International
Tables for
Crystallography
Volume D
Physical properties of crystals
Edited by A. Authier

© International Union of Crystallography 2006
International Tables for Crystallography (2006). Vol. D. ch. 1.6, pp. 154-155

Section 1.6.4.1. The polarizing microscope

A. M. Glazera* and K. G. Coxb‡

a Department of Physics, University of Oxford, Parks Roads, Oxford OX1 3PU, England, and bDepartment of Earth Sciences, University of Oxford, Parks Roads, Oxford OX1 3PR, England
Correspondence e-mail:  glazer@physics.ox.ac.uk

1.6.4.1. The polarizing microscope

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There are countless applications of polarizing microscopy. One of the largest fields of use is in mineralogy and petrology, where the requirement is to identify naturally occurring minerals, the optical properties of which have already been determined elsewhere. Medical applications of a similar sort exist, for instance in the identification of the minerals present in bladder or kidney stones. The chemist or materials scientist who has synthesised a crystalline material may also wish to identify it from known properties, or it may be a new substance that needs to be described. For other purposes it might, for example, be necessary to determine the orientation (relative to crystallographic axes) of mineral specimens, e.g. in the cutting of synthetic corundum for the manufacture of watch jewels. This section explains the point of view of an observer who wishes to record and measure optical properties, for whatever reason. Although much of what follows is discussed in terms of mineral crystals, it is equally valid for crystals in general, whether organic or inorganic.

The polarizing microscope incorporates five major features not found in ordinary microscopes. These are:

  • (i) A polarizer, normally a sheet of Polaroid, which is part of the microscope substage assembly. This produces plane-polarized light before the light reaches the specimen. In some microscopes, the polarizer can be rotated, though applications of this technique are rare. In the commonly used petrological microscope, the vibration direction of the polarizer is set in what is called the E–W direction, that is, as the user of the microscope sees the field of view, the vibration direction is from side to side.

  • (ii) An extra, high-power condenser situated in the substage immediately below the specimen. The condenser is switched in and out of the optical path as required.

  • (iii) A rotating stage, circular in plan and graduated in degrees. For a number of purposes, specimens can be rotated through known angles.

  • (iv) An analyser, a second polarizing device, situated in the microscope tube above the specimen. Its vibration direction is set at right angles to that of the polarizer, i.e. usually N–S. Like the condenser, this can be inserted into the optical path as needed.

  • (v) A Bertrand lens, also in the microscope tube and insertable as required, which has the function of transferring images from the back (upper) focal plane of the objective to the front (lower) focal plane of the eyepiece. The Bertrand lens and the extra substage condenser are used together to convert the microscope from the orthoscopic to the conoscopic configuration (see later).

In addition, polarizing microscopes have slotted tubes that allow the insertion of a variety of extra devices generally known as accessory plates. Most common amongst these are the sensitive-tint plate (or [1\lambda] plate) and the quartz wedge.

Objective lenses of various magnifying powers are mounted in a rotating turret. Apart from magnification (typically ca [\times 5] for low power, and [\times 40] or more for high power), the numerical aperture (n.a.) of a lens is an important feature. This is defined as the diameter divided by the focal length. This is a measure of the angle of the cone of light that can enter the objective. In the conoscopic use of the microscope (see Section 1.6.4.11[link]), this angle is required to be as large as possible so that the properties of rays travelling through the crystal in a variety of directions can be observed. Numerical apertures of more than ca 0.9 can not be achieved with `dry' objectives, but higher values are obtained by inserting a drop of immersion oil between the specimen and the lens.

Eyepieces in polarizing microscopes are set in a short tube, at the lower end of which is mounted a set of cross wires, which lie in the front focal plane of the lens. When the microscope is properly focused, a real image of the specimen, created by the objective, is made to coincide with the cross wires. The cross wires are conventionally oriented vertically (N–S) and horizontally (E–W) in the field of view, and coincide with the vibration directions of the polarizer and analyser.








































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