The focusing of the microscope and adjustment for each eye will correct for most conditions of near or farsightedness, eliminating the need to wear corrective eyeglasses during microscopework. Even moderate astigmatism will not hamper most microscope use; however, for moreserious astigmatism, for certain other conditions of the eyes, or if preferred, prescriptioneyeglasses should be worn. Correction for astigmatism in eyeglasses can easily be determinedby holding the eyeglasses at arm's length and rotating them while looking at an object throughone lens at a time. If the length or width of the object changes while doing this with either lens,then there is an astigmatism correction and wearing the glasses during microscope work may berecommended. Special high-point eyepieces are made with a longer exit pupil distance to easilyaccommodate the extra distance needed when eyeglasses are worn. These eyepieces areidentified in some way by the manufacturer.To take advantage of the optimal resolution and illumination of the microscope, a techniqueknown as Koehlor illumination is used. NOTE: Some settings may have been preset by themanufacturer and will not be adjustable.With a slide specimen on the stage, use a low power (2-10X) objective, and focus with thecoarse and fine adjustments. Low magnification provides a larger field of view for easiersearching of the specimen. It also provides a greater working distance than higher powerobjectives, offering more safety against focusing too low and breaking the slide.
The microscope may have an auxiliary swing-in (or swing-out) lens in the condenser. Follow
manufacturer's recommendations for correct use of the swing-in/swing-out lens with various
objectives. Use the coarse and fine adjustments to focus the specimen. To obtain Koehlor
illumination, close the lamp (field) iris diaphragm, if present, at the base of the microscope, and
bring into focus by vertical adjustment of the condenser. Use the centering screws or knobs on
the condenser, if present, to center the focused circle in the field. If not on the condenser,
centering screws or knobs for this purpose may be on the base near the lamp diaphragm. Open
the lamp diaphragm until it is just past the field of view. This may not be possible with low
power objectives of some microscopes until the auxiliary lens in the condenser is correctly
adjusted.
If the microscope has no lamp diaphragm, place a piece of paper with a small hole cut in it over
the lamp opening and make the same adjustments to focus on its inner edge. Microscopes with
mirror and external light source are not discussed here, but the principles are similar. Follow the
manufacturer's instructions for adjusting the lamp filament if possible.
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