C H A P T E R 4 : C E L L S — U N I T S O F L I F E 65

4.1 Using a microscopeAn average cell is between 0.01 mm and 0.1 mm across. If you could grow a cell 200 times bigger than normal, it would still only be the size of a pin head. To see cells you need to use a microscope.

Using a microscope

AimTo use different microscopes correctly.

Risk assessment and planning

1 Read the investigation and draw two data tables to record the parts of the microscope. See the table on the next page.

2 Explain why the following rules are necessary.• Always carry a microscope with two hands—one

hand holding the arm and the other supporting the base.

• Place the microscope securely on the bench away from direct sunlight.

• Only use lens-cleaning tissue to clean dirty lenses. Never use your fingers or paper towel.

• Look at the microscope side-on to wind the body tube to its lowest point, then focus by winding the body tube upwards. This way the glass slide cannot be broken.

1Apparatus

• different types of microscopes as pictured• microscope lamp• lens-cleaning tissue• prepared slides• microscope slides• cover slips• non-permanent black pen• magazine, newspaper and tissue• thin plastic ruler with millimetres marked on it• dropping bottle of water

Method

Part A: Microscope parts

1 Collect each microscope pictured and become familiar with the parts marked on the diagrams.

coarseadjustment

knob

fineadjustment

knob

arm

arm screw

base

eyepiece

body tube

nosepiece

high powerobjective

low powerobjective

stage

condenser

iris diaphragm

mirror

adjustable eyepiece

knurled ring

low power objective

high power objective

stage with moveablebase plate

focusingknob

A monocular microscope A stereo or binocular microscope

© K.L. Books and Anne Garton 2005

S C I E N C E F O R L I F E 8 66

continued1

Microscope part

Name Function

1 A platform where the slide is positioned with clips to hold it in place.2 Reflects light up through the slide to the eyepiece.3 The lens that the operator looks through.4 This concentrates the rays of light and directs them onto the object being viewed.5 A tube that the eyepiece and nosepiece are attached to.6 Moves the body tube up or down for focusing; can be coarse or fine.7 Lenses used to see the detail of the specimen; can be low power or high power.8 Can be adjusted like a camera shutter to control the light passing through the object and the

body tube.9 Supports and stabilises the microscope.10 Holds the body tube and can be used as a handle for holding the microscope when carrying.

2 Copy the table above into your notebook and as you examine the monocular microscope enter the following parts in their correct position in the table.

eyepiece, body tube, high and low power objectives, stage, condenser, iris diaphragm, mirror, base, arm, coarse and fine adjustment knobs

3 Check that you have the parts in the correct place in the table.

4 Explain how the stereo or binocular microscope is different from the monocular microscope.

Part B: Magnification and focusingThe total magnification of a microscope is found by multiplying the eyepiece lens magnification by the objective lens magnification. Each lens is labelled with an X which indicates the number of times this lens magnifies. For example, the eyepiece may have a 5X on it and the objective lens may have a 10X on it. The total magnification is 5 multiplied by 10, or 50X, so you are seeing the object 50 times larger than normal.1 Place a prepared slide onto the stage of the monocular

microscope. Follow these steps to set up and focus the microscope.• Turn the objective lens to the lowest possible

magnification. What is the total magnification? Record this.

• Look through the eyepiece and angle the mirror so that the field of view, or the area you see as a circle of light, is evenly lit. Use a lamp, not direct sunlight, if there is not enough natural light. If there is too much light, adjust the iris diaphragm.

• Place the prepared slide on the stage under the clips.

• Look at the microscope side-on and use the coarse focus knob to wind the body tube to its lowest point just above the slide.

• Look through the eyepiece and, with the coarse focus knob, wind the body tube upwards away from the slide.

• Once you have the slide in focus, turn the objective lens to high power. If you need to bring the specimen into focus, use only the fine focus knob. What is the total magnification now? Record this.

2 Practise focusing the microscope on high and low power using the different prepared slides available to you.

3 Now try to set up and focus a binocular or stereo microscope. Can you use the prepared slides with the binocular microscope? What sort of things can you examine with this microscope?

Part C: The field of viewWhen looking into a microscope, the field of view changes, depending on the magnification you are using. As the magnification increases the field of view decreases. This means you see less of the specimen you are viewing, but in far more detail.

40xlow power

100xmedium power

400xhigh power

0.4 mm1.6 mm4 mm

As the magnification increases the field of view decreases.

© K.L. Books and Anne Garton 2005

C H A P T E R 4 : C E L L S — U N I T S O F L I F E 67

1 Set up the monocular microscope on its lowest magnification.

2 Place a thin plastic ruler on the stage and bring it into focus.

3 Measure the diameter of the field of view in millimetres and record it in a table like the one below. Depending on the microscope you use, you may need to alter the magnifications in the table or add more columns. Repeat this for higher magnifications as well.

4 Repeat steps 1–3 for the binocular microscope.Low power

Medium power

High power

Eyepiece lens 5X 5X 5X

Objective lens 10X 15X 20X

Total magnification 50X 75X 100X

Diameter of field of view

Part D: Moving the slide

1 On a clean microscope slide draw a small letter a (the size of the print on this page). Use a non-permanent marker.

2 As before, set up the monocular microscope on low power and place the slide on the stage.

3 Look into the eyepiece. What do you notice about your letter? Is it upside down? Back-to-front? Can you see all of it? Describe what you see.

4 Now move the slide to your right. Which way does the letter move? Try moving the slide to the left, then up and down. Which way does the letter move for each new position?

5 Repeat these steps for high power and for the binocular microscope. Record your observations.

Part E: Producing a wet-mountIn preparing a wet-mount the specimen is placed in a thin layer of water between the slide and the cover slip.Water can damage the microscope, so make sure the slide is not running with water when it is placed on the stage. Blot up any excess water with a paper towel. Try not to tilt the microscope in case any excess water runs off the slide onto the stage.1 Set up the monocular microscope as before.2 Place a small piece of newsprint onto a slide

and secure it in place by adding a drop of water to cover it.

3 Place the cover slip at the edge of the water as pictured and slowly lower the cover slip using a drawing compass. If this is done properly you should avoid air bubbles forming on the slide.

4 Practice may be needed to perfect this skill. Try different objects under the microscope, e.g. magazine print and tissue paper.

Place a piece of newsprint on the slide.

Add a drop of waterto the newsprint.

Producing a wet mount

© K.L. Books and Anne Garton 2005

continued

Part F: Drawing specimens

1 Place a prepared slide on the monocular microscope.2 Put the microscope on low power and focus the image.

Now put the microscope on high power and see which is the best view. Draw what you see using the following rules.• Learn to keep both eyes open. Use one eye for

viewing and the other for drawing. This may take some practice.

• Use a lead pencil and blank (unlined) paper.• Draw the specimen in a circle which represents the

field of view, but make it much larger as shown. Show the diameter of the field of view (part C) and try to draw the specimen to scale.

• Draw the outline of the image and don’t shade or colour in the parts. Draw the detail of one or two cells only, not all the cells in view.

• Include the name of the specimen, the date and the total magnification.

• A stain is often used to highlight parts of a cell that cannot easily be seen. A stain is a dye that colours different parts of the cell and makes them more visible. Record any stain used.

• Write a description next to the diagram.3 Now draw the same object using the binocular

microscope. What differences do you notice?

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Results The diagrams and observations are the results of this investigation.

ConclusionFor the conclusion, copy and complete the table below. The examples given may need to be changed.

Parts on this microscope but not on the other

Range of total magnification

Largest field of view

How the specimen is viewed

Description of image seen

Monocular microscope

50X–200X 4 mm Upside down and back to front

Binocular microscope

Moveable base plateKnurled ring for focusing

Larger specimens can be viewed, e.g. parts of organisms, but in less detail

Date:Object:Description:

Stain used (if any):Magnification:

Draw only afew cellsin detail.

© K.L. Books and Anne Garton 2005