Description of Petrological Microscope

1. Department of Geology, GSS College,Belgaum.
Petrological Microscope and its parts
A mineral or a rock is studied under microscope by making a thin section out of it. To
make a thin section glass slide, mineral/rock fragment, canada balsam and section making
tools are required. The mineral/rock fragment is taken and is fixed to the glass slide with
the help of the canada balsam. Fixing of mineral fragment to the glass slide involves -
cooking of canada balsam, where few drops of canada balsam on glass slide is heated
until it gives thread when the sharp pin is put into it and pulled. The mineral section is
placed on it and uniformly pressed to remove air bubbles trapped between the mineral
fragments, Canada balsam and the glass slide. The mineral section now is subjected to
grinding on a section-cutting wheel using emery powder. Once the section is thin
(0.03mm) enough, few drops of Canada balsam is placed on it and heated to the required
temperature and a glass cover slip is placed on top of it. It is uniformly pressed to remove
trapped air bubbles. Once it is done a section is clean and it is ready for the study.
A petrological microscope magnifies the object and also restricts the light to vibrate in a
certain specified direction. The petrological microscope consists of following parts-
1. Eye piece or ocular with cross wires 8.Condenser
2. Bertrand lens 9. Diaphragm
3. Analyser (upper nicol prism) 10. Polarizer
4. Slot for inserting optical accessories (mica
plate, gypsum plate, & quartz wedge)
11. Plane concave mirror
12. Arm of the microscope
5. Objective 13. Coarse adjustment screw
6. Rotating stage graduated with vernier 14. Fine adjustment screw
7. Clips for holding sections 15. Foot of the microscope
1. Eyepiece or ocular:It is placed at the top of the tube of the microscope. It contains two cross
wire one horizontal and other vertical. It is made up of two lenses. It helps in enlarging or

2. Department of Geology, GSS College,Belgaum.
magnifying image of the mineral section. The ocular contains a lens, which is usually a 10x
lens. The magnification of your thin section is the product of the magnification of the ocular
and the objective. For example, if you are looking at a section using the 4x objective, and
your ocular is 10x then the total magnification is 40x.
2. Bertrand lens: It is placed in the tube of the microscope. It can be brought in and out of the
path of rays. This is used when examining interference figures and while studying the
proprties of minerals like sign of the mineral and finding out biaxial and uniaxial nature of
the mineral under conoscopic condition.
3. Analyser (Nicol Prism): This is similar to polarizer made up of transparent calcite crystal
called as Iceland spar in the form of a prism. Two rhombohedral parts of a calcite cut
diagonally, are joined by means of Canada balsam. This can be put into or out of the path of
rays of light as and when required. Below the analyser there is a slot or opening in which
optical accessories like mica plate, gypsum plate and quartz wedge are inserted.
4. Objective: This is placed at the other end of the tube of the microscope and can be removed
easily whenever required. Obectives are used for magnification. There are three types of
objectives- low power, high power and medium. Low power objectives magnify or enlarge 12
to 14 times of the original diameter. Medium and high power objectives magnify 40 to 80
times the diameter.
5. Tube of the microscope: Ocular, Bertrand lens, analyser and objective are fixed in the tube
of the microscope. This tube can be raised or lowered as and when focusing is required by
means of screw.
6. Rotating stage: This is provided just below the objective, which is graduated with vernier
scale. This can be rotated either clockwise or anticlock wise. For holding the thin section on
the stage it is provided with 2 clips. The stage can be fixed by operating a screw.
7. Clips for holding the section: These are two steel clips, which are fitted on the stage of the
microscope. Their one end is fixed to the stage and the other is moveable. These clips hold
the section in place.
8. Condenser: It is a lens, which condenses the rays and illuminates a particular point of the
mineral or rock section on the stage and is fixed below the stage of the microscope.
9. Iris Diaphragm: It is made up of metal piece. It works as a shutter and controls the amount
of light that falls on the object.
10. Polarizer: It is similar to analyser in construction. It is fixed at the lower end of sub stage and
is rotatable. It is also called as nicol prism.When both polarizer and analyser is in the path of
rays they are said to be under cross nicols (under analysed light). With the help of both the
nicol prisms (analyser & polarizer) the light can be made to vibrate in any specified plane.
11. Mirror: This is placed below the sub stage and can be tilted. It is a Plano-concave mirror.
Light can be reflected and made to fall on thin section. Plane mirror is used for studying
sections under low power objective. Below this there is a base which is the heaviest part of
the microscope.
12. Arm of the microscope: One end of the arm of the microscope is attached to the base with
the help of the fastening screw. The other end is attached to the tube of the microscope. The
arm can be moved in back to front for inclined position.
13. Coarse adjustment screws: Rotating clockwise or anti clockwise can bring the object into
rough focus.
14. Fine adjustment screws: By operating these screws the objective studied under investigation
is brought under sharp focus.
15. Subs-tage: The sub-stage arrangement consists of condenser, diaphragm, and polarizer. This
sub-stage can be raised or lowered by operating screws.