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Observing in Biology Index

Observing in Biology Homepage
Observing very small specimens and Calculating the magnification on a compound microscope
Question on the use of the Microscope

Biology Practical Work index

 

Making a slide with a coverslip

Usually we put the specimen in a few drops of water so that it does not dry out whilst we are looking at it. The water also helps the light to pass through the specimen more evenly.

To make a slide you need a glass slide, a coverslip and a needle as well as distilled water

To observe a specimen under a compound microscope you need to support it on a glass microscope slide so that light can pass through the specimen that you are looking at. The light comes up from the lamp or the mirror underneath the microscope.

Place the specimen on a slide and add a drop of distilled water

To protect the specimen in the water drops on the slide, you must cover it with a very thin piece of glass called a coverslip. This keeps everything flat and it also stops the specimen from drying out.

Carefully place one side of the coverslip into the water

When you put the coverslip on the slide you must make sure that you do not trap any air bubbles under it. Air bubbles get in the way of our view and cause confusion.

So that there are no air bubbles trapped under the coverslip, use a mounted needle or a pair of forceps to lower the coverslip slowly onto the drops of water with the object.

Use a needle to lower the coverslip. This avoids air bubbles.

When you have lowered the coverslip onto the slide you may find that there is too much water around it. Take a piece of paper towel or filter paper and soak up the excess water.

If you find that there is not enough water under the coverslip add a drop of water by the side of it. You will see that the water is drawn under.

 

 

Reflected light

Most of the microscopic objects that you will look at are transparent because they are so thin. To see transparent objects under the microscope we shine light through them. This is called transmitted light.

Sometimes you may want to observe a specimen which is opaque. Light will not pass through it. The method to use is to shine the light down on top of the stage from a bench lamp. The light will be reflected off the specimen. You are observing the specimen using reflected light.

Using reflected light with a compound microscope

 

 

 

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Looking at the slide using the microscope

Using a compound microscope (photo by Paul Billiet)

 

Follow these instructions every time that you use a microscope and you will not have any problems in using it. It takes a bit of practice, however, so do not give up; call for help if you have difficulty.

Take the microscope from its cupboard. Be careful; it is heavy. Use bout hands to carry it to your place. Keep it upright all the time.

Put the microscope gently on the bench and plug in the lamp on the bottom. Now turn the microscope and lamp so that the objective lenses are facing away from you.

Make sure that the low power objective lens is clicked into place on the rotating nose piece.

Put the slide and object on the stage of the microscope and hold the slide in place using the spring clips.

 

Compound microscope showing the stage and spring clips

 

Lower the microscope using the coarse focusing knob as low as it can go without touching the slide. You must look from the side of the microscope when you do this.

Now look down the eye-piece lens. Adjust the light using the diaphragm. The diaphragm opens and closes a hole which lets more or less light into the microscope. On low power you will need less light. Close the diaphragm until you see things more clearly. If you leave it right open you will soon get a headache!

Focus the microscope by turning the coarse focusing knob, slowly, as you are looking down the eye-piece. When you look down the eyepiece you will see the object in a circle of light. This is called the field of view and the specimen that you are looking at should be right in the middle of it. The specimen that you are looking at will become clear but it may not be right in the centre; you will have to move the slide.

Now you will find something strange. When you move the slide to the left the specimen appears to move to the right in the eye-piece. When you move the slide to the right the specimen appears to move to the left. Now try moving the slide towards you and away from you. You will get used to this but it is confusing at first.

You can now turn the rotating nose-piece to the medium power objective lens. Make sure that the lens clicks into place.

You will need to refocus a bit using the coarse focusing knob and you will have to increase the light by opening the diaphragm. You will also find that the object is bigger but not in the centre of the field of view. Move the slide to centre the object.

For really small specimens you will need to turn to the high power objective lens. This needs some practice. Everything will be magnified 400 times or more, so when you move the slide a fraction of a millimetre it is magnified 400 times!

Before you turn to the high power lens make sure that the specimen is right in the centre of your field of view. Turn the rotating nose-piece again so that the high power lens clicks into place. Be careful; the lens will be very close to the slide this time. Refocus again using the fine focusing knob. You will need more light so open the diaphragm some more.

 

Taking care of your microscope

It is very easy to get finger marks on the eyepiece lens and water on the high power objective lens. The lenses of the microscope need to be kept clean so that you can see clearly. Do not use a paper towel or a handkerchief. Use the special lens cleaning tissue which is soft and will not damage the lenses.

Sometimes water spills onto the stage where the slide is held. You should wipe this dry. You can use paper towel for this.

 

Putting the microscope away

  1. First switch off the lamp. It will be hot so leave it to cool down before removing it.

  2. Turn the rotating nose piece to low power.

  3. Take the slide off the stage and dry the stage if necessary.

  4. Lower the body of the microscope by turning the coarse focusing knob.

  5. Now unplug the lamp and put it away.

 

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