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.
