Compound microscope

 A compound light microscope is made up of two sets of lenses of which one is known as the objective and the other as eye piece. It is one of the most commonly used and suitable one in the biology laboratory. The different parts of a compound microscope are separated into two groups on the basis of their function-Mechanical parts and Optical parts.

1. Mechanical parts:

It includes (a) Foot, (b) Pillar, (c) Arm, (d) Stage, (e) Body tube, (f) Draw tube, (g) Coarse and fine adjustment screws and (h) Nose piece. A brief description of the different mechanical parts is given below-

(a) Base or foot: A basal V or horse shoe-shaped part on which the whole body of microscope stands.

(b) Pillar: It is a short vertical limb- like structure stands at right angle to the foot.

(c) Arm or handle: It is a sickle- shaped portion of the microscope. The lower portion of the arm stands on the pillar and upper portion connect the body tube. The point where the pillar and arm meet is called inclination joint. This is provided with a sliding screw on two sides of it, with the help of which the arm can be adjusted backward and forward as desired.

(d) Stage: A round or rectangular or square stage is attached to the top of the pillar and in front of the handle. There is a small hole at its centre. The stage is provided either with movable- sliding system or only spring clips on two sides of the hole. The former helps to hold the slide and permits forward, backward and sideways movement with the help of screw provided with the stage. The latter holds the slide firmly in one position.

(e) Sub-stage: A sub-stage is attached directly below the stage. It may be movable or fixed. It consists of mainly two parts- iris diaphragm and condenser lens. Iris diaphragm is a wheel- shaped metal disc which regulates the entry of light by changing its aperture. Condenser is a system of two or more lenses which receives parallel light rays from the mirror and converge them. Both the iris diaphragm and condenser lens are placed within the bangle-like metallic ring of the sub-stage.

(f) Body tube: It is composed of a tube and attached to the upper end of the arm by means of a short connective called the bridge. The body tube carries the draw tube and the ocular at its upper end. The lower end of the tube carries a revolving nose piece with about three objectives viz, low power, high power and oil immersion libroрмоэ и да подаризн

(g) Draw tube: This is a graduated small tube placed on the upper side of the body tube and partly remains inside the body tube.

(h) Coarse and fine adjustment screws: There are the two pairs of screws placed on the lateral sides of the bridge connecting the body tube and handle. The coarse pair is placed above the fine pair. With the help of coarse adjustment screw, the body tube can be moved up and down rapidly to bring the object into focus under low power. Fine adjustment screws are responsible for slow vertical movement of the body tube and is used when the object is under high power. (i) Nose piece: It is a circular disc-shaped structure, generally having three objective lenses screwed at different positions.

The draw tube, body tube and nose piece together constitute the mechanical tube. The length of the mechanical tube of all types of compound microscope is 160 mm. Mechanical tube, objective and eye piece constitute the optical tube through which light is traversed. The length of the optical tube is 180 mm.

2. Optical parts:

Mirror, condenser, objective and eye piece constitute the optical part. A brief description of the different parts is given below-

(a) Mirror: It is a plano-concave mirror, with one side plain and other concave. The plain side is used to reflect sunlight rays while concave is used for the rays from a lamp. A mirror focuses the light on the object through the condenser.

(b) Condenser: It is made of several plano-convex lenses and converges the beam of light focused by the mirror or artificial light. Below the condenser mechanical diaphragm is present only to control the entry of light.

(c) Objectives: It is the most important lens system of the microscope controlling both the available magnification and quality of the image. These lenses are attached to the nose piece. Usually there are two objectives, which provide magnification power of 10X and 45X. An oil

immersion objective (100X) is generally present in research type of microscope in addition to the above.

(d) Eye piece or ocular: Eye piece is a small tube provided with the plano-convex lenses at its two ends. It is attached to the top of the body tube. Eye pieces normally range in magnification from SX to 15X.

Magnifying power of a compound microscope:

Magnifying power of objective lens

Magnification of an object is equal to the magnifying power of an eye piece lens multiplied by the magnifying power of an objective lens. For example, if you are viewing an object through an eye piece lens of 10X and an objective lens of 10X, the object that you see under the microscope is magnified by 10x10 that is equal to 100 times.

Procedure for microscope operation:

Setting up of the microscope-

(a) When taking the microscope from its case, carry it with both hands. Hold the arm with one hand and place the other below the base to give support. Set the instrument down gently on the table, keeping the arm towards you, the stage away from you and the base several inches from the edge of the table.

(b) Rotate the nose piece until the low power objective (the shorter one) is in line with the body tube and clicks into position.

(c) If your microscope has a mirror instead of a sub-stage illuminator, you must look through the ocular and move the mirror around until it reflects light upward through the opening in the stage. Use the flat side of the mirror, not the concave side. Do not let sun-light strike the mirror directly.

(d) Open the iris diaphragm if the condenser is present. Look through the eye piece, adjust the mirror and diaphragm to get a complete and evenly brightly illuminated round field of vision.

(e) If the ocular or objective is cloudy or dusty, clean the lenses gently with a small piece of lens paper. Do not use any other kind of paper or cloth.

How to focus the microscope-

(a) A prepared slide is placed on the centre of the stage and fix it with clips.

(b) Now the low power objective is lowered down over the slide by rotating the coarse adjustment screw. While doing this, objective should be viewed from the side, so that the contact between the lens and the slide is avoided.

(c) Now while looking through the ocular, use the coarse adjustment to move the body tube upward or downward until the image becomes visible and sharp.

(d) When higher magnification is desired, first focus the object under the low power of the microscope and then rotate the nose piece until the high power objective (the longer one) is in lines with the body tube and clicks into position. Now with the help of fine adjustment move the body tube upward or downward to bring the image into the sharpest focus. While doing this, always look carefully so that the contact between the lens and the slide is avoided.