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Micro = Small, Scope = to view.

It magnifies the image of the object to be visualized

through it. Normally, the laboratory microscopes provide

18 Concise Book of Medical Laboratory Technology: Methods and Interpretations a magnification of 40x (scanner), 100x (low power),

400x (high power) and 1000x (oil immersion). The total

magnification is obtained by multiplying the magnification

of the objective with that of the eyepiece.

Parts of the Microscope

It has three sets of parts. They are the:

1. Stand,

2. Mechanical adjustments, and

3. Optics or the lenses.

Stand

It consists of:

1. The tube—supports objectives and eyepiece.

2. The body—gives support to the tube.

3. The arm—gives correct height and angulation to the

body and the tube.

4. The stage with a pair of spring clips or a mechanical

stage.

5. The substage holds the condenser lens with its iris

diaphragm and a holder for light filters and stops.

6. The foot on which other parts rest, can be in tripod or

horseshoe shape.

Mechanical Adjustments

Focusing Adjustments

These are coarse and fine adjustments.

Coarse Adjustment

Controlled by a pair of large knobs, one on each side of

the body. On rotating this, the tube moves with its lenses.

Some microscopes have this attached to the stage; so that

instead of the tube, the stage moves up and down. Coarse

adjustment is enough for low power lenses.

Fine Adjustment

Necessary for high power and oil immersion lenses. This is

usually controlled by two smaller knobs on each side of the

body. They may be graduated to indicate the movement in

microns.

Draw Tube

It is used to adjust the distance between the objective lens

and the eyepiece lens.

Inclination

The arm can be tilted upon the foot by a hinge.

Condenser Adjustments

Focusing of condenser is done by rotating a knob present

on one side below the stage.

Aperture Adjustment

It is done by the iris diaphragm (made up of leaves).

Centering of Condenser

It is done to bring the light beam accurately through the

instrument. In some microscopes, it is permanently fixed.

Mechanical Stage

It has knobs for moving the slide across or along the stage.

Monocular, Binocular and Digital Microscopes

Monocular—has only one eyepiece (Fig. 1.7).

Binocular—has 2 eyepieces, the only advantage it offers is

that it causes less strain on the eyes (Fig. 1.8A). Nowadays

digital microscopes are available, here digital image is

projected onto a digital display device (Fig. 1.8B).

FIG. 1.8A: Binocular microscope with substage lamp

FIG. 1.7: Monocular microscope with substage lamp

Laboratory 19

3. Apochromatic (Apo)—are very highly corrected and

costly and are only of value in special work.

Spring-loaded Objectives

The high power objectives (40X and 100X) of most modern

microscopes are spring loaded, i.e. the front mount of the

objective will be pushed in rather than pushed through

a specimen, if such an objective is accidentally pressed

against a specimen when focusing (Fig. 1.9).

Working of Oil Immersion Objectives

A beam of light passing from air into glass is bent; and

while passing from glass to air, it is bent back again. The

bending effect and its limitations can be avoided by

replacing the air between the specimen and lens with an

oil which has optical properties similar to that of glass,

i.e. immersion oil. When an appropriate oil is used, the

light passes in a straight line from glass through the oil

and back to glass as though it were passing through

glass all the way. Whenever possible, the immersion

oil recommended by the manufacturer of a microscope

should be used (Fig. 1.10).

Eyepiece

The most commonly used eyepiece is known as Huygens

eyepiece which has 2 lenses mounted at a correct distance

apart, with a circular diaphragm between, which give a

sharp edge to the image. These are available in different

magnifications. Lesser the magnification, brighter and

sharper is the image. For routine work, a 10X Huygens is

good enough. The 15X eyepieces are also available, as are

wide field ones.