The Microscope

The average human cell is one hundredth of a millimetre in diameter (0.01 mm). A lens or a combination of lenses can be used to produce a much bigger image of the object (magnified) in our eye.

Eyepiece Lens: magnifies the image produced by the objective lens; is marked on it. The usual level of magnification is 10X (magnified to make it look ten times bigger)

Nosepiece: allow different levels of magnification by allowing easy change to a different objective lens.

Objective Lens: magnifies the image of the object under study; the level of magnification is engraved onto the lens casing e.g. 4X, 10X, 40X. The 4X is the low power and the 40X the high power objective lens.

Total magnification is calculated by multiplying the magnifying power of the eyepiece by the objective lens. Therefore the usual three levels of magnification are 40X, 100X and 400X.

Coarse Focus Wheel: used for focusing at low power when using the low power objective lens.

Fine Focus Wheel: used for exact clear focusing producing the sharpest image at all levels of magnification; this is the only focus wheel to use with the medium and high power objective lens!

Stage: the glass slide is placed on the stage with the object for study it carries placed directly over the hole in the stage so light can pass through it; clips on the stage may be used to hold the slide in a steady position.

Light Source: this may be a reflecting mirror or a lamp below the stage.

Condenser: focuses the light from the light source onto the object on the slide.

Diaphragm: used to vary the amount of light passing to the object to get an image of the best clarity – not too bright or too dark.

Investigation:Using the Microscope to Examine Animal and Plant Cells

1. Switch on light or adjust the mirror to get good light.

2. Place a prepared glass slide of plant or animal cells onto the stage.

3. Make sure that the tissue is centred directly above the hole in the stage.

4. Turn the low power objective lens (4X) until it is ‘clicked’ into position.

5. Turn the coarse focussing wheel to bring the lens as close to the slide as possible.

6. Look through eyepiece and turn coarse focusing knob to slowly move lens away from slide until the tissue is in focus.

7. The diaphragm may have to be used to change the brightness to give a better view.

8. Observe the cells and move the slide to scan the tissue.

9. Before increasing the magnification, move the area of greatest interest into the very centre of your view.

10. Click the next higher objective lens, the medium power, (10x) into position.

11. Only turn the fine focusing wheel to sharpen the image.

12. Note the greater detail – draw a labelled diagram of part of a cell.

13. Before increasing the magnification, move the area of greatest interest into the very centre of your view.

Click the high power objective lens (40X) into position. draw a labelled diagram of a plant or animal cell.

Only turn the fine focusing wheel to sharpen the image.

MandPractivity 6 Prepare a slide from plant tissue and sketch the cells under magnification.

1. Cut an onion bulb downwards in half.

2. Place a thumb between two of the thick flesh leaves of one half and pull out a portion.

3. On the facing concave surface is a thin ‘skin’ of tissue only one cell thick.

4. Use a blade to make shallow horizontal and vertical cuts to make small squares of plant tissue.

5. Remove two squares of the fine tissue with a forceps and place them close on a glass slide – one will be left unstained and the other will be stained.

6. Place a drop of iodine stain on one plant tissue square and after a minute soak up the excess with paper.

7. Place a few drops of water on the plant tissue squares and then cover both with the same cover slip – the cover slip makes the view brighter and clearer.

8. Examine the plant tissue under low, medium and high magnification using a microscope as in the last investigation.

9. Compare the unstained and the stained tissues samples – the nucleus, cell wall and cytoplasm show up more clearly in the cells of the stained tissue.

10. Draw labelled diagrams of the tissue as seen under medium and high magnification.