The Light Microscope
Biology – Leaving Cert
Experiments
Materials and Equipment
Microscope
Prepared Microscope Slides
Procedure
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Familiarise yourself with all procedures before starting.
Switch on the light source and remove the eyepiece cover, if present.
Rotate the nosepiece so that the low power lens is used.
Put a prepared microscope slide on the stage of the microscope.
Move the slide until the object is above the hole in the stage.
Use the stage clips to hold the slide in place.
Using the coarse adjustment wheel, ensure that the low power lens is at the closest
setting to the slide.
Look down the eyepiece. Keep your eye about 2 cm from the eyepiece. Adjust the
iris diaphragm so that the field of vision is bright but not dazzling. Adjust the
position of the slide, if necessary.
Use the coarse adjustment wheel to focus the object as sharply as possible. Then use the
fine adjustment wheel to sharpen the focus. If necessary, readjust the iris diaphragm so
that the specimen is correctly illuminated.
To increase the magnification, rotate the nosepiece so that the next highest power
objective lens is above the specimen.
Refocus using the fine adjustment wheel. Readjust the illumination if necessary.
To further increase the magnification, rotate the nosepiece again so that the highest
power objective lens is immediately above the specimen.
Focus using the fine adjustment wheel only.
Magnification is calculated by multiplying the magnification of the objective lens by
the magnification of the eyepiece. The magnification of microscope lenses is engraved
on the lens casing.
Draw labelled diagrams of your observations under low power (L.P.) and high power
(H.P.).
BIOLO
16. To further increase the magnification, rotate the nosepiece again so that the highest
power objective lens is immediately above the specimen.
17. Focus using the fine adjustment wheel only.
18. Magnification is calculated by multiplying the magnification of the objective lens by
the magnification of the eyepiece. The magnification of microscope lenses is
engraved on the lens casing.
19. Draw labelled diagrams of your observations under low power (L.P.) and high power
(H.P.).
Result
Conclusion/Comment
SKILL ATTAINMENT
BE FAMILIAR WITH AND USE THE LIGHT MICROSCOPE
Following instructions
Familiarise yourself with all procedures before starting
Follow instructions step by step
Listen to the teacher’s instructions
Correct manipulation of apparatus
Provide a light source
Remove eyepiece
cover
Rotate nosepiece so that the low power objective lens is being
used Place the slide on the stage
Use the stage clips
Use the coarse adjustment
wheel Use the iris diaphragm
Use the fine adjustment wheel
Use the high power objective
lens
Refocus using the fine adjustment wheel
Observation
See a clear image
Appreciate the importance of the correct placement of the
slide Notice the effect of magnification
See the effect of adjusting the iris diaphragm
Appreciate the use of the coarse adjustment
wheel Appreciate the use of the fine adjustment
wheel
Recording
Write up the procedure
Draw labelled diagrams of your observations
Interpretation
Draw reasonable conclusions from your observations and results
Application
Become aware of any other application(s) of what you learned in this
activity
Organisation
Exercise caution for your personal safety and for the safety of
others Work in an organised and efficient manner
Label as appropriate
Work as part of a group or team
Clean up after the practical
activity
Background information
Microscopes are fundamental biological tools. There are two basic types of light microscope,
the compound microscope and the stereoscopic microscope. The compound microscope is
used to examine cellular material and gives a magnification of up to 400. The specimen
has to be translucent. The stereoscopic microscope is used to view whole structures in 3
dimensions.
The most important feature of any lens system is its resolving power. The resolving power of a
lens system is the smallest distance separating two objects that can be distinguished by the lens
system and that allows them to be seen as two distinct objects rather than as a single entity. For
example, most humans see two fine parallel lines as two distinct markings if they are separated
by 0.1 mm. If they are closer together we see them as a single line. Thus the resolving power of
the human eye is 0.1 mm.
The light microscope has a resolving power of about 0.0002 mm so it gives useful views of
cells and can reveal features of some of the sub-cellular contents of eukaryotic cells.
Most microscopes are fitted with three objective lenses, which magnify 4, 10 and 40.
The eyepiece lens is usually 10. The total magnification is obtained by multiplying the
number on the eyepiece by the number on the objective lens.
Helpful hints
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It is advisable to clean the lenses of the microscope regularly using lens tissue.
This minimises confusion between debris and unstained cells.
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The objective lenses are preferably cleaned with ethanol-diethyl ether (30:70) on lens
tissue or with lens cleaning tissue.
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A common mistake is to have too much light coming through the specimen particularly
when viewing under low power. Opening or closing the iris diaphragm will vary the light
intensity.
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To locate small objects e.g. single cells, reduce the light intensity by closing the
iris diaphragm and traverse the slide methodically.
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Never remove a slide while the high power objective lens is in position. Turn back to the
low power first.
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Always treat the microscope with great care. Carry it in both hands and cover when
not in use. Allow the lamp to cool before covering and storing.
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Students who wear glasses can remove them for viewing, as microscope adjustments
will accommodate most deficiencies in eyesight (except astigmatism). This is more
comfortable and stops the spectacle lenses being scratched by the eyepiece holders.
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