I'm afraid I can't add a great deal more than what Chris says! A couple of times I have seen similar things appear on Ebay, but never won one so I haven't seen them up close before. But I do know that the S-1 Mercury Sunlight lamp was made in this bi-post format as well as the more usual mogul screw based version. Probably that is the lamp you saw in Dr. Hicks' collection Tim.
The different electrodes in this lamp clearly suggest that it is intended for DC operation - my guess is you would want to make the ring electrode the cathode, and the heavier one the anode because it will get much hotter there. The bulk of the light will be created by incandescence of the anode.
Whereas Pointolite lamps use the filament purely to assist in striking the arc to a tungsten bead, in this arrangement it would appear that the arc is iniatiated in the same way as in an S-1 lamp. After ignition it would move down to the pair of electrodes and I expect the lamp would create light almost completely by incandescece of the anode. This would indeed make it a very desirable source for photomicrography, where a small point source is of paramount importance to deliver crisp images.
The peak intensity will be on the surface of the anode and I think you would set up the optical equipment to view its incandescent surface by looking through the annular cathode. This could also have been a nice way of aligning the lamp in the optical system.
Tungsten blackening on the envelope clearly shows that the bulb has been burned cap up and contains a high pressure gas filling. Are you able to test the gas Tim, perhaps on a Tesla to see if it is argon or based on another gas? If it is anything other than argon, the light output from the discharge would be negligible compared with the electrode's incandescence at 30 Amperes, and I think this would confirm that it is an incandescent electrode lamp.
By the way, be careful if you do light it that you put sufficient ballasts in circuit to limit the current to 30 Amps. Lighting it on a car battery would probably cause it to draw too much current once the arc has struck, resulting in melting of the electrodes and filament tails. Also you must make sure that the arc does ignite if you are running it up slowly. If you light it to the point that the arc is not quite starting, this could overload the filament. I dont think that a tungsten coil this thin would be happy operating at 30A so you need to let the discharge strike quickly and take some loading off the coil!
Almost certainly it would have been superseded by the short arc mercury lamp in the mid 1950's, at the same time as Pointolite lamps began to lose their popularity. Although the source size of this and Ediswan Pointolite is very small, the incandescent beads emit a continuous spectrum of visible light. Super pressure mercury lamps emit light of only a few wavelengths and this is found to be of far greater importance in creating sharp photomicrographic images.
Still, its a very desirable looking lamp and thanks for posting the pictures Tim!
