Procedure

1. Observe a compound microscope, and study figure 3.1 to learn the names of its major parts. Note that the lens system of a compound microscope includes three parts—the condenser, objective lens, and eyepiece.

Light enters this system from a substage illuminator (lamp) or mirror and usually is concentrated and focused by a condenser onto a microscope slide or specimen placed on the stage. The condenser, which contains a set of lenses, usually is kept in its highest position possible.

The iris diaphragm, which is located between the light source and the condenser, can be used to increase or decrease the intensity of the light entering the condenser. Locate the lever that operates the iris diaphragm beneath the stage, and move it back and forth. Note how this movement causes the size of the opening in the diaphragm to change. (Some microscopes have a revolving plate called a disc diaphragm beneath the stage instead of an iris diaphragm. Disc diaphragms have different-sized holes to admit varying amounts of light.) Which way do you move the diaphragm to increase the light intensity?_Which way to decrease it?_

After light passes through a specimen mounted on a microscope slide, it enters an objective lens system. This lens projects the light upward into the body tube, where it produces a magnified image of the object being viewed.

The eyepiece (ocular) lens system then magnifies this image to produce another image which is seen by the eye. Typically, the eyepiece lens magnifies the image ten times (10x). Look for the number in the metal of the eyepiece that indicates its power (fig. 3.2). What is the eyepiece power of your microscope?_

Figure 3.1 Major parts of a compound microscope with a monocular body and a mechanical stage. Some compound microscopes are equipped with a binocular body.

Eyepiece (ocular)

Body tube

Eyepiece (ocular)

Body tube

Mechanical stage control knobs

Substage Illuminator Lamp

Fine adjustment knob

Mechanical stage control knobs

Coarse adjustment knob

Fine adjustment knob

Slide holder finger of mechanical stage

Stage

Condenser

Iris diaphragm lever

Substage illuminator (lamp)

Illuminator switch

Figure 3.2 The powers of this 10x eyepiece (a) and this 40x objective (b) are marked in the metal. DIN is an international optical standard on quality optics. The 0.65 on the 40x objective is the numerical aperture, which is a measure of the light-gathering capabilities.

Figure 3.2 The powers of this 10x eyepiece (a) and this 40x objective (b) are marked in the metal. DIN is an international optical standard on quality optics. The 0.65 on the 40x objective is the numerical aperture, which is a measure of the light-gathering capabilities.

Numerical Aperture 40x

The objective lenses are mounted in a revolving nosepiece so that different magnifications can be achieved by rotating any one of several objective lenses into position above the specimen. Commonly, this set of lenses includes a scanning objective (4x), a low-power objective (10x), and a high-power objective, also called a high-dry-power objective (about 40x). Sometimes an oil immersion objective (about 100x) is present. Look for the number marked in the metal of each objective that indicates its power. What are the objective lens powers of your microscope?__

To calculate the total magnification achieved when using a particular objective, multiply the power of the eyepiece by the power of the objective used. Thus, the 10x eyepiece and the 40x objective produce a total magnification of 10 x 40, or 400x.

2. Complete Part A of Laboratory Report 3.

3. Familiarize yourself with the following list of rules for care of the microscope:

a. Handle the microscope with great care. It is an expensive and delicate instrument. To move it or carry it, hold it by its arm with one hand and support its base with the other hand.

b. Keep the microscope under its dustcover and in a cabinet when it is not being used.

c. To clean the lenses, rub them gently with lens paper or a high-quality cotton swab. If the lenses need additional cleaning, follow the directions in the lens cleaning technique section that follows.

d. If the microscope has a substage lamp, be sure the electric cord does not hang off the laboratory table where someone might trip over it. The bulb life can be extended if the lamp is cool before the microscope is moved.

e. Never remove parts of the microscope or try to disassemble the eyepiece or objective lenses.

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Responses

  • jacob
    Which way do you move the diaphragm to increase the light intensity?
    2 years ago

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