Light from the light source passes into the condenser lens, which is mounted beneath
the microscope stage in an upright microscope (and above the stage in an inverted
microscope) in a bracket that can be raised and lowered for focussing (Fig. 4.3). The
condenser focusses light from the light source and illuminates the specimen with
parallel beams of light. A correctly positioned condenser lens produces illumination
that is uniformly bright and free from glare across the viewing area of the specimen
(Koehler illumination). Condenser misalignment and an improperly adjusted con-
denser aperture diaphragm are major sources of poor images in the light microscope.
Thespecimen stageis a mechanical device that is finely engineered to hold the
specimen firmly in place (Fig. 4.4). Any movement or vibration will be detrimental
to the final image. The stage enables the specimen to be moved and positioned in
fine and smooth increments, both horizontally and transversely, in theXand the
Ydirections, for locating a region of interest. The stage is moved vertically in the
Zdirection for focussing the specimen or for inverted microscopes, the objectives
themselves are moved and the stage remains fixed. There are usually coarse and fine
focussing controls for low magnification and high magnification viewing respectively.
The fine focus control can be moved in increments of 1mm or better in the best research
microscopes. The specimen stage can either be moved by hand or by a stepper motor
attached to the fine focus control of the microscope, and controlled by a computer.
The objective lens is responsible for producing the magnified image, and can be the
most expensive component of the light microscope (Fig. 4.4). Objectives are available(a) (b)Fig. 4.3Two basic types of compound light microscope. An upright light microscope (a) and an inverted light
microscope (b). Note how there is more room available on the stage of the inverted microscope (b). This
instrument is set up for microinjection with a needle holder to the left of the stage.104 Microscopy