MICROSCOPE

There are three main types of microscopes.

BIOCULAR MICROSCOPE

This is like a fancy magnifying glass. It is only used to look at fairly large objects like an ant, etc. It is also called a stereomicroscope because it uses two lenses to create a 3D image. The advantage is that you can see the whole object in three dimensions. The object is not sliced up; therefore, you can see its surface pretty well, but it does not get as magnified as other microscopes. We don’t have one to look at in this lab, so you can skip the questions on pp 12-13, but read the whole chapter and answer the questions about the binocular microscope on other pages.

COMPUND MICROSCOPE

This is what we will be using here. It is used for examining much smaller objects, such as bacteria or single celled organisms found in pond water. Usually, larger objects such as tissues (or an onion in today’s lab) are sliced in half to look at the inside.

ELECTRON MICROSCOPE

This is a very expensive device that is generally only found in research institutes. It can view the very smallest of objects, such as a virus (which are small enough to fit inside the nucleus of a bacteria cell!). Generally, the object is not sliced up, so it can view the surface of an object, but in much greater detail since it has the greatest magnification. It can also be used on objects that are sliced in half to look at the inside.

RULES FOR MICROSCOPE USE

  1. Pick up and carry the microscope with two hands; one hand lifts the arm of the microscope, and the other hand supports the bottom.
  2. The shortest objective lens (lowest power) should be positioned right above where the slide would go before and after you use the microscope. Make sure the short objective is in place before you put a slide on. Make sure it is also in that place when you put the microscope away.
  3. Make sure the stage is down all the way before returning the microscope.
  4. Wrap the cord up again before putting it away.

Each of you should go get a microscope (use the tan ones in the back right corner of the lab; the grey ones next to them are not as good). When you are back at your table, don’t plug them in yet; we’re going to go over the parts of the microscope.

SLIDES

LETTER “e”

COLORED THREADS: Which color is on top?

SPIROGYRA

NORMAL BLOOD SMEAR

SICKLE CELL ANEMIA SMEAR

PARTS OF THE MICROSCOPE

  1. Base
  2. Light source
  3. Condenser: focuses the light on the slide
  4. Diaphragm (with control lever): regulates the amount of light coming through
  5. Stage: where the slide sits
  6. Stage Clips: hold the slide in place
  7. Mechanical stage control knobs: allow you to move the slide back and forth
  8. Arm: provides a carrying handle
  9. Body tube: connects the nosepiece to the eyepieces
  10. Eyepieces: these have a magnifying lens in them that is 10x
  11. Nosepiece: holds the objective lenses
  12. Objective lenses
  13. Scanning power: this objective is 10x. Since the eyepiece is also 10x, the total magnifying power of this objective is 10 times 10, or 100x normal size. When you look at an onion cell with this lens, it will look 100 times larger than it is.
  14. Low power: this objective is 25x. What is the total magnifying power?
  15. High power: this objective is 65x. What is the total magnifying power?
  16. Oil immersion (we will not be using this one). For your information, this objective is 85x, and when used with a drop of special oil, it magnifies even more. It is used to view objects smaller than we will be using.
  17. Coarse adjustment knob: With the slide in place and the scanning objective in position, you look through the eyepieces and use the coarse adjustment knob to get the object in focus. AFTER THAT, NEVER TOUCH THE COARSE ADJUSTMENT KNOB again until you are done with the slide. If you switch objectives and then use this knob, the slide will get crushed into the objective and damage both. These instruments cost about $1000 each, and the slides are expensive, too! Get out your letter “e” slides and look at them on scanning power.
  18. Fine adjustment knob: After the initial focus, switch objectives to the next largest one (low power). Now use the fine adjustment knob to get focus back. Then move to high power and use fine focus again. Do not use the oil immersion objective. When you are done with the slide, MOVE THE SCANNING OBJECTIVE BACK INTO POSITION FIRST, then switch slides.

NOTE that objects are upside down and backwards in a microscope. You can see this easily on the slide of the letter “e”. Also, when the slide is moving to the right (as you look with your naked eye), the slide appears to be moving to the left when you are looking in the microscope. Also, when an object is in focus at low power, it will also be in focus at high power; the term for this is PARFOCAL (bottom of p16).

BROWNIAN MOTION DEMONSTRATION

Molecules of a liquid or gas are constantly in motion because of the energy in all atoms and molecules. In 1827, Robert Brown, a Scottish botanist, noticed that pollen grains suspended in water on a slide appeared to move by a force that he was unable to explain. In 1905, Albert Einstein, searching for evidence that would prove the existence of atoms and molecules, used this experiment to prove his theory.

I’ll take a slide and put a drop of tap water on it. Then I’ll wet the tip of a probe and touch it to the carmine red powder, and touch it to the water on the slide and mix it. Then I put a cover slip on it and look under the scanning power lens. See the particles moving around? That’s called Brownian motion…we just proved that atoms and molecules exist! They bounce around and bounce off the powder and cause it to move. Now I’ll switch to higher power.

WET MOUNT SLIDE PREPARATION

For your first wet mount, you will get a GLASS SLIDE and take the flat end of a TOOTHPICK and scrape the inside of your cheek. Put the toothpick on the end of the slide and smear it from one end to the other, and try to get a long, thin smear. Place the slide on a PAPER TOWEL. Put a drop of METHYLENE BLUE DYE on the slide, and place a COVER SLIP on top. Blot the excess stain off with another paper towel.

DISPOSE OF YOUR CHEEK CELL SLIDES AND COVER SLIPS in the broken glass container because it is considered hazardous material since it has been inside your mouth.

For your next wet mount, carefully SLICE AN ONION on the cutting board with a razor blade so that it is THIN ENOUGH to see through. Repeat the previous method except USE IODINE FOR DYE.

For your third wet mount, you will observe a living, single celled organism called EUGLENA found in pond water. Put your dropper deep into the jar into the green area at the bottom and suck up some water with it. Put a drop on a slide (on a paper towel). Add one drop of METHYL CELLULOSE which is a thick substance that will slow the organisms down so they don’t swim too fast for you to see. Place a cover slip on top and gently blot any excess so the slide is not wet. Look under the microscope right away, and move the slide around frequently if you can’t see them because they swim away from the light. Keep the jar to the pond water on but not screwed down because they need air.

For the onion and live organism, rinse the slides and cover slips at the sink, holding them over the plastic basin in case you drop something, to prevent the glass from going down the drain. There is no need to dry them; just return them to the alcohol jar.