent scratch marks
(sleeks) on the front lens element with a single swipe. Once present, scratches cannot be
removed, and their effect (even if hardly visible) is to scatter light and permanently reduce
image contrast. Further, most lenses contain an antireflection coating composed of layers
of a dielectric material; each layer is just a few atoms thick. Although antireflection sur-
faces are protected with a layer of silicon monoxide, you should use only high-quality lens
tissue and apply only a minimum of force to wipe off drops of excess oil.

Mechanical Force

Never apply strong physical force to an objective lens or other optical component. To
bring another objective into position, move the rotating turret; do not grab and pull on a
lens to bring it into position. Also, never remove a stuck objective with a vice-grips or a
pipe wrench! If the threads of an objective become stuck to the rotating turret from dried
culture medium, oil, or corrosion, apply a drop of water or lens cleaner or penetrating oil
to loosen the objective and avoid using force. Likewise, never drop an objective onto the
lab bench or floor. Also, do not allow an objective to strike exposed edges of the micro-
scope stage or the condenser (easy to do on some inverted microscope designs). Impacts
of this kind cause two forms of irreparable damage: (1) They can crack the compounds
that seal the top lens element and metal lens cap, thus allowing immersion oil to penetrate
into the lens and coat internal lens elements, causing permanent damage. (2) They can
induce permanent stresses in the glass lens components of an objective and may severely
degrade its performance in sensitive forms of light microscopy that use polarized light.

Exercise: Constructing and Testing an Optical Bench
Microscope

Microscope Construction and Testing
• Determine the focal lengths of the lenses using the method described in the
text and label them with pieces of lab tape applied to their edges. Use three
50 mm lenses for the objective, ocular, and illuminator’s collector lens to
construct the optical bench microscope.
• Mount in order of sequence on the optical bench: a tungsten lamp illumina-
tor, 50 mm collector lens, specimen holder, objective lens, and ocular.
• Handwrite the letter a with a fine marker pen on a sheet of lens tissue or on
a microscope slide, and tape it to the specimen holder, centering the letter on
the optic axis.
• Position the collector lens about 75 mm away from the lamp filament. Posi-
tion the specimen about 20 cm away from the collector lens. The light from
the illuminator should be focused into a 1–2 cm diameter spot centered on
the object (the letter a).
• Using the lens equation 1/f 1/a 1/b, calculate the object-lens distance
that gives an image-lens distance of 30 cm, and mount the objective lens at
the calculated position.