Types Of Light Microscopes
The ordinary microscope is called a bright-field microscope because
it forms a dark image against a brighter background. The
microscope consists of a sturdy metal body or stand composed of
a base and an arm to which the remaining parts are attached.
A light source, either a mirror or an electric illuminator,
is located in the base. Two focusing knobs, the fine and coarse adjustment
knobs, are located on the arm and can move either the
stage or the nose piece to focus the image.
The stage is positioned about halfway up the arm and holds
microscope slides by either simple slide clips or a mechanical
stage clip. A mechanical stage allows the operator to move a slide
around smoothly during viewing by use of stage control knobs.
The sub stage condenser is mounted within or beneath the stage
and focuses a cone of light on the slide. Its position often is fixed
in simpler microscopes but can be adjusted vertically in more advanced
The curved upper part of the arm holds the body assembly, to
which a nose piece and one or more eyepieces or oculars are attached.
More advanced microscopes have eyepieces for both eyes
and are called binocular microscopes. The body assembly itself
contains a series of mirrors and prisms so that the barrel holding
the eyepiece may be tilted for ease in viewing . The
hosepipe holds three to five objectives with lenses of differing
magnifying power and can be rotated to position any objective
beneath the body assembly. Ideally a microscope should be parfocal—that is, the image should remain in focus when objectives
The path of light through a bright-field microscope is
shown in figure 2.4. The objective lens forms an enlarged real
image within the microscope, and the eyepiece lens further
magnifies this primary image. When one looks into a microscope,
the enlarged specimen image, called the virtual image,
appears to lie just beyond the stage about 25 cm away. The total
magnification is calculated by multiplying the objective and
eyepiece magnifications together. For example, if a 45 objective
is used with a 10 eyepiece, the overall magnification of
the specimen will be 450.
Fig: Bright Field Microscope