He told me that he had purchased a Goerz substage condenser that is intended for providing a novel type of oblique lighting. Unfortunately it had a broken diaphragm and also does not fit his Zeiss Standard microscope. I offered to have a look at the diaphragm and also see if it was possible to fit to a Zeiss Standard.
It proved to have only 9 of the 12 diaphragm blades present, and although it would work with 9 blades, this was not ideal.
Condenser diaphragm, dismantled, showing 9 blades
I decided to try and replace the missing blades. I did not have any steel shim of the right thickness, but did have some 0.005" brass sheet. I stuck some electricians PVC tape to one of the good blades and carefully trimmed it to make a pattern. I did this three times and stuck the patterns to the brass sheet. I then masked off the sheet with a combination of more tape and a circuit board resist pen. I masked off the rear of the plate completely with PVC tape. The plate was then immersed in ferric chloride solution and occasionally agitated until the sheet had etched through. This was then rinsed thoroughly in water, and the three new brass blades peeled off from the PVC backing and drilled in the correct positions for pivot pins. These were made from brass rod. I first tried to fix them in the holes by using a pin punch on the end of the rod to expand it and jam in the hole. This was only partially successful, so in the end I soldered them and then filed the raised soldered region flush with the blade surface. This left just enough solder to hold (I hope!!).
Here is the reassambled diaphragm with new brass blades:
...but the blades were bright brass. To blacken them, I used cuprammonium carbonate solution. To make this, I bought some copper sulphate on eBay, some household ammonia and some washing soda (sodium carbonate). Firstly a spoon spatula full of copper sulphate was dissolved in water in a test tube. Separately, a couple of spatulae full (to ensure excess) of washing soda were dissolved in water.The two solutions were mixed and the resulting precipitate of copper carbonate filtered off and washed. This was then dissolved up in about 30 ml of household ammonia solution. My daughter loved the beautiful rich blue colour of the cuprammonium carbonate solution (but not the smell!). [Did you know than ammonia was named after the god Ammon because of the smelly substance formed when the ashes of sacrifices at the Temple of Ammon in Egypt were moistened with water?...]
The brass blades were degreased by wiping with a cotton bud moistened with toluene and gently cleaned with fine emery cloth. They were then immersed in the cuprammonium carbonate solution for about 30 mins. This gives a gunmetal black finish. Here is the finished and assembled diaphragm:
To fit the condenser to a Zeiss substage, I tried a number of configurations. The most successful was to build a "nest" for the condenser out of sheet plastic that enables it to sit on top of the substage mounting ring. An aluminium ring is attached to the underside of the plastic nest and fits into the Zeiss substage ring, enabling the condenser to be located and centred. This ring should really be the Zeiss dovetail type of ring, but as I do not have a lathe, is at present a plain ring. It works, however!
Here is the condenser mounted in its "nest":
and -
Note the half-silvered slit filter
Here it is mounted in the substage ring of a Zeiss Standard GFL (same size substage ring as the other Standards):
The condenser is sited rather higher than a Zeiss condenser would be, so to prevent crashing into the underside of the slide, it is advisable to fit an extension to the height stop.
Now to describe how the condenser functions:
The main part of the condenser is a normal condenser with clip on top lenses. These are either plain, or spot lenses as used for darkground illumination. The condenser has a displacement control that can throw it off centre in a single linear direction. In the lower focal plane of the condenser is a clear slit formed in a half silvered filter. You can see this swung out in one of the images above. If the slit is aligned with the direction of the displacement control it is possible to move the darkground spot along the line of the clear slit, but maintain constant lighting through the semi-silvered region of the filter. It is claimed this gives superior oblique light images.
As for the images, with less than a x40 objective, very satisfactory normal darkground can be achieved:
Zeiss Plan 25/0.45 Objective
At higher powers, COL can be used with the slit side lighting and variable offset:
Zeiss Plan 40/0.65 Objective
Zeiss 63/0.8 Objective
I've posted a lot of pictures in this topic, but understand I'm allowed to in the technical areas by the guidelines. I hope this topic gives some interest and maybe some ideas and techniques that others may find useful, as well as introducing the Hungarian Goerz condenser.
Thanks to Cactusdave for a fascinating few weeks of experimentation!
