It is an optical device used to study minerals and rocks in thin sections. It makes use of polarised light for the purpose of identification of minerals. It is equipped with the following mechanical and optical parts:
1. Mechanical parts:
(a) Base or foot.
(b) Pillar, attached to the base.
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(c) Joint for making the upper part of the microscope inclined.
(d) Arm, attached to the upper end of the pillar.
(e) Tube, attached to the upper part of the arm, about 10″ in length with slots for inserting certain optical devices, such as mica plate, gypsum plate, quartz-wedge etc.
(f) Stage, which is a circular disc with a hole in the centre, above which the mineral section, i.e., the slide is clipped.
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(g) Adjusting screws, the tube can be moved up or down by means of two screws known as Coarse and Fine-adjusting screw.
(h) Mirror Arm and Fork to rotate and turn the mirror.
2. Optical parts:
(a) Mirror, one side plane and the other side concave to reflected light.
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(b) Lower Nicol-prism, i.e., polariser to produce plane polarised light.
(c) Iris Diaphragm lessens illumination of field, if slightly closed.
(d) Condenser, to increase the intensity of light.
(e) Objectives, two or more lenses of different focal lengths,, used to magnify the image of objects.
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(f) Upper-nicol (Analyser) or the Upper Nicol Prism used to analyze the plane of vibration of light which passes through the mineral section.
(g) Eye-piece or Ocular. It is with cross wires at right angles and a system of lenses fitted into the upper end of the tube which magnifies the image of objects.
(h) Bertrand lens. It is used to magnify interference figures.
Optical Accessories:
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(a) Quartz wedge:
It is used for the determination of interference colour, optical sign and nature of vibration-direction of the mineral.
(b) Mica plate:
Also known as quarter-wave plate or glimmer plate, which gives a pale neutral gray interference colour, when put into the slot of the tube, used for determining the optical sign of a mineral. ,
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(c) Gypsum plate:
Also known as sensitive-tint plate, as it produces sensitive violet red interference colour of first order ; used for determining the optical sign of a mineral.
(d) Bereck compensator:
An optical device made of calcite.
Through the examination by a Petrological-microscope, the following characteristics of a mineral section are identified.
(i) Colour.
(ii) Form of a mineral, i.e., whether euhedral, subhedral or anhedral.
(iii) Cleavage.
(iv) Relief of a mineral.
(v) Refractive index
(vi) Inclusions of any other substance.
(vi) Alteration.
(viii) Pleochroism.
(ix) Interference colour.
(x) Extinction.
(xi) Extinction angle.
(xii) Twinning.