Explains the parts and how to focus a microscope
The Microscope
Parts and Specifications
Historians credit the invention of the compound microscope to
the Dutch spectacle maker, Zacharias Janssen, around the year
1590. The compound microscope uses lenses and light to
enlarge the image and is also called an optical or light microscope
(vs./ an electron microscope). The simplest
optical microscope is the magnifying glass and is good to about ten
times (10X) magnification. The compound
microscope has two systems of lenses for greater
magnification, 1) the ocular, or eyepiece lens that one looks into and
2) the objective lens, or the lens closest to the object. Before
purchasing or using a microscope, it is important to know the functions
of each part.
Eyepiece Lens: the
lens at the top that you look through. They are usually 10X or 15X
power.
Tube: Connects
the eyepiece to the objective lenses
Arm: Supports
the tube and connects it to the base
Base: The
bottom of the microscope, used for support
Illuminator: A
steady light source (110 volts) used in place of a mirror. If your
microscope has a mirror, it is used to reflect light from an external
light source up through the bottom of the stage.
Stage: The
flat platform where you place your slides. Stage clips hold the
slides in place. If your microscope has a mechanical stage, you
will be able to move the slide around by turning two knobs. One
moves it left and right, the other moves it up and down.
Revolving Nosepiece or Turret:
This is the part that holds two or more objective lenses and can be
rotated to easily change power.
Objective Lenses:
Usually you will find 3 or 4 objective lenses on a microscope.
They almost always consist of 4X, 10X, 40X and 100X powers. When
coupled with a 10X (most common) eyepiece lens, we get total
magnifications of 40X (4X times 10X), 100X , 400X and 1000X. To
have good resolution at 1000X, you will need a relatively sophisticated
microscope with an Abbe condenser. The shortest lens is the lowest
power, the longest one is the lens with the greatest power. Lenses
are color coded and if built to DIN standards are interchangeable
between microscopes. The high power objective lenses are
retractable (i.e. 40XR). This means that if they hit a slide, the
end of the lens will push in (spring loaded) thereby protecting the lens
and the slide. All quality microscopes have achromatic,
parcentered, parfocal lenses.
Rack Stop:
This is an adjustment that determines how close the objective lens can
get to the slide. It is set at the factory and keeps students from
cranking the high power objective lens down into the slide and breaking
things. You would only need to adjust this if you were using very
thin slides and you weren't able to focus on the specimen at high power.
(Tip: If you are using thin slides and can't focus, rather than adjust
the rack stop, place a clear glass slide under the original slide to
raise it a bit higher)
Condenser Lens:
The purpose of the condenser lens is to focus the light onto the
specimen. Condenser lenses are most useful at the highest powers
(400X and above). Microscopes with in stage condenser lenses
render a sharper image than those with no lens (at 400X). If your
microscope has a maximum power of 400X, you will get the maximum benefit
by using a condenser lenses rated at 0.65 NA or greater. 0.65 NA
condenser lenses may be mounted in the stage and work quite well.
A big advantage to a stage mounted lens is that there is one less
focusing item to deal with. If you go to 1000X then you should
have a focusable condenser lens with an N.A. of 1.25 or greater.
Most 1000X microscopes use 1.25 Abbe condenser lens systems. The
Abbe condenser lens can be moved up and down. It is set very close
to the slide at 1000X and moved further away at the lower powers.
Diaphragm or Iris: Many
microscopes have a rotating disk under the stage. This diaphragm
has different sized holes and is used to vary the intensity and size of
the cone of light that is projected upward into the slide. There
is no set rule regarding which setting to use for a particular
power. Rather, the setting is a function of the transparency
of the specimen, the degree of contrast you desire and the particular
objective lens in use.
How to Focus Your Microscope:
The proper way to focus a microscope is to start with the
lowest power objective lens first and while looking from the side, crank
the lens down as close to the specimen as possible without touching it.
Now, look through the eyepiece lens and focus upward only
until the image is sharp. If you can't get it in focus, repeat the
process again. Once the image is sharp with the low power
lens, you should be able to simply click in the next power lens and do
minor adjustments with the focus knob. If your microscope has a
fine focus adjustment, turning it a bit should be all that's
necessary. Continue with subsequent objective lenses and
fine focus each time.
What to look for when purchasing a microscope.
If you want a real microscope that
provides sharp crisp images then stay away from the toy stores and the
plastic instruments that claim to go up to 600X or more. There are
many high quality student grade microscopes on the market today.
They have a metal body and all glass lenses. One of the most
important considerations is to purchase your instrument from a reputable
source. Although a dealer may give you a great price, they
may not be around next year to help you with a problem. One
dealer that we can highly recommend is Microscope World.
They offer a wide variety of instruments at very competitive prices.
Find them at www.MicroscopeWorld.com
http://www.microscope-microscope.orgExplains the parts and how to focus a microscope
