The Light Microscope
A microscope magnifies objects which are too small to see with the naked eye. The microscope used in this lab is
called a compound light microscope because it uses light to magnify objects and has compound lenses (ie. more than one
lens). The standard unit of measurement in a microscope is the micrometer (μm) which is equal to 1/1000 of a millimetre. The
modern compound microscope is able to magnify objects up to 2000 times and if it could magnify higher it would add no more
detail due to its limits in resolving power which is the ability of an optical device to reveal detail. To test the resolving power
of your eyes move backwards from two parallel lines. The limit of your resolving power is the distance at which you can no
longer distinguish the lines as two separate entities. It is due to the nature of light that light microscopes cannot resolve objects
separated by less than 0.2 μm. Without improving resolving power, higher magnification produces blurry and useless
information. The electron microscope uses electrons rather than light and uses magnets rather than lenses to magnify objects.
This simply allows more magnification with resolving power.
The Component of a Standard Compound Light Microscope
Some Key Components
A. The Diaphragm
Many microscopes have an adjustable disk under the stage used to vary the intensity and size of the cone of light
projected through the specimen.
The setting you select for your viewing is a function of the amount of transparency of the specimen and the degrees of
contrast that makes viewing the specimen optimum. Here is a comparison of different settings so you can see the impact.
B. The Condenser
This is an optional lens between the diaphragm and the specimen and is used focus light onto the specimen. It most
useful at the highest powers (400X and above) as it provides a sharper image by improving the ability to distinguish objects. It
is set very close to the slide at highest power and moved further away at the lower powers. Since it affects the cone of light
focussing on the specimen, it not only improves quality of viewing, it also impacts on resolving power of the light microscope
along with the objective and ocular lenses. Below is a comparison of condenser settings.
How to Use a Compound Light Microscope
To ensure that it is not damaged and that your work is optimum quality, follow these steps:
1. Obtain a microscope by carefully removing it from the cupboard avoiding hitting the body tube.
2. Carry the microscope with two hands: one placed under the base and one holding the arm.
3. Set the microscope down on the lab bench. Look from the side and adjust the course adjustment knob until the objective
lenses are as far from the stage as possible.
4. Make sure the revolving nosepiece is set on the low power objective lens. If it is not, turn the revolving nosepiece until it
is in place.
5. Make sure the diaphragm is as close to the stage as possible and that the condenser is fully open to allow as much light
through the stage as possible.
6. Place your specimen on the stage and look through the ocular lens. Turn on the microscope light source. Slowly adjust the
course adjustment knob until the specimen is clearly focussed in the field of view.
7. Watching from the side, slowly rotate the revolving nosepiece until the medium power objective is in place.
8. Look through the ocular lens and slowly adjust the course adjustment knob until the specimen is clearly focussed in the
field of view.
9. Watching from the side, slowly rotate the revolving nosepiece until the high power objective is in place. Make sure the
objective lens moves freely into place without scratching the slide or hitting the stage. Do not use the oil immersion lens (a
very high objective lens) if your microscope has one as it will scratch the slide.
10. Look through the ocular lens and slowly adjust the fine adjustment knob until the specimen is clearly focussed in the field
of view.
11. Watching from the side, slowly rotate the revolving nosepiece backwards until the low power objective is in place.
12. Turn off the microscope, unplug the power supply, wrap the cord around the base, place the cover on the microscope and
return it to where it was stored using two hands to carry the microscope.
How to Make A Proper Microscope Drawing
A good diagram takes considerable time and includes the following characteristics:
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use blank white paper and do not put more than two diagrams on a page
include an overall centred and underlined title for the entire report and include an underlined date
include a very specific, underlined title including the viewing magnification (usually on a prepared slide) for example a
title could be e.g. Cross-section of a Hydra at 25 X Magnification for every diagram; if there is only 1 diagram it should be
centred and if there is more than 1 diagram, the title can be centred or flush with the left margin
only use a sharp pencil to make your drawing and if you need to show shading use dimpling (small pencil dots)
you can draw or trace a large circle to represent your field of view (the area you are seeing in the microscope)
make your drawing is to scale so that objects are proportional (for objects present in high numbers, draw a proportion of the
specimens) and use a ruler to label any identifiable features to the side of your diagram
use a viewing magnification that reveals a significant amount of information, such as a large, single, whole cell
include the drawing magnification (underlined) at the bottom of the diagram
Microscope Measurements.
A. Determining Viewing Magnification A Compound Light Microscope
1. Record the ocular magnification from the side of the ocular lense (typically 4, 5 or 10X).
2. Record the objective magnification from the side of the specific objective lense being used (typically, 10, 20 or 40X).
3. The viewing magnification is simply the ocular magnification times the specific objective magnification.
e.g. if the if the ocular = 5X and objective = 10X, the viewing magnification = 50X. This means you are seeing the specimen
50 times larger than it is actually. This information is included in the title.
B. Determining Low Power Field Diameter of A Compound Light Microscope
1.
2.
3.
4.
The field diameter is the distance across the viewing area of a microscope.
Record the ocular magnification and the low power objective lens magnification.
Obtain a transparent ruler and place it on the stage. View the ruler on low power.
Place the edge of the ruler exactly over the center of the field of view. Place one of the mm lines at the left edge of the field
of view. Determine how many mm fit across to one decimal place. For example, 2.3 mm.
5. Record your diameter of the low power field of view in mm.
C. Determining Higher Power Field Diameter of A Compound Light Microscope
1.
2.
3.
4.
At higher powers, math is involved.
First you need to determine the low power field diameter (see above).
Record the higher magnification.
Use the following formula to solve the higher power field diameter.
higher power field diameter
low power viewing magnification
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low power field diameter
higher power viewing magnification
e.g. if the lower power field diameter is 2.3 mm with a magnification of 25X, then the field diameter at 40X magnification
would be 1.4 mm
D. Specimen Size
1. Estimate how many cells fit across the diameter of your field of view (include 1 decimal place).
2. Now calculate how many mm the specimen is at this higher power by using the following formula:
specimen size 
diameter of field in micrometers
number of specimens that fit across
e.g. if the higher power field diameter is 1.4 mm and you estimate that 5.5 specimens fit across this diameter, each specimen is
0.255 mm (you could work in micrometers if the object is very small; in which case, multiply mm by 1000)
E. Drawing Magnification
1. This is an indication of how much you magnified the specimen when you drew it on paper.
2. Simply measure your drawing of the cell along the same dimension measured in the microscope.
3. Make sure you use the same units in te following formula:
drawing magnification 
drawing sizein mm
specimen sizein mm
e.g. if your drawing dimension is 50 mm and the actual specimen size is 0.5 mm, the drawing magnification is 100X (means
you enlarged the specimen 100 times in your drawing and this information is included at the bottom of your diagram)
Prepared by K. Zuber