ZEISS VERTICAL ILLUMINATOR WITH PROVISION FOR DARK GROUND ILLUMINATION

DESCRIPTION:
This is a vertical illuminator which uses a prism to direct light from a horizontal source down through the obective. The prism traverses one side of the inside of the illuminator. An oval-shaped opening allows light to be directed towards most of the prism. The prism can be adjusted via a tiny knob to direct light down through the objective which acts as a condenser. Light returns through the rest of the aperture of the objective. With this kind of illuminator, the entire field of view cannot be evenly illuminated, and the prism blocks part of the light returning, but the intensity of the illumination is brighter than with a coverslip type of illuminator. The top screws into the nosepiece and the objective then screws into the bottom of the illuminator. The top knurled fitting can rotate independently of the rest of the illuminator, so that the opening of the illuminator can be fixed in a particular direction.

This model also has provision for dark ground illumination. Although the standard model without the provision for dark ground illumination was sold from about 1895 to 1939, this model was not listed in any Zeiss catalog that I have seen. It is therefore unclear to me when it was first introduced, and for how long it was offered. I can only assume it was first introduced after the plain model, likely sometime in the 1920's. It has included with it two tin stops for dark ground vertical illumination. They each incorporate a half-circle stop. The illuminator has a thin slit cut in it, which allows the introduction of one of the dark-field stops to allow dark ground illumination. Standard bright field illumination is available without the stops in place. For more examples of vertical illuminators and their history, please see the vertical illuminator page.