The image at the left is that of a dissecting or
stereo microscope. Notice that it has only two
sets of lenses, including the eyepiece. The
specimen to be observed is an opaque object (light
does not pass through it). The observer sees
the surface of the dissection specimen or other specimen
being studied. This specimen is placed in a
container on the stage of the microscope.
Biologists use the light microscope to observe microscopic
specimens. This microscope is also called the
compound microscope is given its name because it has more than
two sets of lenses. It has an eyepiece lens (or ocular)
and two or more sets of objective lenses (this microscope has
three lenses) on a nosepiece that usually revolves.
This kind of microscope is also called a light microscope as
it requires a source of light to pass through the specimen.
The specimen observed with this kind of microscope is usually
microscopic and has to be translucent (allows light to pass
through it). The specimen to be observed is placed on
the stage of this microscope.
Parts of the Light
- eyepiece or ocular
- body tube
- fine adjustment knob
- high power objective
- low power objective
- mirror (many microscopes have a
- coarse adjustment
- stage clip
- inclination joint
Functions of the Light Microscope Parts
eyepiece (ocular) -
where you look through to see the image of your specimen.
body tube-the long tube
that holds the eyepiece and connects it to the objectives (not
fine adjustment knob-small,
round knob on the side of the microscope used to fine tune the
focus of your specimen after using the coarse adjustment knob
part of the microscope at the bottom of the body tube; it
holds the objectives
high power objective --
used for high power magnification of the specimen (the longer
low power objective --
used for low power magnification of the specimen
amount of light going through to the specimen
light or mirror-source
of light usually found near the base of the microscope; makes
the specimen easier to see
coarse adjustment knob
-- used for focusing on low power
arm-part of the
microscope that is grasped when one carries the microscope
clips on top of the stage which hold the slide in place
(The specimen is placed on the stage for viewing.)
inclination joint -is
used to tilt the microscope
Microscope Usage Rules
- Always carry the microscope with two hands - one on the
arm and one underneath the base of the microscope. Hold it
up so that it does not hit other objects.
- Do not touch the lenses. If they are dirty, ask the
teacher for special lens paper or ask your teacher to clean
the lenses for you.
- If using a microscope with a mirror, do not use direct
sunlight as the light source. Blindness can result. If using
a microscope with a light, turn off light when not in use.
- Notify teacher if a slide or cover slip breaks. Students
should not handle broken glass.
- Always clean slides and microscope when finished. Store
microscope set on the lowest power objective with the
nosepiece turned down to its lowest position (using the
coarse adjustment knob). Cover microscope with dust
cover and return it to storage as directed by your teacher.
About the Compound Microscope
Always begin focusing on the lowest possible power.
Remember to center the specimen you
are observing in the field of
view before switching to a higher power. Make
certain that you
move the objectives away from
the specimen when focusing so their is no collision between
the objective being used and
the slide/cover slip which may damage the objective lens.
2. As you switch from low to high power, the
field of view becomes darker. To deal with this
the diaphragm needs to be
opened to allow in more light. (Frequently on
low power the
diaphragm needs to be partially
closed as it is too bright.)
3. As you switch from low to high power the
field of view becomes smaller.
Images viewed under the light microscope are reversed
(backward) and inverted (upside down).
This is a compound light microscope view of the letter F
placed on a slide in its normal position.
Paper chromatography is a procedure used to separate
substances in a mixture. In the Living Environment/Biology
lab, this mixture is usually a solution of liquid plant
pigments containing different kinds of chlorophylls and other
colored photosynthetic pigments.
A small concentrated sample of a mixture is placed on the
chromatography paper above the line of a solvent mixture.
The paper is contact with a solvent solution at its bottom.
This solvent moves through the paper due to capillary action
and dissolves the mixture spot. Some parts of the
solvent mixture to be separated have a greater attraction for
the chromatography paper, so they move a lesser distance,
while other parts of the solvent mixture have a lesser
attraction, so they move a greater distance up the paper.
Chromatography of a Plant Pigment
The specific mixture placed on chromatography paper will
separate into consistent patterns as long as the same solvent,
paper, and amount of time allowed for the separation are not
changed.. Different solvents will change the separation
pattern of the mixture. Mixtures that are colored can be
separated into component colors by paper chromatography.
The Rf value of a pigment is a statistic often computed
from a chromatography separation. Each component
of a solution. Each pigment in the solution will
have a specific Rf for the same solvent when the
chromatography occurs for a specific length of time.
Calculation of Rf
|| distance the pigment travels from
the original spot of solvent
distance to the wetting front of the solvent
Gel electrophoresis is a procedure used to separate charged
molecules of different sizes by passing them through a gel in
an electrical field. The gel serves to act as a
support for the separation of the molecules of different
sizes. The gel is usually composed of a jelly-like
material called agarose which is made from seaweed.
Molecules such as DNA fragments of different lengths and
proteins of different sizes are often separated in the gel.
Holes are created in the gel which serve to hold the
particular DNA mixtures to be separated. The
DNA fragments are then loaded into the wells in the gel.
Separation of DNA
contains very small holes which act to regulate the
speed which molecules can move through it based on
the size of the molecules. The smaller
molecules will move much more easily through the
small holes in the gel. As a result,
large fragments of DNA lag behind small fragments,
thus allowing the experimenter to separate these
molecules based on their size.
Sometimes molecular weight markers
are electrophoresed along with the specimen, so the
experimenter may know the size of the DNA fragment which has
been separated. Different individuals or organisms form
different banding patterns in the plate when their DNA has
been separated. DNA is cut into pieces for
separation for electrophoresis by restriction enzymes.
These enzymes were originally discovered in bacteria and were
used by the bacteria to defend themselves from invasion by
other bacteria and viruses.
Some Uses for
the Gel Electrophoresi DNA Separation
- It may be used to determine an individual's genetic
relationship to his or her ancestors, as the more closely
matched the banding pattern between two individuals, the
more closely they will be genetically related.
In theory, no two individuals will form the same DNA
banding pattern when the electrophoresis is completed.
- It may be used to identify an individual that have
committed crimes based on the ability to match the
suspects DNA to evidence which has been collected at a
- It may be used to determine evolutionary
relationships between organisms, as organisms with a
closer genetic relationship will form more similar
Setup with Power Supply
Gel Electrophoresis for Separating DNA Molecules (Dr.
Karen Hughes/University of Tennessee at Knoxville)