Lighting Techniques for the Novice Part I - Polarised Light

by David Walker, UK

There are a number of lighting techniques in microscopy that can easily be used by the amateur with little cost to enhance the viewing of many objects. These include dark ground illumination, Rheinberg illumination, oblique illumination and cross polarisation. This is the first in a series of articles that attempts to provide a gentle introduction for the novice on how to use these techniques.
An emphasis is placed on their practical application, rather than the theory because many books describe the theory but not necessarily how the amateur can use these techniques.

Part I discusses how the novice can obtain and use polarising filters with the compound microscope (not the stereo) and is illustrated with images of crystals viewed using polarised light. How these images can be obtained and the type of subjects that can benefit from polarised light will be discussed in more detail in Part II.
 
 

Polarised light - the bare essentials
Normal light from the sun or microscope lamps vibrates in all directions perpendicular to the axis in which the light travels. If this light is passed through a polaroid filter only light vibrating in one plane is transmitted... it is now polarised light. If this light is then passed through a second piece of polaroid filter which has been orientated so that the plane of polarisation is parallel to that of the first filter, the light is transmitted. However, if the second filter is then turned through ninety degrees about the axis of light travel then no light is transmitted... the filters are in a situation called cross polars.

 If the above is unclear, then try taking two pairs of polaroid sunglasses and view through them both as you turn one....... at the point of cross polars very little light will pass through.

 To use polarising filters, place one filter, the polariser in the swing out filter holder beneath the microscope condenser and the second smaller filter, the analyser on the eyepiece (see below). If you don't have a filter holder or condenser ( some student microscopes don't ) place the filter in any convenient place between the light source and the specimen e.g. stick with plasticine under the stage.

 Obtaining polaroid filters
Polaroid filters can be purchased quite cheaply in the small quantities required for the amateur, and a number of sources are suggested below.

 Clip-on polaroid sunglasses
Most larger chemists (drugstores) or opticians sell the clip-on polaroid sunglasses for prescription glasses. In the UK they are usually less than five pounds and are cut to shape by the user to fit their glasses. Although not optically flat and of the highest quality, they are ideal for the amateur microscopist wanting to try polarised light techniques.

 All the images on this page were taken using these type of polarising filters.

 A circular filter (or polariser) is cut out to fit the swing out filter holder under the microscope condenser found on most microscopes (use a paper template or existing filter for this holder to get the size right ). A second much smaller disc (the analyser) is cut out to fit on the microscope eyepiece. The author prefers to lightly stick the filter on the eyepiece with plasticine, 'Blu-Tak' or similar thus allowing this filter to be rotated in use by turning the eyepiece. It is preferable to use a monocular head and single eyepiece for simple polarised light studies.

 Microscopy clubs
Some microscopy clubs purchase stocks of polarising filters, and resell in smaller quantities to their members. This is a good way of obtaining the higher quality optically flat filters. The Quekett Microscopical Club for example sell the high extinction HN22 polaroid sheets in 50mm squares for less than five UK pounds which is sufficient to make a polariser and analyser.
 
 

Microscope dealers
The suppliers who cater for the amateur also usually sell small quantities of polaroid sheets at a reasonable price and should also stock the pre-cut filters that should fit specific microscopes although these will be more expensive e.g. typically 25 UK pounds for a polariser and analyser.

 What can be viewed in polarised light?
Try viewing the subject under the microscope using polarised light and see! Focus the subject first with the filters off, then use the filters and turn the eyepiece to pass through the cross polar position. A wide variety of subjects benefit from viewing in polarised light, these include:

 

• 1) crystals of organic and inorganic compounds
  2) natural minerals
  2) insect parts
  3) fibres
  4) components fabricated in some plastics
  5) certain biological subjects e.g. those containing keratin, chitin, starch grains, wood sections.

 More details and illustrations of subjects that can benefit from viewing under polarised light will be discussed in part 2 of this series.

 Image details

 In order of appearance in article (as described on the slide labels):
 

• 1) Salicine crystals
  2) Magnesium platinocyanide
  3) Salicine crystals - analyser turned to create a different set of colours
  4) Cholesterine

 All images by the author, David Walker of Victorian slide preparations.
 
 Acknowledgement: Thanks to Christopher Redding for loaning me his selection of Victorian slides from which the above images were taken.

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