Favourite Microscopes: The Jena Ng

by René van Wezel, UK

Big-name microscopes from a couple of decades ago were built to last a lifetime. The workmanship is unrivalled in today’s mass market economy, and it is therefore no surprise to see many of the former laboratory workhorses ending their ‘career’ in the hands of the serious amateur.

My personal favourite is the Jena Ng, sold as a routine laboratory microscope in the 1960’s and made by the Carl Zeiss works in Jena, Eastern Germany.

As many will know, Zeiss has quite an interesting history, and managed to keep their quality at top level even at their most difficult of times just after WWII, when the allies took the ‘brains’ out of Jena to Western Germany. Following up on this, the Russians took practically all that was left in inventory, machinery and manpower from Jena to Russia.

The instrument described here is a typical example of the equipment produced by the Jena factories in the first couple of decades after WWII. It is in fact a copy of the famous L stand, designed by Zeiss in the 1930’s (Zeiss-Jena still had the patents on those designs). The L stand was at the time a major design breakthrough from the traditional inclined type. From then on, every major manufacturer turned at some point to this design. The stage here is always horizontal, and moves up and down by an ergonomically low positioned fine focus while the objective lens stayed stationary. This made application of extra equipment to be carried on top of the microscope (particularly for photography) far less troublesome compared to the old designs were focussing was solely done by moving the tube with lenses.

  

Stand

The stand is almost equal to the Lumipan stand produced 30 years earlier. (Editor's note added Jan. 2016. Link is to Stefano Barone's later article on the Lumipan.)

As with the L-series, the N-series was supplied in different versions, like the Lg/Ng, sold as routine laboratory microscope and a larger version Lu/Nu, sold as research model.

For all of them, the coarse focus moves the arm up and down; the movement can be set by an adjustable ring for when heavy equipment is attached.

The fine focus is lifting the stage via an internal clockwork, and even hard pressing on the stage does not affect the feel of the fine focus up- or downward.

The stage is of the old style gliding type, at the time already replaced by ball bearing stages in the West. Although the system works fine, care has to be taken to keep the microscope clean as lubricant thickening by collection of dust dramatically increases friction. I do not know the preferred lubricant for this job, and even the lightest greases like Vaseline give more friction then is comfortable. At the moment I am using thick bicycle oil (which includes PTFE and adherents) which works well and is sticky enough to keep to its place.

The stage can be changed for other types (no less than 5 types were available) and is mounted on a round rack. The stage can be rotated fully over this rack and even with the highest magnifications the field is not completely lost (which shows the accuracy during manufacture). This makes in my opinion a dedicated rotating stage only necessary for serious polarization work.

Illuminator: base

The illuminator consists of a Köhler base and the condenser. The remarkably simple base uses a 6V 15W Tungsten lamp of which the filament is factory centred: there are no separate centring screws for the lamp. This filament is projected onto the condenser iris by a single collector lens, via a centrable first surface mirror. Half of the base is actually covered with a metal plate against dust, which is removed for this picture:

The collector lens is a single very strong lens, minorily corrected for spherical aberration. The image of the filament however does not fill the whole condenser opening and is therefore insufficient to fill the aperture of an oil immersion lens fully. Therefore, the system is also fitted with a swing-in opaline diffusion sceen just behind the lamp (this is nowadays more or less the standard configuration for all major microscope manufacturers). It has several advantages: it evens out the illumination, both in the image (which is needed with lowest magnification) as well as in back focal point of the objective (needed with the highest magnifications). Furthermore, by using the large illuminating surface of the diffusion screen rather than the filament-point source, correction of the collector lens becomes practically non-critical. This may explain why the single-lens collector from the Jena does quite well compared to the far better corrected 3-lens collector system of the post-war Zeiss-West designs.

Correct Köhler setup: the filament is projected sharply on the condenser iris (left). When looking in the tube without an ocular, the filament is again visible in the back of this oil immersion objective 90/1.30 (middle). With the lamp diffuser in place, the whole of the objective receives light (right). The condenser was water-immersed to the slide.
 

Illuminator: condenser

The condenser supplied with this microscope seems a rather simple 2-lens type 1.4 NA (or maybe with a doublet as lower lens). The front lens is rather large and close to the slide, which make it easy to use immersed with either water or oil. The illuminating system with diffusion filter in place is efficient enough to fully illuminate the whole aperture of a 1.3NA oil immersion lens, although at those apertures remnant spherical aberration is clearly apparent. Even so, the correction is a big step up from the ubiquitous uncorrected Abbe condenser, and compared to that only a minor step away from the ultimate achromatic-aplanatic condenser.

 

Modern condensers nowadays do often have a swing-out top element for use with low power objectives. For this condenser however, this is not a good option as even without the top lens it is still rather powerful (with NA of 0.6) and still not able to fill the field of the 8* objective.

Therefore, an extra swing in lens is supplied underneath the condenser, which increases the size of the illuminated field to a massive 9 mm (which is the diameter of the front lens) and that is sufficient for as low as the average 2,5* objective. For best performance, the condenser should be racked down a bit and is then able to just about show the fine line pattern of the test diatom Gyrosigma balticum with a 20/0.4 lens (which means that resolution is barely impaired with this extra lens).

By careful comparison I have not seen notable image degradation by this swing-in lens, either resolution wise or contrast wise compared with the full condenser or the condenser with top lens removed, so I do not see any reason to recommend either system.

A disadvantage here is unfortunately the lack of a filterholder under the condenser. Although a filter place is reserved for on the illuminating base, for experimental illumination a filterholder near the back focal plane of the condenser is preferable for easy access.

Overall, the illuminating system is more then adequate for most uses, including darkfield and phase contrast. The illumination is strong, with the lamp generally run on 3.5V for normal BF work, daylight corrected with an extra thick blue daylight filter.

Zeiss-Jena produced around 8 different condensers for this model. They also produced a condenser revolver, with the ability to flick between 3 condensers in an instance. It was often fitted on the research models with another parade horse: the aplanatic ‘pancratic’ (zoom) condenser, with which the NA was continuably variable  between 0.16 and 1.4, but due to high costs not often found.
 

The pancratic (zoom) condenser from a later ‘ausJena’ microscope series, with interchangable top lenses mounted on a revolving nosepiece. The three top lenses mounted here are for normal functioning with an aplanatic 1.4 top, a low power system for objectives from x2.5 and a top lens for darkfield illumination. Pictures courtesy of Fr. David Evans.

 

Optics (imaging system)

Optics supplied with this microscope were the typical short barrel (33mm) achromat series 8/0.20, 20/0.40, 40/0.65 and an apochromat 90/1.30.  Again, they are of typical pre-war Zeiss design. Of course newer developments like the anti-reflective coating technology were quickly incorporated, but many of the lenses were still principally based on designs from Abbe’s times.

This is not necessarily a bad thing: early lens designs were as simple as possible, which in turn created bright and contrasty objectives that were relatively easy and therefore more accurately to assemble then more intricate designs with many lens elements. Of course, the lenses show strong field curvature as the manufacturer did not compromise its design to incorporate a flat field (a cut glass lens by nature shows an image in a curve rather than a flat field). Not surprisingly therefore, this achromat series compares very well with modern lenses in the same class, and is still produced by LOMO for their Multiscope/Biolam student microscopes.

The bino-head is coupled to the microscope via a ‘D’ elbow piece. This incorporates a Telan lens system, so that it does not increase the optical tubelength. Indeed, a straight monocular tube can be fitted either via the D-joint or directly on top of the arm of the microscope. The latter is actually my preferred option for critical microphotography where I do not want any prism or other glass in the way between the objective and the ocular.

Due to the standarization of the system, parts from the more modern –val series of microscopes (Amplival, Ergaval, Jenaval) can be used on this microscopes, as well as LOMO components. 

The microscope in use with a Coolpix camera via the straight tube (left), and modularity possibilities with components from newer lines of microscopes. Here, an ausJena beamsplitter for creating a trinocular for photography, or to look at the image with more then one person (here with a LOMO 45°tube).
 

The binocular head does show its age of design, with one eye tube receiving slightly less light then the other. While that sounds compromising, it has to be remarked that the brain is a very good fault corrector so it is not noticable in practise.

Oculars supplied with this microscope were a set of 10 times PK (plan-compensation) oculars, with high eyerelief and therefore are usable with glasses. In fact the oculars are very similar to the well-known Leitz Periplan 10* oculars in construction. However it has to be said that the Leitz oculars are clearly superior with better defenition outside the center of field.

This instrument has been in careful use in a botanical institute in the Netherlands. It finished its professional career however rather quickly and ended up in the cupboard when priorities in the laboratory shifted to other techniques and more modern instruments.

After 25 years the grease in the fine gear was getting rather stiff and had to be replaced, which was a job that had to be done by a specialist microscope service company. The resulting price tag effectively rendered the instrument a write-off for the institute, but certainly not so for the amateur.

In a rather gossipy tone, the service technician told me that he thought the greases used at the time in Eastern Block microscopes would better suit army vehicles then optical instruments, and I was pleased to hear that the new lubricant should last practically forever.

Different generations: the Jena Ng next to a more modern
research microscope from Zeiss-West (Axiophot, late 1980’s).

 

Thanks to Dr W. Wimmer from the Zeiss web archives (http://www.zeiss.de/archiv) for a pdf download of the instruction manual for the Jena Ng. This manual can be accessed at http://www.science-info.org/micro/docs/zeiss/

Also thanks to Fr. David Evans, for the use of two images from his ausJena pancratic condenser system.

- "while the objective [of the L stand] lens stayed stationary". In fact, in case of the L stand, the stage is fully stationary and both the coarse and fine focus moves the arm up and down. In addition, the Lu stand allows the stage lowered for using the Auflicht kondensor (for epi illumination).

 

- "larger version Lu/Nu", the corresponding version in the N series is called Nf, with lowerable stand (see above), interchangeable stage console, and a special interchangeable base for the pancratic condenser. The Nu model is a later and much larger infinity corrected design with M19 objectives, even though most accessories (stages, condenser, binocular heads etc.) are compatible.

 

- Coarse focus friction. It is in fact adjustable by a collar behind the coarse focusing knob.

- Swing-in lens for low magnification. In my opinion it is slightly advantageous over the usual swing-out front lens design, for it allow changing back to low magnification temporarily even when condenser immersion is applied.

- The pancratic condenser cannot be used with Ng, for it needs a special element in the base consisting of an iris and an auxiliary lens. The latter is inevitable for correct working.

- The later models in the Ng/Nf series were fitted with long barrel lenses.

- Both Ng and Nf could be used for polarization microscopy using a special accessory. Nf could also be used for epi illumination using the Auflicht kondensor.

- There was a third model in the N series called POLMI-A. It is a special polarization microscope. Its construction is similar to Ng, but it has a higher arm, which allows inserting either a complex analyzer module with centering slide-in lens holder, or the Auflicht kondensor without lowering the stage. It is also equipped with a rotating stage and a very versatile polarizing condenser with swing-out top lens and also a swing-out frosted auxiliary lens for the lowest magnifications. Finally, its (interchangeable) monocular tube has a built in Bertrand lens and an iris for confocal microscopy.

 

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Published in the December 2004 edition of Micscape.

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