Stops
Both types of microscope
may have two adjusters on the light input. One is the
aperture stop, which is usually marked by the letter A.
The other, rarer stop, is the field stop, usually
marked by the letter F. These are both used to restrict the
amount of light getting to the specimen, but have different
effects.
The condenser is a
collimated light source which produces parallel rays of
light. The aperture stop is close to the condenser and it
reduces the numerical aperture of the lens. As such it has
the effect of reducing the resolution, increasing the depth
of field and may increase the contrast. However, if the stop
is closed to a very small hole, it has the effect of
producing diffraction fringes, which can be misleading.
Generally it is bad practice to operate the microscope with
such a small aperture
The field stop restricts
the size of the circle of light that reaches the specimen
surface. This is used to reduce flare and unwanted
reflections within the microscope.
As a general rule both of
these stops should be kept nearly completely open, and it is
rare that adjustment of these will be needed for standard
observations.
Use of scale bars
The easiest way of
measuring the size of a feature under a microscope is to
relate it to the size of the field of view. The simplest way
of achieving this is to measure the size of the field of
view at a low magnification, and then scale the size
appropriately as the magnification is increased. The field
of view can be measured approximately by looking at a ruler
under the lowest magnification lens.
Accuracy can be improved by
using a graticule. A graticule is a slide with a very fine
grating which, if metric, will usually measure 1mm across,
and is divided into 100 segments, i.e. each segment
is 10 µm across. This allows
much greater accuracy in measuring the field of view, and so
greater accuracy in measuring features.
Metric
graticule in polarised light
On some microscopes, a
scale bar is superimposed on one of the eyepieces, which can
be used to further improve the accuracy of measuring feature
sizes. The scale bar can be calibrated by observing either a
graticule or a ruler at a low magnification. For example, if
1 division is equivalent to 20 µm
with a x5 magnification lens, each division is equivalent to
2 µm with a x50 magnification
lens. By measuring a feature using the scale in the
eyepiece, the actual size of the feature can be calculated
by knowing the width of the divisions in the eyepiece. The
scale bar on the eyepiece is particularly useful because it
can be rotated and so both widths and lengths can be
measured without rotating the specimen.