Optical staining - an impressive gimmick

You need a clear coloured glass filter fitting to the filter-support of the condenser, a condenser that can be adjusted vertically by means of a gear-drive, some circular cardboard-pads (4 mm - 12 mm they can easily be made with a core drill), coloured transparent varnish as used for glass paintings, black varnish and a rotatable stage used to protect microscopic slides with a ring of varnish. For testing purposes use starch dispersed in water.

First try to obtain a good dark-field illumination: Fix a cardboard-pad with a drop of glycerol in the middle of the glass filter and open the diaphragm fully. For weak objectives (up to 20x) it is easy to get a good dark field illumination this way. Once the appropriate diameter of the pad is established remove the pad, clean the filter and draw a ring of black varnish of exactly the same diameter around the centre of the filter using a slide ringing table (slide ringer). When the varnish has dried completely cover the inner opening with transparent varnish of different colour (see left fig.). The result is a dark field image with the background now not black but coloured. In the example given objects are displayed yellow against a green background. If the background is too bright reduce the internal opening by another black ring of varnish - this re-adjustment of the background brightness is critical!

Much more impressive results are obtained using a coloured sector disc (see right fig.). Such filters were available many decades ago, but then they were no longer produced, since this illumination is inferior to proper dark field illumination because of its lower contrast. Nevertheless, this kind of illumination is often preferable because the contrast of dark field illumination sometimes is too extreme. The colour combination is of course a matter of taste, adjacent rainbow colours are recommended.