Most illumination forms tend to be used legitimately, and by that I mean used alone and not in combination with other techniques and illumination types. I have written about oblique and diffuse lighting before and stressed in both articles the advantages of experimenting with illumination.

In this article I illustrate the use of two well known illumination techniques and show how to use them together to advantage. I must point out that the technique is purely for improving visual observation and I do not make any claims that any advances in interpretational observations can be made. If the visual presentation of the specimen can be improved in any way, then I think this is legitimate practise regardless of convention.

Methodology

The technique is very simple and consists of standard polarised lighting, but also having partially obstructed light masked in the condenser zone using the traditional off-axis stop. Thus the specimen is illuminated with polarised light from an oblique source, which is then imaged and analysed in the usual way.

The degree of obliqueness, and extent of crossing of the polarisers is left to the observer, and these two combinations promise a wide variation of image enhancement. 

Sponge Spicules. x150 Achromats. Brightfield.	As left, partially crossed polarising filters plus oblique.

The image above left uses straightforward bright field with semi-closed substage iris. Although the general form of the spicules is shown, some subtle surface features and colour bandwidth are missing? Also the view through the eyepiece is more offensive to the eye since the bright background does not add any information to the image. With the polarised/oblique technique the background can be made almost neutral and therefore help the eyes to concentrate on the subject.

The photo right shows how colour enhancement from the polarised lighting using semi-crossed polarising filters, and bass relief using oblique lighting helps to produce more attractive images. The degree to which either of these techniques can be adjusted is infinite, so observers can adjust to their hearts content. I prefer a darkish background, but not black as when the polarising filters are crossed at 90 deg. The darkened background helps provide lower contrast and so the surface features of these interesting spicules can be seen more comfortably.
 
  

Radiolarians, x150 achromatic, polar/oblique.

This photo of Radiolarians was also illuminated in a similar fashion using polar/oblique settings, but the purpose of the polarised light was indirect in that all I wanted was a way of reducing the background intensity so that it would be easier to photograph. This reduction of light intensity is infinitely variable from black to almost no restriction. Thus the contrast often encountered in photomicrography can be tamed with subjects which are not sensitive to polarised light.
 
  

Boric acid crystals x125 achromatic. Plain polar.	As left, with oblique lighting.

Crystals of boric acid seen above demonstrate the effectiveness of using both polarising and oblique illumination. The depth of the crystal structure perceived through the binocular head is not present in the photo on right, and thus the differences between these two lighting techniques is unfortunately not as dramatic as it is in reality. Again I stress that the variations of settings of both the oblique lighting and the degree of crossing of the polarising filters is endless, and in the final analysis has more to do with personal tastes and preferences than scientific exactness.

Technique

There are a number of ways of accomplishing this technique, and those observers who already have polarising facilities present on their 'scopes can easily introduce oblique in the normal way by inserting an off-axis patch stop. Those who like to use polarised lighting techniques can refer to the Micscape Article Library (lighting techniques section).

For best results the patch stop for the oblique lighting needs to be as high up the substage as possible, in the filter tray preferably, otherwise the side lighting is only partially effective.

Summary

Two simple benefits can arise from the use of the two lighting techniques. The first is to visually enhance the original polarised image using oblique lighting. The second is using the variable light intensity effects of crossing polarising filters to aid both visual and photographic observations, using specimens not responsive to polarised light.

Pro's

1) Simple to setup using standard existing sundries and apparatus.

2) Can be very effective, enhancing subject viewing and showing specimen's form more clearly.

3) Useful for certain photographic situations too.

Con's

1) None ??

All photo's taken with Olympus C830-L digicam using Zeiss Photomicroscope I with in-built polarising facilities.