The Compound Microscope Purpose: To learn the structure and basic use of the compound microscope. Materials: Compound microscope Slides Cover slips Newsprint Pipette Scissors Clear plastic ruler Introduction: The microscope is the biologist’s basic tool. It has been developed to help explore the world of living things too small to be seen with the naked eye. Early microscopes, like the one Antony van Leeuwenhoek made, had only one les and were difficult to use. The biggest problem was magnification. The more powerful the lens-for greater magnification-the closer the viewer’s eye had to be to the lens. At very high magnification, the lens almost touched the eye. The early microscope user had to be very steady indeed. A major advance in microscopes came with the invention of the compound microscope. It has two sets of lenses, which magnify objects much more than a single lens. Procedure: Part 1: Structure of the Microscope The compound microscope has four basic parts: the lens system, the focusing system, the stage, and the lighting system. The Lens System: One of the two sets of lenses are the objective lenses. They work much as did the lens of the early, simple microscope. The objective lenses make the initial or primary magnification. They are located in the nosepiece of the microscope. Inscribed on each objective is the magnification or power of that lens. This tells the number of times the lens magnifies the image. Microscopes usually have 2 – 4 objective lenses. Draw a diagram of the microscope and label the nosepiece and label the power of each objective lens. Does the higher power have a longer lens? Why? The second kind of lens in the microscope is the ocular, sometimes called the eyepiece lens. This lens is located at the top of the body tube. The ocular serves as a small telescope, magnifying the image made by the objective lens. This enlargement is called the secondary magnification.. Label the ocular lens on your diagram and record the power of the ocular lens as well. The total magnification of the microscope is determined by multiplying the primary magnification (objective lens) by the secondary magnification (eyepiece lens). Record the total magnification for each of the lenses in your diagram. The Stage A specimen to be viewed throught the microscope is mounted on a glass slide and covered with a coverslip. The slide rests on the stage, the flat surface beneath the body tube. Label the stage and body tube on your diagram. Stage clips hold the slide in place (if your microscope has them). Label the stage clips as well. The stage clips also help in making slight adjustments in the slide’s position by holding the slide steady. Do not allow water to get on the stage. The Lighting System For you to see the specimen, light must pass through it and lenses to your eye. The lighting system is located under the stage of the microscope. The lighting system can be a concave mirror which faces a light source or a light under the stage that turns on when the microscope is plugged in. Under the stage you will also find the diaphragm. It is used to adjust the amount of light that passes through the specimen. The diaphragm works like the aperture on a camera. Practice opening and closing the diagram while looking through the eyepiece. Notice how the amount of light increases and descreases. Label the light source and diaphragm in your diagram. Part 2: Using the Microscope Put the low power objective in place. Look through the eyepiece and adjust the light so that you see a uniformly bright field of view. Prepare your slide to view under the microscope. Cut a lowercase “e” from a newspaper and place it in the center of a clean slide. Put a drop of water on the top of the letter with a pipette. Next, place the edge of a coverslip against the water, and with a pencil gently lower the coverslip over the “e”. Place the coverslip in this manner prevents bubbles from forming. Be sure that the bottom of the slide is dry. This type of slide is called a wet mount. Place the slide under the stage clips, so that the “e” is right side up. You are now ready fo focus on the “e”. Focus always begins with the lowest plower objective. First, click the low power objective into position in the nosepiece. Then, looking at the side of the microscope, turn the course adjustment knob until the objective is as close as possible to the slide WITHOUT touching it. Now look through the eyepiece and turn the course adjustment know in the direction that will move the objective lens away from the stage. The “e” will come into approximate focus. To sharpen the focus, turn the fine adjustment knob back and forth. Draw what you see looking into the microscope. Describe the features of the newspaper letter “e”. While looking through the microscope, move the slide to the right. Which way does the letter “e” move? Push the slide away from you on the stage. Which direction does the letter appear to move when viewed through the microscope? Now, look at the “e” under high power. First, under low power, center the “e” in the field of view. Switch to high power by turning the nosepiece until the high power objective clicks into place. Sharpen the focus by turning the fine adjustment knob. If you cannot find the “e” under high power, try this. Look through the eyepiece and move the slide slightly, If this does not bring the “e” into view, move the slide in the other direction. Remove the slide from the stage when you are finished and clean the slide and coverslip with a paper towel. Part 3: Measuring Microscopic Objects Return the objective lens to the lowest power. Place the metric ruler on the stage. The edge marked with the division lines should be over the stage opening. This distance from the center of one line to the center of the next line is 1 millimeter (1000 microns). Look into the eyepiece. Move the ruler under the center of one of the division lines is at the left of the field. The diameter of your field of vision is the length from one side of the field to the other. This length is represented by the line AB in the illustration. The right edge of the field of vision may not be on the line. If it is not, estimate the distance from the last line to the right edge as a fraction of a millimeter. What is the diameter of your low power field of vision? With the ruler still in place, put the next highest objective in place. Look into the eyepiece. Note that the field of vision is now less than it was under the low power. To determine the diameter of the field of vision under the next highest objective lens you find the magnification factor between them. If the low power objective magnifies 4X and the next higher magnifies 40X, then 40X divided by 4X = 10. The higher objective magnifies 10 times more than the low objective. The diameter of the low power field of vision must be divided by 10 to get the field of vision for the higher power objective. Find the magnification factor for each of the higher objective lenses as well as the size of each field of vision. Record this in table form. Make a wet mount of your hair. Find the diameter of your hair in microns. Record your hair diameter.
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