Viewing Slides

It is stating the obvious to say that a microscope is necessary to view microscope slides, and the type best suited is a POLARISING or PETROLOGICAL (same thing more or less) model. Fossils and some sedimentary rocks can be viewed in a quite satisfactory way with a standard microscope, but it is the analysis and identification of the crystal minerals that make a polarising microscope essential.

Polarising Microscopes

This differs from a standard microscope in having 2 filters, one beneath the stage of the microscope called the POLARISER and one above, usually mounted on a slide, somewhere in the tube of the microscope above the objective lens. This is called the ANALYSER. A rotating stage is the other main difference, and while not essential for general viewing, it is needed for proper study and mineral analysis.

Polarising filters

Crossed filter view

It is beyond the scope of this brief description to explain the nature of light and its properties, those wishing to study in greater depth are referred to the numerous text books on optics. Let it be sufficient to say that the lower filter, the polariser, removes all light except that vibrating in one direction, and the analyser, being set at right angles to the polariser, removes the rest. In this situation no light reaches the eyepiece of the microscope. This is called CROSSED FILTER or XPL view.

If a slide containing a crystalline rock is placed on the stage of the microscope, inbetween the filters, the regular atomic structure of the crystal has the effect of twisting the polarised light as it passes through the crystal, so that on reaching the analyser some light is transmitted, and the crystals appear illuminated against a dark background. Furthermore in certain instances light is split on entering the crystal and travels through it at slightly different speeds, recombining slightly out of synchronization with itself as it emerges. This gives rise to a phenomenon known as OPTICAL INTERFERENCE, and is the same effect that can be seen when light is reflected from a thin film of oil on water. (although the cause is different)

Different minerals split and slow the light by different amounts, a property of minerals called BIREFRINGENCE. The resulting colour seen in a thin section of standard thickness, 30 microns, can be used as a means of identifying the minerals in a rock. Unfortunately it is rarely as simple as this. The orientation of the mineral has a marked effect on the amount of slowing, and hence the colour, also many slides are not exactly 30 microns thick, and a thicker section will slow more, so this has to be allowed for. Texts on optical mineralogy contain charts converting colours seen to birefringence values at various thicknesses, and these can be consulted, but after viewing a representative selection of slides the common minerals soon become familiar without recourse to the colour charts.

Basalt from Stromboli

BASALT FROM STROMBOLI

Colourful Pyroxene crystals, white Feldspar, black glassy matrix

Plain Polarised light

The ‘analyser’ is generally mounted on a slide and can be easily withdrawn from the view. The slide can then be viewed in PLAIN POLARISED LIGHT or PPL, and appears to be as in normal light. This is not quite the case, the light is still polarised, and still splits in the crystal. On rotating the slide some coloured crystals appear to change colour, a phenomenon known as PLEOCHROISM, and this again is used as a means of identification.

Standard Microscopes

For those possessing a standard microscope it is possible to view interference colours by placing a small piece of polarising film on each side of the slide. Polarising film can be inexpensively purchased (ebay) and works well enough, but can be fiddly to position correctly. Polarising adaptors can be purchased to convert standard microscopes, but inexpensive monocular polarising microscopes are available, and usually contain more specialist features.

Slides of fossils and those of some sedimentary rocks can be viewed with a standard microscope, as the important features are visible without the necessity of polarised light.

One feature not required for general viewing is high magnification, I use a 4x objective and 10x eyepiece. If I could get a lower magnification I would use it, but microscopes seem to focus better across the field of view at higher magnifications and this seems to be the lower limit. For wide field of view low magnification photography I have reverted to taking photos without the microscope, using polarising film and super macro settings on a digital camera. This gives better focus across a wide field of view than I can obtain through a microscope.