Parts of the microscope:
Objectives: these are the lenses
used to magnify the specimen on the stage. Do not let the objectives ever touch
your slides. To change magnification, rotate the nosepiece on which the
objectives are fitted. Touching the objectives will put the centering out and
requires repair by a specialist… so don’t touch them!
Rotating stage: the stage can be
rotated 360° and is graduated in degrees. For most microscopes the stage
diameter is 120 mm.
Condenser: the polarising
condenser is in a cylindrical sliding mount with swing-out top part and aperture
iris diaphragm. Normally the condensing lens is swung out, and only swung in,
when viewing an interference figure. The condenser varies the amount of light
hitting the specimen.
Polariser: vertically removable
filter polariser in rotatable mount.
Focussing: the image is focused
by raising or lowering the rotating stage. There are single-knob coarse and fine
focussing controls on both sides of the stand.
Interchangeable tube: the
eyepiece ocular is the lens fitted to the top of this tube. The eyepiece
magnifies and focuses the image produced by the objective lens. The eyepiece
assembly contains two cross-hairs which should be oriented E-W and N-S (parallel
to the vibration directions of the polariser and the analyser). The cross-hairs
can be focussed by raising or lowering the eyepiece in its mount. The eyepiece
have a standard magnification of 10x or 8x. The magnification produced by both
eyepiece and objective is obtained by multiplying the two separate
magnifications.
The Bertrand lens: the Bertrand
lens magnifies and focuses interference figures. It swings in and out.
Pinhole stop: swings in and out,
for singling out small object details in conoscopic observations.
Analyser: Removable filter
polariser that can swing in and out. It allows light to pass through vibrating
N-S, i.e. perpendicular to the polariser vibration direction. Periodically check
that the polariser and analyser vibration directions are at right angles to each
other.
Revolving nosepiece: use this
nosepiece when changing to a higher or lower magnification. Pushing the
objectives can cause damage to the objectives and will put the centering
out.
Accessory plates: accessory
plates are used for studying interference figures and the retardation produced
by mineral specimens. When required, they are inserted into the microscope tube
in a slot between the objectives and the analyser. One standard accessory plate
with the microscope is the gypsum plate.
When one leaves out the
polarisers, the orthoscopic image formation is the same as in a normal
microscope. (Fig. 1.4). Light from a (low voltage) microscopy lamp is comprimated through the collector lens, which is build in the lamp holder. The
light is polarised by the polariser. Then it passes through the aperture
diaphragm and the condenser, which determines the intensity and the top angle of
the through the sample going lightcone. The desired top angle depends basically
on the aperture of the chosen objective.
Now the light passes through the sample, objective and analyser,
respectively. The vibration direction of the analyser is perpendicular to that
of the polariser. Finally it passes through the eyepiece. This is called
orthoscopic because the light rays are roughly parallel and vertical as they
pass through the sample, which is
the determining factor for the image formation.
This is different when using
conoscopic viewing, which is obtained by bringing in the condensing lens below
the stage and the Bertrand lens above it. Now the eye does not see points of the
object but directions through the object. The centre of the image now represents
the properties of light going through the sample perpendicularly, while the
edges of the image represent the outer radiation of the through the object going
light cone. For further explanation see chapter IV.
I.3.1 Orthoscopic and conoscopic
viewing
The petrographic or polarising
microscope is the most important instrument for the determination of the optical
properties of minerals and their textural relationships. Microscopy is the
easiest, quickest and least expensive way of obtaining information about
minerals. The image formation in a normal microscope is considered to be known.
