December 18, 2019

My Leitz Wetzlar polarizing microscope

This was an Ebay purchase 10-13-2019 for $700. The seller tells me: 
It was the personal microscope used by a professor of geology.
A couple of years before he passed away he gave it to me for my work.
But I have a more modern research scope so I really didn't have any use for it, so eBay it is!
This is a very good quality scope for its time, and the optics are still excellent.

Resources

The scope itself

It came with literature describing a Leitz Ortholux Pol. However the base is V shaped rather than T shaped. The focus knob has fine focus inside of coarse focus (not two separate knobs). The fine focus does have limits. This may actually be a Dialux Pol, but I am not able to say at this point. It is unfortunate that the manufacturers do not place a model name or number on these scopes.

The serial number on my scope is 6xxyyy -- this indicates it was made in 1962.

Reading various articles (see the above), this is a classic "black enamel" microscope, likely made entirely of brass. These were produced sometime between 1937 and 1972. It has the following objectives:

The scope was damaged in shipment. Both the condenser assembly and the turret with objectives was banging around loose in the box. Visible damage was that the lever to adjust the diaphragm on the condenser was broken and the knob to adjust the interocular distance was bent. There also seems to be some hidden damage to the optics. The 10x objective seems to work fine, but the 25 and 63x objectives cannot reach focus using the microscope stage. If the slide is lifted up towards these objectives, they will focus, but at an extremely small working distance. In addition the binocular assembly seems to have become disaligned -- the right eye is offset upwards from the left and there is some eye strain using binocular viewing.

The shipper regrets all this and has offered every possible measure to give me satisfaction. He paid the USPS $150 to ship the microscope (from New York state to Tucson, Arizona) and does not wish to see me pay the same to ship the damaged instrument once again. USPS took 9 days to accomplish this. Clearly they handled this roughly, but it was not packed to tolerate rough handling, so here we are.

Repairing objectives

I have a Leitz microscope of the same era (not a polarizing scope, it is probably an SM model). I has objectives that look identical (except that they lack the "P" marking that indicates that the objective is strain free and suitable for use on a polarizing microscope). I am using this to check various things with my objectives.

The SM scope has a 10/0.25 objective. On the SM scope this has a working distance of 0.206 inches. The "P" objective on the Pol scope has a working distance of 0.197 inches. When I transfer the "P" objective to the SM scope, I measure a working distance of 0.195 inches. All of this seems reasonable and within expected tolerances.

I remove a 40x objective from the SM scope and mount it on the Pol scope. I can achieve focus using the stage! This is a Leitz 40/0.65 170/0.17 objective. It does not have the "P" marking of course,

I notice that the working 40x objective has "spring action" to prevent breaking slides. The P 63x does not. So I investigate, first by disassembling the 40x to see how this is set up. Then I take apart the 63x -- it does have "spring action"! Apparently it had just been banged hard enough during shipping to jam it in the compressed position. I continue to take apart the 63x, and with the optics removed, use a dowel to unjam the "piston". I work it enough times to get it moving freely, then reassemble the whole thing and now it works! I can get proper focus on the Pol scope within the range of the stage.

Let us see if the same problem explains the P 25x objective. I unscrew the outer sleeve, and yes it does have a spring, and yes the mechanism is jammed in the compressed position. This one is really stuck! Happily the optics are all in a little barrel assembly that slides out. It was necessary to use a dowel and small hammer to get this mechanism apart. I removed the little stop screw, drove it completely out, and cleaned it up. Now it is back together and also works properly giving me focus within the stage range.

And of course the 100x Oel objective is also jammed. This one the worst of all, but it yields to the same treatment.

My analysis after finishing all this is as follows. I believe that a layer of corrosion had been building up on the slip sleeve that allows the "spring action". These objectives probably were not providing spring action before the microscope was shipped to me because of this, but of course they worked fine since they were fully extended. But in the trauma of shipping they were forced back and the corrosion then jammed them in position

An extra objective

I had a Nikon 4x objective laying around, and found it handy to get a "wide view" to get oriented when looking at a slide. But it was for a 160mm tube length and was not parfocal. So I looked on Ebay and found a Leitz 3.5x/0.10 objective marked for a 170mm tube length. It is not a "P" objective, but I don't expect to be doing demanding observations using polarized light with it. And frankly it seems entirely fine. It is parfocal, so I can jump to any of the other Leitz objectives without having to do hardly any refocusing. Also it matches the other Leitz objectives on the scope and looks like it belongs there. Cost was only $27 ($35 with shipping included). A worthy addition.

Overhaul the turret

I was told before I purchased the scope that the objective turret was very stiff. And indeed it was! It seemed natural to tackle this after getting the objectives themselves working. This should have been easy. First I removed all the objectives, then the large screw in the center. Under this is a small ball bearing with balls loose in a race! Using tweezers and patience I manage not to loose any balls. The turret is too stiff to just spin off (though this should work), so I resort to my wooden dowel again and tap firmly through the objective holes. This does drive the turret off. There is a gigantic ball bearing around the entire periphery of the turret!! Again I manage not to loose any balls (though a couple are already missing). Once I simply wipe off old crud with a paper towel, it can be installed by hand. But I make a couple of turns with 600 grit wet/dry, feeling more for burrs than actually trying to remove anything. I reassemble it using a top quality grease with a lot of molybdenum disulfide (Brownells action lube). It now spins effortlessly.

Disassemble the binocular head

I am not doing this for amusement and exercise. The left and right eyepieces are not in proper alignment and I want to investigate and see if there is an obvious problem. This also allows me to straighten the bent knob for interocular distance adjustment. Once I get the mechanism separated from the optics, I get brave, clamp the knob in a vise and bend it back straight. This works fine, but too much of this sort of thing will eventually break the shaft.

I also have opportunity to investigate the rod with ball on the end that can be pushed in or pulled out. When pushed in, a beam splitter sends light to both the photo tube and to the eyepieces. This yields a somewhat dimmer image in the eyepieces. When pulled out, nothing goes to the photo tube, and a prism delivers an image solely to the eyepieces. So pulled out is the prefered mode when not doing photography.

My inspection reveals no obvious problem (such as a loose or displaced prism). I am probably faced with a tedious trial and error process of shimming the prisms in order to get both eyes working together without strain.

To reassemble it requires driving the shiny knob off the shaft for the interocular adjustment. First a screw is removed. Then the knob slides (or should slide) onto a shaft with two flats. Then I wrap tape around a black tube and use pliers to unscrew it. The tape avoids marring the paint. Once this is off, assembly is easy.

The eyepiece mechanism can be taken off by just removing 4 screws. All four screws are only exposed if the eyepieces are made as narrow as possible. The Knob and tube must also be removed to slide it out. It comes out with prisms attached so take care! Do not remove the eyepiece tubes, and in particular do not remove the four screws hidden below them (if you do, the prism will come loose and possibly be damaged).

On reassembly, I realized there is significant "latitude" for how the eyepiece tubes are mounted to the head. I was pleased to discover that this is sufficient to remedy my left/right alignment issue! All that is necessary is to loosen the four screws and slide the tube up, down, left, right to get the images to match to your satisfaction, and you are done!

I expected to need to experiment with shims behind the prims and to have to go through a tedious exercise of trail and error, repeatedly removing and attaching the eyepiece assembly. Thankfully nothing like this is necessary. Unlike other microscopes I have worked on, Leitz apparently took into account the need to perform this alignment without pain and suffering. My thanks to whoever intelligently designed this instrument.

So, apart from straightening the shaft for the eyepiece separation mechanism, none of this disassembly was necessary. Quite likely one of the eyepiece tubes got bumped out of alignment during the shipping trauma. Of course it all seems easy and simple now, but it was a full days work carefully disassembling and reassembling the binocular head. Another full day was spent on objectives and the objective turret, so I really have 2 full days invested so far in repairing this microscope.

Eyepieces

This microscope came with several sets, and in some cases sets of 3. I have read that in microscopes of this era, some aberrations are not fully dealt with by the objective and the eyepiece "finishes the job". This means that if the photo tube is used, a proper eyepiece is necessary here to give a full corrected image, hence the sets of 3. I have: I am using a pair of American Optical 10x WF (175A) and these give a superb wide image and have lots of eye relief. So far, I prefer these to any of the eyepieces that I received with the microscope. I put one of these in the left eye, and one of the Leitz Periplan in the right to compare them. They do give exactly the same image. The AO eyepieces though have more eye relief, making them pleasant to use with eyeglasses. They also do take in a wider field, which is nice.

I am learning more, and the "Periplan" are the Leitz compensating eyepieces that should be used with the Leitz objectives for best results. No telling about how the results will be with the others (but there is certainly no drastic issue).

Orientation of the polarizer

When I observe biotite grains in plane polarized light, they are darkest when the cleavage is oriented N-S (pleochroism). According to Nesse (page 250) "grains are darker when the trace of cleavage is parallel to the vibration direction of the lower polarizer". Perkins and Henke state (page 7) "most modern polarizing microscopes ... allow light vibrating in an east-west direction to reach the stage. Older microscopes, however, have the lower polarizer oriented in a north-south direction."

Apparently this microscope is set up using the older convention. The microscope has a calibrated scale on both the polarizer and the analyzer, so this could be changed. At this time they are both set at the "0" mark. I don't feel an urgent need to change this.

Conoscopic observations

This was entirely "not working" the first time I tried to tackle it. The first thing I needed to do was to center the condensor, and there are two handy knobs under the stage to do exactly this. Then I needed to center all the objectives, and I got the two tiny wrenches to do this with the scope (thank goodness, as there are few alternatives to these things for turning square shafts down in holes.

After this, some repair work was required. The betrand lens swings in just fine, but the tiny focus knob at the end of the lever was all jammed up. I took the microscope head apart far enough to access this. Two tiny screws enclose the betrand lens "can" and after removing these the tiny lid comes off. I used a toothpick to apply a tiny bit of fine oil to the shaft and most importantly the piston like thing that holds the betrand lens. After working this a bit and giving it some time, it freed itself up and now is working fine. The oil I used is the infamous "Break Free CLP". Using the least possible is highly advised as you do not want oil running inside the microscope and getting on optical elements. Two knobs are provided to center the bertrand lens.

Full disassembly of the head is a mystery. There is an adjustment for the upper polarizing element (the "analyzer") -- it is marked in degrees and there is a vernier provided, which suggests that it ought to move, but it may as well be glued in place. Prudence dictates leaving well enough alone and it stays fixed at 0 degrees for the forseeable future.

Have any comments? Questions? Drop me a line!

Tom's microscope pages / tom@mmto.org