58 Chapter 3
LABORATORY
Cell Structure continued
EXERCISE:up and down. As you do this, notice how
different parts of the threads and different
threads become distinct. When one thread is
in focus, the others above and below are
blurred. By continually fine focusing up and
down through the threads, you can perceive
the depth dimension that is not evident when
the focus is resting at one point.^- Now turn to high power and notice that you
can see much less depth than under low
power. In fact, you may not be able to distin-
guish one whole thread completely clearly
under high power.^ - The vertical distance that will remain in
focus- at any particular time is called the
depth of focus or depth of field. The medium
in which the threads are embedded between
the cover slip and the microscope slide is like
the water depth in a swimming pool. When
the specimen is near the top close to the cover
slip it is in focus; it will go out of fo-cus if it
swims down to the bottom of the mi-croscope
slide (if it is a live specimen like a
Paramecium that you will observe- later on).
II.. The Dissecting Microscope- Another common microscope in use in
anatomy and other biology courses is the
binocular dissecting microscope. Your in-
structor has set one up on demonstration at
the front of the lab. This microscope has a
pair of oculars, one for each eye. The dis-
tance between the oculars can be adjusted by
pulling them together and by pushing them
apart until you have adjusted them for the
correct position of your eyes. The ocular pair
has a magnification of 103. The mov-able
large single objective lens usually goes from
13 to 23.^ - Place a small moss plant on the stage of the
microscope (usually a large round glass
plate). Move the plant around and change the
objective lens to observe the arrangement of
the small whorl of leaves on the stem.
III. The Electron Microscope
Light microscopes can only magnify to about- Most of our knowledge of cellular fine
structure has been derived from pictures taken
with an electron microscope. Electron micros-
copy uses a beam of electrons rather than light and
magnets instead of glass lenses. Electron beams
have a much shorter wavelength than visi-ble
light, and refracting them with magnets yields
resolutions thousands of times greater than light.
Special training is required to use an electron
microscope.
Observe photographs taken of cellular details
with an electron microscope. Your in-structor will
put on demonstration a number of electron
micrographs.
IV.. Preparing Your Own Wet
Mounts^
Wet mounts are prepared by placing a drop of
material on a clean microscope glass slide. If the
material is dry, then place it in a drop of water on
the center of the slide. A cover slip is then placed
on top of the material by holding it at a 45° angle
until the edge of the cover slip touches the drop of
water. Then gently drop the cover slip on top of
the material. The water will push air in front of it
to prevent air bubbles.A. Human Epidermal Cells- Gently scrape the inside of your cheek with a
clean flat toothpick. Prepare a microscope
slide by placing a drop of water in the center
of the slide. Place the scrapings in the drop of
water in the center of the slide. Add a small
amount of methylene blue stain to the drop of
water by touching an eye droplet full of stain
to the drop of water. Mix the stain, water, and
cheek scrapings with your tooth-pick. Cover
with a cover slip. This is the only slide we
will use a stain on.^ - You have removed some of the protective
epithelial cells that line your mouth. These
cells are constantly being worn off and