Each time the microscope is to be used it should be set up correctly
to give a good image. Most often users forget to adjust the iris diaphragm
and focus the condenser to give its optimum performance, this could result
in objects in the preparation being undetected.
There are many microscope set up procedures for specific functionality
but the most versatile for general use is ‘Critical Illumination’ as detailed
below.
Examine the microscope and identify the parts as siting of some controls
may vary and some have ‘Fixed’ or specialised condensers.
Ensure the light intensity control is set to low, turn on the light
and increase the brightness to give a white light output.
Raise the substage condenser fully and open the iris diaphragm fully.
Rotate the nosepiece to the lowest power objective (x4).
Rack the stage down using the coarse adjustment control and place
the specimen slide on the stage using the retaining mechanism to hold in place.
While looking carefully rack the stage up close to the objective
lens (do not rack the lens into the slide; this cannot normally occur when
a low power objective is selected).
Set the eye pieces to the correct width for your eyes so you can
look down both together comfortably.
Focus the Microscope.
Look down the eyepieces with both eyes open and slowly lower the
stage to bring the specimen into focus, (use the coarse adjustment control
initially and then the fine). Slight movement of the specimen while focusing
by using the mechanical stage controls may aid you to find the object. If
the image is not sharp focus adjustment of the eyepieces may be necessary.
If one eyepiece is fixed and one adjustable, close the eye over the adjustable
one. Now focus using the fine control until a sharp image is obtained, (if
you cannot achieve this ask for assistance as the problem may lie elsewhere).
Now open the other eye (keep both open) and using the adjustment on the eyepiece
itself rotate to obtain a sharp image.
Focus the Condenser.
The condenser should be fully raised to start with. Rotate the nose
piece to select the x10 objective and check the focus using fine adjustment.
Position the point of a pencil into the middle of the light source over
the lamp housing. Using the condenser control, slowly rack it down until
the outline image of the pencil becomes sharp and crisp, (this is the ‘Critical’
point for the condenser for optimum performance from where the technique
derives it’s name). Note this must be done only after a specimen has been
focused on the stage.
Set the Iris Diaphragm.
When the iris diaphragm is fully open the image is flooded with light
and definition is lost due to ‘white-out’. As the diaphragm is closed controlling
the amount of light passing through the condenser the image is darkened
and outlines appear thickened and defined as more contrast is achieved. Remove
one eyepiece and place safely to one side. Close the diaphragm down ,then
open slowly until you achieve a dark ring blanking off about 1/5th of the
original field of view to give a ‘bicycle tyre’ effect. The correct amount
of light should thus be obtained.
Examine the specimen, moving it by means of the mechanical stage
controls. No further adjustment to the condenser or iris diaphragm is usually
necessary to view other slides of the same type/density at low power. The
Vernier scale readings may be used to identify and re-locate a point of interest
on a slide provided it is a ‘mounted’ preparation and the slide is placed
on the stage in the same orientation, i.e. well labeled.
