BY ALL THAT'S DECENT AND HOLY, I'M NOT COLLECTING OLD MICROSCOPES! But sometimes one thing just leads to another...
For some time, I'd been enjoying harassing the local protozoa with my old brass Spencer, occasionally slipping a slide under my slightly later binocular Spencer model 54 for the novelty of a 3D show. Then, getting curious about dark field views, I bought a set of those plastic dark field occulting discs or whatever they're called that you slip into the filter holder under your condenser to give it a try. This worked very well, and gave an interesting view, giving more of an "outside view" of my little creatures with sometimes better visualization of fine detail such as flagella than the usual bright field view. But, it only worked at low powers, up to a 10x objective. Above that, the view deteriorated into poorly defined, shapeless masses of light.
Then I ran across a self illuminated dark field condenser on ebay... A Spencer Model 330 US Army Medical School Model. I knew the chances of it fitting my old brass Spencer were pretty iffy, but if it didn't fit, it came in an elegantly made wooden box with a beveled lid to keep it in, as such stuff often did back then, until I figured out what to do with it . And fortunately, it came with it's instructions. Otherwise, it would've taken me considerable time and frustration to figure out the importance of getting it exactly centered, how to center it, the need for oil contact between the condenser and slide, the recommended thickness of the slide, etc. This stuff was evidently kinda persnickety. And suffice it to say, it didn't fit my old brass Spencer. Which is just as well. It would be pretty cumbersome switching out and continually adjusting condensers if it had fit.
I set about getting the illuminator on it to work until I figured what to do. It required a 6 or 4 volt bulb with a 15mm bayonette base with 2 contacts on the bottom, and recommended a "Hot Shot Battery." Every place was fresh outa both. After considerable searching, the closest I could do was a 15 volt automotive bulb. The same size and configuration, just different voltage. I bought several to last me the rest of my life, and then purchased an AC-DC 12 volt adapter and built it into a little stand with a knob to control brightness. At first, I was a little concerned about heat, but this thing didn't seem to get too warm.
I started looking for another microscope to permanently mount it on. My dark field condenser was listed in the 1924 and '27 catalogues available free online, so I figured that a scope from one of those would be a good gamble. Due to the fact that I'd also figured out that the position of the condenser was pretty exacting, I decided to go for one with the recently improved rack and pinion substage thingy. I settled on looking for what appeared to be a Model 44 from the '27 catalogue and started watching ebay. This was a very popular stand, so it didn't take long for a few to show up. Also, the fact that they were common made them less valuable, which made them affordable. Good for my purposes. Before long a decent one turned up and it was mine. But... the existing condenser had the beveled adapter to snap into the substage cast right into it. I had to find another adapter to fit my dark field condenser. More ebaying. I found what should work on a similar dark field condenser which was missing it's illuminator. Problem solved, and everything now fit together.
Now it was time to learn to USE this thing... In it's description of Spencer's Dark Field Illuminators, the 1924 Spencer catalog says in part, "Dark Field Illuminators have now come to be accepted as a necessary accessory to the microscope for certain types of work, such as the detection of minute colloidal particles held in suspension in fluids or for examining unstained and living bacteria --" My main interest was more along the lines of protozoa, and I had previously seen hints of advantages at lower powers. However, this higher power dark field stuff was more "technique sensitive." The directions specified a slide thickness of 1.5mm, although a thinner slide would work by lowering the condenser as long as oil contact was maintained. I ordered some slides off ebay that claimed to be 1.5, but they were only 1.34mm. A shallow well slide I had came closer at 1.44mm on the flat sides. Either worked, but I had to use a considerable amount of immersion oil, which tended to wind up smeared all over the stage as I followed my animalcules around. I had no problems at the age of 8 getting my father's old microscope to work well with traditional bright field stuff, but now as a 69 year old, I was having to figure my way into this dark field thing. The usable depth of field seemed to be smaller, and the out of focus areas were more distracting being "lit" against the dark background. If the water was too murky, an annoying amount of halation erupted. The effect was less effective if there was too much "stuff" in the way, such as the lacework of algae that I often used to confine my microscopic critters as I looked at them... although the threads of algae were often beautiful. As time went by though, I was gradually getting more adept at this thing... When everything fell into place, fine external details, cilia, and flagella showed up wonderfully, and I was seeing my little creatures more like they might see each other, from the outside, although "lit up." A nice tool to have in the box. Then, the day came when I was looking at a few smaller protozoa in a few drops of rather "depleted" water from a jar I''d left sitting for too long using a shallow well slide... Most of the interesting stuff was gone. I don't know why I pulled out the dark field scope, but when I did, dropping the condenser a bit to compensate for the thin well after slopping on the immersion oil, I saw BACTERIA lit up like starlight against the dark background! Mostly bacilli, but a few flashy spirochetes winding through the field sort of like John Travolta in "Urban Cowboy." Cool... I wondered how I'd missed them before. I put the slide back on the other scope for bright field, and yes, they were there, but not nearly as conspicuous... I'd missed them as "background noise" while looking for my larger protozoa. This socked home the above old catalog description, and I'd learned a little more. I now find myself developing a little more interest in bacteria, too. Like I said earlier, one thing leads to another.