A sequence of steps to
There is a clear sequence of steps to take to achieve perfect
viewing. These are detailed in the box below. A word of warning: Take
care when adjusting the focus knobs that you do not advance the
objective lens onto the slide! It is very easy to break the slide
and possibly damage the objective.
When setting up the focus it is best to view from the side and lower
the objective so that it is nearly, but not, touching the slide. Now
adjustments can be made while viewing through the eye-piece and slowly
winding the objective UP and AWAY from the slide - this
way you avoid any potential damage to either the slide or microscope.
Getting things in focus
Once the specimen is in focus, fine adjustments in illumination and
iris aperture can be made to improve viewing.
The slide should be scanned systematically, usually by finding the
top corner of the cover glass and then moving the slide slowly across
the stage to the adjacent corner. When the opposite side is reached the
slide is moved up until a new field of view is visible and then moved
slowly across to the other side. This is repeated until the bottom
of the slide is reached.
Higher magnification are obtained by rotating the nosepiece turret
and selecting another objective and then re-focusing.
Parasites are often transparent
Since many parasites are transparent to light it is often necessary
to use various techniques to highlight them. The two most popular
methods are phase contrast and darkfield. Both of these methods are
outside the scope of these pages, but essentially they manipulate the
light so that transparent objects are more readily visible. These
specialist methods usually mean adding special condensers of objectives
to your microscope. While these methods are useful they are not
essential for fish disease diagnosis.
If there is a problem with viewing any specimens with an ordinary
brightfield microscope it is possible to increase the contrast by
racking down the condenser or closing up the iris aperture, although it
does reduce resolution.
Scheme for setting up a simple monocular microscope
make sure the 10x eyepiece is in place
at the top of the draw tube
raise the body tube a few inches above
the stage - by looking from the side and turning the course focus
rotate the nosepiece and click the
lowest power objective into place above the stage (usually a 10x)
adjust the illumination if using a
mirror, turning the flat side of the mirror towards the light source
so that light is reflected up towards the condenser
rack the condenser up to within 2mm
below the stage and adjust the iris diaphragm until it is half open
place the specimen on the stage making
sure that the cover glass is uppermost and secure it with either the
stage clips or the mechanical stage arms
adjust the angle of the mirror so that
a spot of light appears on the slide directly below the objective
looking from the side and using the
course control knob, lower the objective until it is just above the
look through the eyepiece. Adjust the
mirror to give an even amount of illumination
use the course control knob to slowly
rack the objective upwards and look through the eyepiece until the
specimen is in focus. (Tip) it is
sometimes easier to focus on the edge of the cover slip to start
with as this gives a nice clean edge when in focus - whereas mucus
can sometimes be difficult "to find"
use the fine focus to obtain the
sharpest possible image
if the light is too bright either use a
bulb with a lower wattage (if using a table lamp to illuminate the
mirror) or adjust the iris diaphragm to reduce glare
focus the light source onto the slide
by slowly racking down the condenser - watch that this does not
affect the mirror angle. Adjust the condenser and iris diaphragm to
give optimum illumination. Ideally, once the condenser is set in the
optimum position, there shouldn't be any need to keep altering it.
While this long list may seem daunting, it
is because I have tried to cover every step. You will also note that
much of it revolves around optimizing the light source if it is mirror
based. With a fixed light source many of these steps can be
ignored. After you have set up the microscope a few times it
should become second nature.