There are traditionally two ways for microscope illumination¹: critical, where the light source is imaged into the specimen, and the so-called Köhler illumination, where the light source is imaged into the near focal point of the condenser (its iris diaphragm).

In the late 1960s Carl Zeiss Oberkochen introduced a third system: the light source was directly below the specimen. The system was called LUCIGEN and was intended mainly for the successor of the popular Standard Junior, the Standard K, although it could be used equally well with any Zeiss Standard microscope with a condenser sleeve 39.5mm (Lucigen S) or a centrable substage (Lucigen Z). The odd model I have is neither but has an integrated sleeve that screws directly to the underside of the stage. The sleeve has a helical slot that allows the illuminator to be vertically adjusted by rotating the main body. Furthermore, my Lucigen lacks the lamp socket, so, in order to test it, I had to rig up a suitable light source.

The instructions² read: The advantages of this unique illuminating system are its quick and easy manipulation as well as its high performance.

A special opal-glass (milk-glass) disk of 9 mm diameter , the heart of the system, is illuminated by a 6V 5W tungsten bulb via a collector lens and a daylight filter. Control of the illuminating aperture, and with it the contrast, is achieved by simply adjusting the height of the Lucigen. Field of view and correct illumination of the aperture are automatic. Uneven illumination, reduced resolution due to extreme stopping down, or undesired off-centre illumination are avoided. The user can concentrate totally on his object, even a change from a low power 2.5x objective to an oil immersion 100x requires no additional manipulation: the illuminating aperture ranges from any dry objective to a maximum of 1.0 (dry) and 1.4 (immersed) as the opal-glass gives off light in 180 degrees in all directions. The maximum illuminated field is 9mm = the diameter of the opal-glass disk.

In fact this particular illuminator combines condenser and light source in one, eliminates the necessity of a swing-out front lens or even an immersed front lens (the limit is, however, the light intensity, which even at a maximum of 8.2V may be insufficient in extreme cases, but, then, the system is intended for simple routine work).

The price in Canada at the time was $30.50 compared to an equivalent condenser 0.6 ($46.00) when a basic monocular Standard K with three objectives sold for $258.50 plus illuminating/condenser system.

Despite its attractive price and obvious advantages, the Lucigen never became popular. That may simply be because of prejudice. Its production was soon discontinued. Not having a microscope where my Lucigen could be attached, I rigged up my “vertical optical bench” to replicate a microscope. By means of double-sided adhesive tape I attached the Lucigen to the underside of a stage and inserted a small battery-operated torch from below as light source. As my set-up lacked a fine focus control and was, admittedly, a bit shaky, obtaining sharp pictures with the high power was a bit of a problem. Still, the illustrations show a comparison between a specimen illuminated by Lucigen and a normal 0.9 condenser (Zeiss Planachromat 10x and Leitz Periplan10x/18 wide angle eyepiece for the Nikon Coolpix 995). The specimen is the paw (foot) of a 13 day old rat foetus. The low power picture gives an idea of the object, its slightly uneven illumination is due to my improvised light source. Visually there was no discernible difference in resolution or contrast between the two systems.

I think a hobby microscopist with an old microscope lacking a condenser could easily replicate the Lucigen system by using a piece of thin transparent white plastic directly under the specimen.

All comments to the author Fritz Schulze are welcomed.

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