Proper use of the Compound Microscope 
Objectives
The magnification of an image is
primarily controlled by the objectives which are housed in a rotating nose piece. To
change objectives you rotate the nosepiece, starting with the 4X objective. Do not start
viewing by swinging in the 20 - 100 X objectives. These may be damaged if they hit the
specimen.
The magnification is indicated by a number on each objective. Furthermore there is a
progression in size such that the longest objectives have greatest magnification. The
distance between the objectives and the cover slip (working distance) decreases
dramatically as the magnification of the objective increases.
The 100 X objective is an oil
immersion lens. Note the black line near the tip of the objective. This is used to
identify an oil immersion lens.
Place a small drop of oil on the objective lens.
A small drop of oil must also be placed on the cover slip.
The lens should be carefully lowered
into the oil prior to focusing.
Observe this with your naked eyes focusing on the objective and the specimen. Do NOT look through the
oculars.
Oil improves the optics because it unites the glass cover slip and the objective. It
replaces air with oil. The oil has the same refractive index as glass. Thus less light
scattering & refraction occurs.
Be sure that the specimen was in focus at 40X before switching to 100X.
Avoid focusing down on the specimen with an oil immersion lens. Change the focus so that
the objective is traveling away from the slide. If the image does not come into focus,
reverse the direction until it does. When in doubt, STOP!!! & ask for HELP!!!
The lens might be dirty or there may be some other problem.
The oil also protects the objective lens from scratching.
Notice that we have 4, 10, 20, 40 & 100 X objectives. Always start with the 4 X objective to prevent damage to the other objectives which may collide with the specimen.
Once one of the lenses is focused an
a specimen, the others should also be in focus when they are swung into place. This
property is referred to by the term parafocal. However, in actual practice some adjustment
is required when you switch from one objective to another. This usually presents little
difficulty. However, you must be especially careful when switching from 10X to 40X and
from 40X to 100X.
We will often be using fresh sections and whole mounts in the class. These can be thick
and irregular. Consequently, greater care must be taken when changing objectives. When in
doubt, play it safe and ask for help until you get acquainted with the material you are
studying.
Oculars
The oculars should be adjusted to
suit both of your eyes. Note that there is a scale on the tube holding both objectives. We
will label microscopes so that each student can work with the same instrument throughout
the course.
Grasp the adjustable knurled ring below each ocular with your thumb and forefinger and
gently rotate it so that each is set at 64 which is its midpoint.
Before you make any adjustments, place a slide on the stage and focus on part of the
specimen.
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You
are now ready to achieve Kohler illumination. Oh happy day!
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