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A compound microscope is an indispensable instrument in any biological laboratory. It is used for passive observation of structural details of a cell, tissue or organ in sections.
A modern compound microscope has following structural components.
Non-Optical Components:
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1. Base (foot):
It is U or horseshoe-shaped metallic structure that supports the whole microscope.
2. Pillar:
It is a short upright part that connects to base as well as arm.
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3. Arm (Limb):
It is a curved metallic handle that connects with the arm by inclination joint. It supports stage and body tube.
4. Inclination Joint:
It is used for tilting the microscope if required for observation in sitting position.
5. Stage:
It is a metallic platform with a central hole fitted to the lower part of the arm. Microscopic slides held on the stage by either simple side clips or by a mechanical stage clip.
6. Body tube:
It is meant for holding ocular and objective lenses at its two ends. The end holding ocular lens is called head while the end containing 3-4 objective lens is called nose piece. The body tube has an internal pathway for the passage of light rays which form the enlarged image or microscopic objects.
7. Draw tube:
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It is a small tube that remains fixed at the upper end of the body tube. It holds eyepiece or ocular lens.
8. Rack and pinion:
The microscope has a rack and pinion attached either to body tube or the stage for bringing the object under focus.
9. Adjustment screws:
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There are two pairs of screws for moving the body tube in relation to stage, larger for coarse adjustment and smaller for fine adjustment. In fine adjustment the body tube or stages moves for extremely short distances. In coarse adjustment the body tube or stage can move up and distance. In coarse adjustment is meant for briefly objective lens at a proper distance from the object so as to form image of the same at the ocular end. Fine adjustment is required to obtain sharp image.
10. Automatic Stop:
It is a small screw fitted at lower end or rack and pinion. It is meant for stopping the downward sliding of the body tube so as to prevent the damage of objective lens and the slide.
Optical Components:
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11. Diaphragm:
It is flitted just below the stage for regulating the amount of light failing on the object. Diaphragm is of two types, disc and iris.
12. Condenser:
It is attached below the diaphragm. Condenser can be moved up and down to focus light on the object.
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13. Reflector (Mirror):
It is attached just above the base. Both its surface bear mirrors, plane on one side and concave on other side. Plane side is used in strong light and concave side in weak light. Reflector directs the light on the object through the condenser and diaphragm system.
14. Objective Lenses:
They are fitted over the nose piece. Objective lenses are of two 10 three types – low power (commonly 10X or 5X), high power (commonly 45X) and oil immersion (commonly 100X, can be more). An objective lens is not a simple lens but compound lens. It forms real inverted image of the object inside the body tube.
15. Ocular Lens or Eyepiece:
It is lens through which image of the microscopic object is observed. It also takes part in magnification. Depending upon magnification, the eye piece is of four types-5X, 10X, 15X, and 20 X. Advanced microscope has two eye pieces so that both the eyes can be used (Fig. 1.4). Microscope head having device for using two eye pieces is called binocular head. It contains a number of internal mirrors and prisms for the passage of light.
Working Principle of Compound Microscope:
The compound microscope is most commonly used in clinical and educational laboratories. It has a combination of lenses that enhances both magnifying power as well as the resolving power. The specimen or object, to be examined is usually mounted on a transparent glass slide and positioned on the specimen stage between the condenser lens and objective lens.
A beam of visible light from the base is focused by a condenser lens onto the specimen. The objective lens picks up the light transmitted by the specimen and create a magnified image of the specimen called primary image inside the body tube. This image is again magnified by the ocular lens or eye piece.
When higher magnification is required, the nose piece is rotated after low power focusing to bring the objective of higher power (generally 45X) in line with the illuminated part of the slide. The objective lens comes very near the cover slip but it does not touch the same. Only fine adjustment it moved for proper focusing. More light may be required. After observation under high power, the nose piece is rotated to bring back the slide under low power.
Occasionally very high magnification it required (e.g. for observing bacterial cell). In that case, oil immersion objective lens (usually 100X) is employed. After focusing under low power a drop of immersion oil (e.g. cedar oil, olive oil) placed over the illuminated part of the cover-slip.
The nose piece is rotated to bring the oil immersion lens in line with die specimen. It comes in contact with the oil. By using fine adjustment only, the specimen is brought under focus. Immersion oil increases the sharpness of the image. Soon after observation, both the lens and the slide are cleared of the oil by fine cotton cloth or lens paper.
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The common light microscope is also called bright field microscope because the image is produced amidst a brightly illuminated field. The image appears darker because the specimen or object is denser and somewhat opaque than the surroundings. Part of the light passing through or object is absorbed. Bright field microscope is used [or study or preserved and stained material as well as live and unstained object or material. However, differentiation is poor in case of live unstained specimen. Special microscopes for their study are dark field, phase contrast and differential interference contract microscopes.
