MODULE #1: Biology: The Study of Life

EXPERIMENT 1.2


Introduction to the Microscope

Supplies:

¨Microscope
¨Lens paper
¨Slides
¨Coverslips
¨Cotton swabs
¨Eyedropper
¨Water
¨Small pieces of bright thread
¨Methylene blue stain

Object: To learn the various parts of the microscope and to learn to use the microscope properly

Procedure:

A. Place the microscope on your table with the arm of the microscope nearest you. With the aid of the illustration, locate all the parts of the microscope and become familiar with them.

What a weird and oddly complicated instrument . . . kinda creeps you out, doesn't it? It doesn't? Must just be me . . .

The odd instrument
Click for an animation that goes through the parts of a microscope

  1. The eyepiece (called the ocular) is what you look through. It usually contains a 10x lens.
  2. The body tube starts at the eyepiece and runs to the part that holds the revolving nosepiece.
  3. The revolving nosepiece is the disc that holds the lenses (which are called objectives).
  4. The coarse focus is controlled by two large knobs on each side of the microscope. It allows for quick focus, but it does not make the image as sharp as it could be.
  5. The fine focus knobs are used to produce sharp focus. They are usually smaller and lower than the coarse focus knobs, but in some scopes they are mounted on top of the coarse focus knobs.
  6. The arm supports the body and stage and is attached to the base.
  7. The base is the heavy structure at the bottom that supports the microscope and makes it steady.
  8. The stage with clips is a platform just below the objectives and above the light source. The clips are used to hold the slide in place.
  9. The objectives are found on the revolving nosepiece. They are metal tubes that contain lenses of varying powers, usually 4x, 10x, and 40x. Some microscopes have a 100x objective as well.
  10. The diaphragm regulates the amount of light that passes through the specimen. It is located between the stage and the light source. It might be a disc that has several holes (a disc diaphragm), or it might be a single hole whose diameter can be varied (an iris diaphragm).
  11. The condenser is also located between the light source and stage. It is a lens system that bends and concentrates the light coming through the specimen.
  12. The light source is on the base and provides necessary light for the examination of specimens.
Magnification is an important feature of any microscope. In your laboratory notebook, write down the magnifications of the objectives on your microscope. You calculate the total magnification of the scope by taking the power of the ocular (usually 10x) and multiplying it by the power of each objective. Thus, if your ocular is 10x and your objectives are 4x, 10x, and 40x, your three magnifications are 40x, 100x, and 400x. Label your three magnifications as low, medium, and high.


B. Now that you are familiar with the parts of the microscope, you are ready to use it to view thread.
  1. Rotate the low-power objective so that it is in line with the eyepiece. Listen for a click to make sure it is in place.
  2. Turn your light on. If you have a mirror instead of a light, look through the eyepiece and adjust the mirror until you see bright light.
  3. Using the coarse focus, raise the stage (or lower the body tube) until it can move no more. (Never force the knobs.)
  4. Place a drop of water on a clean slide and add several short pieces of brightly-colored thread.
  5. Add a coverslip (a thin piece of plastic or glass that will cover the water and press it against the slide). This works best if you hold the coverslip close to the drops of water and then drop it gently. If air bubbles form, tap the coverslip gently with the lead of your pencil.
  6. Put the slide on the stage and clip it down, making sure the coverslip is over the hole in the stage.
  7. Looking in the eyepiece, gently move the stage down (or body tube up) with the coarse focus. If you do not see anything after a couple of revolutions, move your slide a little to make sure the threads are in the center of the hole in the stage. This indicates that the threads are in the field of view.
  8. When you have focused as best you can with the coarse focus, “fine tune” the image with the fine focus.
  9. Place the threads in the very center of the field of view by moving the slide as you look at it through the microscope. Make sure that the threads are at the center of the field, or you will lose them when you change to a higher magnification.
  10. Turn the nosepiece so that the medium-power objective is in place. Until you are very familiar with any microscope, do not turn the nosepiece without checking to make sure it will not hit the slide. Always move the nosepiece slowly, making sure that it does not touch the slide in any way. A lens can easily be damaged if it hits or breaks a slide.
  11. Once the medium-power objective is in place, you should use only the fine focus to make the image sharp. Once again, move the slide so that the thread is at the center of the field.
  12. Again, watching to make sure you don't hit the slide, turn the nosepiece so that the high magnification objective is in place. You should use only the fine focus to refocus.
  13. (Optional) If you like, repeat steps 1-12 using a strand of your own hair rather than thread.
If we wanted to look at the threads at high magnification, why didn't we just start with the high-power objective? Had we tried to bring the threads into focus under high magnification without first looking at them under low and then medium magnification, we almost certainly would have never found the threads. When you look at the slide at high magnification, you are looking at a very, very tiny portion of the slide, and it is unlikely that what you are looking for will be there. As a result, you should always start your microscope investigation with the lowest magnification and then work your way up, centering the specimen in the field of view each time before you increase magnification.


C. Now it is time to get your first look at cells!
  1. Collect some cheek cells by rubbing a cotton swab back and forth on the walls of your cheek inside your mouth. Use only one side of the swab.
  2. Remove the swab carefully without getting a lot of saliva on it.
  3. Rub the wet side of the swab on the slide. You should see a smear where you rubbed the slide.
  4. If you were to look at the cells under the microscope right now, it would be hard to find them, because they are almost transparent. To help make them easier to see, you will add a dye to them. This dye is called a stain, and it will help contrast the cells against the light, making them much easier to see. Place a drop of methylene blue stain on the area where you placed the cells. (This stain will not come out of most fabric, so use it with care.)
  5. Add the coverslip carefully.
  6. Place the slide on the microscope and begin the procedure outlined in section B, looking at the cells under low, then medium, and then high magnifications. At low magnification, the cells will look like dots. Once you find some dots, center them and increase the magnification. At high magnification, you should see a dark blob (the nucleus) and a ring outlining the cell (the plasma membrane). Note the irregular shape of the cells. Draw what you see at each magnification.
  7. Rinse the slides that you used in water and wipe them dry with a paper towel. Wipe the lenses of the scope with lens paper, and put everything away. Clean up any mess you made.