are also commonly used. The microscope most often used in the laboratory is a compound microscope that consists of the vorious pieces identified and discussed briefly below:
Basestructure on which the microscope rests.
Armstructure that supports the magnification and adjustment system; it is the handle by which the microscope is carried.
Stageplatform on which a preparation is placed for examination. In the center of the stage is the aperture or hole to allow passage of light from the condenser. Mechanical stagemeans by which the preparation may be moved about on the stage.
2. Illumination System:
Mirrorusually double, a flat surface on one side, and a concave surface on the other side. The concave surface is used in the absence of a condenser. Many microscopes have a built-in light source instead of a lamp and mirror.
Internal light sourcebuilt into the base of the microscope, and provides a more precise steady source of light into the microscope.
Condensercomposed of a compact lens system located between the mirror and stage. The condenser (usually an Abbe condenser) concentrates (condenses) the light through the aperture in the stage to the objective lens.
Iris diaphragmcontrols the amount of light reaching the condenser. The size of the iris diaphragm opening should approximate that of the face of the objective lens.
Thus, as a general rule, the diaphragm is completely closed when liquid preparations are observed with the low-power objective, and wide open when stained preparations are observed with the oil-immersion lens using natural light.
3. Magnification System:
Revolving nosepiececontains openings into which objective lenses may be fitted and that may be revolved to bring an objective into the desired position.
Objective lenses usually a set of three consisting of a low-power lens (approximate focus 16 mm, magnification 10X), a highpower lens (approximate focus 4 mm, magnification 45X), and an oil-immersion lens (approximate focus 1.8 mm, magnification 100X). Numerical aperture (NA) refers to the angle of the maximum cone of light that may enter the objective. The greater the numerical aperture, the greater the resolution, or ability of the microscope to separate small details clearly.
The body tubethrough which light passes from the objective to the ocular lens. The
ocular lenses (eyepieces)usually a 10X is provided: the number indicates the magnification (in diameters) produces by the ocular of the image formed by the objectives. Magnification is determined by the ratio between the size of the virtual image and the real size of the object. It is expressed in diameter multiples, for example 100X. By multiplying the magnification engraved on the objective by that engraved on the eyepiece, one can determine the total magnification. The total magnification resulting from the systems of lenses is determined by the combination of objectives and oculars:
4. Adjustment System (composed of two parts, both of which raise or lower the body tube together with the lens system):
Coarse adjustment the larger and innermost knob; by rotating the control knob, the image appears and is in approximate focus.
Fine adjustment the smaller and outermost knob; by rotating this control knob, it renders the image clear and well-defined.
The process of focusing consists of adjusting the relations between the optical system of the microscope and the object to be examined so that a clear image of the object is obtained. The distance between the upper surface of a glass slide on the microscope stage and the faces of the