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It is also called a Petrological Microscope and differs from an ordinary microscope in having been provided with two prisms called the nicol prisms.
The general parts of a simple petrological microscope are as follows:
(i) The body, which may or may not be of curved outline.
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(ii) The base, which is often provided with adjustments for absolute leveling at the time of use;
(iii) The tube, which is fitted in the upper part of the body and has adjustment screws for raising or lowering the tube.
(iv) The stage that is fitted in the lower half of the body. It may be circular or rectangular in outline. It has a central opening. The stage is generally graduated, revolving and detachable.
(v) The sockets are provided at different places in the body of the microscope for attaching the objectives (above the stage), polarising prisms (below the stage) and reflecting mirrors, also below the stage. The analyser prism and the eye piece are inserted into the tube above the stage. (Fig. 9.17)
When a mineral section, called the thin section is placed on the stage and focused under the eye piece through an objective with both the nicols out of adjustment, it is said to be examined under ordinary light.
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When, however, nicols are brought in the way of light reflected from the mirror fitted at the base of the microscope, the examination is said to be under polarized light.
When the lower nicol, called the polarizer is so adjusted that its shorter axis is at right angles to the shorter axis of the upper nicol called the analyser, the nicols are said to be mutually crossed and examination of a thin section in this condition is called as under crossed nicols.
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The position of crossed nicols is indicated by the field turning totally dark when there is no mineral on the stage because the ordinary light thrown up is cut out by the nicols. But when there is a mineral section being examined under crossed nicols, the field may or may not turn dark depending upon the behaviour of the mineral towards light.
The study of minerals under cross nicols is of great help in their identification under the microscope.