Microscopes and Other Equipment: Restoring the Nikon S

Nikon S-KT
1971 Nikon Model S with KT base, providing Kohler Illumination

The Nikon Model S microscopes from the 1970s, with their quality all-metal construction, excellent craftsmanship and optics, shiny black finish, and multiplicity of options, are still some of the best general-purpose and research microscopes ever developed:

“…the black Nikon S scopes that were first released in the 1960s … are still hallmarks of versatility and quality and have developed a faithful following despite their age. The Nikon SBR was the first, the Nikon Ske and Nikon Skt, were research grade microscopes, and the Nikon Lke with constant Kohler illumination and changeable nosepiece was the top of the line in its day. Nikon designed these scopes with many interchangeable components so that a scope could be customized to every application…”
(Courtesy Fred Durette, http://snakesafe.jalbum.net/)

The image above is of my own, recently-restored Nikon S-Kt.  However, this wonderful line of microscopes suffered from one serious design flaw, ironically resulting from one of the few plastic (nylon) parts on this otherwise superbly-crafted instrument:  the infamous Nikon S fine focus spur gear.

This nylon gear, used probably because it never needed lubricating, was constructed as a 107-tooth nylon ring surrounding an inner brass collar pressed onto an eccentrically-mounted steel ball bearing on the fine focus shaft.  Since molded plastic materials were not as structurally sophisticated in the 1970s as they are today, with time this plastic ring contracted and broke, disabling the fine focus mechanism and causing thousands of service calls.  Brass replacements for this gear could be purchased as aftermarket parts for a number of years, but are almost completely unavailable today.  Hundreds of otherwise excellent Model S microscopes have been shelved or broken up for parts for no reason other than this one defect.  This problem is well known in microscopy circles.

I purchased this Model S-Kt scope with lenses on eBay for $75.  It had not been used for 20 years and was advertised as “…not turning on…”  This problem was fixed in 10 minutes by re-soldering a flange on the bulb, and, apart from some dirt on the knobs and a few paint chips and dust, the instrument was in beautiful shape.  However, the fine focus mechanism moved unevenly, revealing a broken fine focus gear and a potentially serious problem.

Changing The Model S Spur Gear

After a week of fruitless phone calls all over the United States, Bob Lair of ScopeDoc, a fellow member of the international Amateur Microscopy group, suggested that I contact an old associate, Mark Morris:

Mark Morris, Micro-Maintenance Inc.
2648 Simpson Circle, Duluth, Ga 30096
770 449-6896 (Fax 770 446-5761)

Mark, the only North American source for these gears, works with a machinist who hand manufactures replacement Nikon S brass spur gears,  and had two remaining from the last lot – probably a significant fraction of the world’s remaining supply.  With $50 and a few days’ patience, I had my gear .  Replacing it turned out to be a relatively simple process, as described below.  Do NOT lose any of the irreplaceable small parts!

Note: This gear was one of the last of a group manufactured in 2004.  The latest batch of hand-crafted gears is more expensive to manufacture and will sell at $75 US (Joan Morris, personal communication).

First, viewing the microscope from the back, remove the right fine focus knob by removing the single screw holding it onto the shaft.  Then, using a specialized spanner inserted into the two small holes in the underlying cover (a lens wrench works well), unscrew the circular cover that forms the center of the larger coarse focus knob:

Removing the fine focus mechanism cover
Removing the fine focus mechanism cover

After the cover is removed, the spur gear is visible, with the crack appearing as a dark space in the white nylon gear:

Cracked spur gear and brass replacement
Cracked spur gear and brass replacement

Then remove the left fine focus knob, being careful to retain in the proper order the two small washers beneath the knob:

Left side of fine focus shaft with two small washers
Left side of fine focus shaft with two small washers

Once the left knob and washers are removed, the whole fine focus shaft and spur gear assembly should easily pull out from the right side:NIKON SPUR GEAR 10

Spur gear assembly
Spur gear assembly

The toothed nylon gear should pull off easily, but this leaves the underlying brass collar that has been firmly pressed onto the eccentrically-mounted steel ball bearing.  This can be cut off carefully with a small carborundum wheel mounted on a Dremel tool.

Cutting the brass inner collar of the spur gear
Cutting the brass inner collar of the spur gear

Cut carefully through this brass ring until only a thin bridge of brass remains, then crack the bridge by inserting a screwdriver into the slot and twisting it.  This protects the bearing from being cut by the grinding wheel.  The collar can then be readily slid off:

Spur gear collar after removal. Note the thin inner bridge of brass that has been cracked to remove the collar.
Spur gear collar after removal. Note the thin inner bridge of brass that has been cracked to remove the collar.

The final challenge is to press on the new brass gear.  This can be done by one of three methods:  Tapping the carefully-sized new cylindrical gear onto the bearing (recommended by Mark), using concentric rings to press or tap on the gear, or employing the thermal expansion method.

A combination method proved most effective.  I cooled the bearing for an hour in the freezer, then heated the cylindrical brass gear on a stove element until it was just uncomfortably hot.  Holding the gear with a tea towel, it could be pressed half way onto the cold bearing with firm finger pressure before temperatures equalized and it seized in place.  This oriented the two pieces.  I then placed the assembly flat on the edge of a counter and tapped the gear gently with a small hammer, rotating with each tap.  This proved effective but slow, so I grasped the edges of the gear and bearing between the jaws of a pair of plumber’s pliers.  Applying gentle pressure at intervals around the gear and bearing, rotating the assembly between each gentle squeeze, quickly but gently pressed the gear onto the bearing and leveled the two surfaces.  The gear proved to be very carefully machined and was easily guided to a smooth and precise fit.

The whole shaft and gear assembly was then carefully cleaned, lubricated with a sparse amount of grease, and reinserted into the scope.  The washers, right cover, and knobs were quickly replaced.  The reassembled fine focusing mechanism now moved silkily, with absolutely no hesitation.

NIKON S-Kt 02 BW.jpg
The finished scope



Note:  Original Nikon S System advertisement 1963:
(Anal. Chem., 1963, 35, pp 91A–91A)Free first page

Microscopes and Other Equipment: Restoring the Nikon S

Moss, Rain, and Rocks: Pond Life Outside the Pond

Desmid Gel on Moss, Mt. Richards.
Desmid Gel on Moss, Mt. Richards.

Not all of water-based life lives in ponds, ditches, and lakes.  In this damp climate, there is water everywhere except in the driest season – within cracks in tree bark, in moss, on rain-drenched leaves, and in the damp of the forest floor.


While hiking around midnight in our constant rain on misty Mount Richards, I noted this gelatinous mass on a dripping mossy rock face:

Desmid Gel. Note water droplets on lower surface of the mass - this is a constantly dripping rock face in winter and spring.
Desmid Gel. Note water droplets on lower surface of the mass – this is a constantly dripping rock face in winter and spring.

I believe that this is the desmid Mesotaenium, which is known to grown on mossy rocks with a constant water supply:


It has been documented in both England and the Netherlands, and consists of dispersed cells in a mucilaginous matrix forming a gelatinous mass among moss stalks. Note that there


Mesotaenium in Gel Matrix, 40X. Note smaller desmid species barely visible in background.

is also a population of smaller algae, likely also desmids, seen at 400X  together with one Mesotaenium cell.

Mesotanenium and Smaller Desmid Species.
Mesotanenium and Smaller Desmid Species.

Images of the larger desmid species show shape and chloroplast structure typical of Mestaerium:

Mesotaenium, 400X. Note chloroplast structure and surrounding cell wall.

In some images, the refractile mucilaginous sheath surrounding individual clusters of desmid cells is visible:


This is a link to an older but informative article discussing mucilaginous algae as well as 1980s theorizing about diatom movement – 1980s knowledge and one of the more pompous and verbose articles in the literature (“…palmelloid cell masses of coccolithophorids…”), but also very infomative:


Note on pages 120-122 the comments on the significant role played by mucilaginous gels in Mesotaenium and other algae:BONEY MUCILAGE 01

as well as the potential size of some of these sheets of algal gel habitats:


Nice result from my mountainous wanderings with a headlamp at midnight in the cloud layer.


If you explore all possible things that are common and unnoticed, the world will never a boring place.

I have one old car –  a beloved 1993 Mercury Topaz bought for a dollar from my uncle’s estate, 22 years old and with only 60,000 miles on the odometer.   From the era when Detroit thought it could build cars from rubber bands and old sardine tins, it is grossly underpowered, occasionally blows a tired old engine seal, and has a back window that repeatedly leaks – yet it has proven to be a faithful and undemanding servant.  After a few months of continuous rain, its rubber window seals turn green with a film of algae.

This note is for Kelly Averill Savino, to answer your question regarding my picture of ugly little desmid blobs in a pot.
The implication behind your question was “Whatizzit? What desmid mass? Surely not those ugly little blobs that you can barely see??? Wipe them off with a Kleenex, PLEASE!”
That’s the point. Desmids (see Wim’s lovely images: http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/wimsmall/algdr.html) are some of the most beautiful objects that you can see under the microscope. However, in their appearance in nature, they can take the prize in the category of “ugly-little-masses-that-only-a-mother-could-love.”
The point of my post on Triple Tree (and one of the least appealing pictures ever posted on this site) is that they are easily missed and not very appealing when you find them.
Gel formation and mucilaginous coatings are very important for algae, and some desmids seem to take this to the extreme. I find them on midnight rainy mountain walks, forming ugly little snotty blobs on shaded, drippy rock faces. Or, as in the last note, forming equally ugly, snotty little blobs in an overwatered nursery pot.
However, under the microscope, these nasty little blobs form an amazing and complex little world. The one in the images, formed on lichen on a drippy rock face, contained gels within a gel – a mucilaginous blob, containing multiple tiny gel spheres of higher refractive index (looking rather like a tapioca pudding at low magnification), each containing its tiny cluster of bright green desmid cells.
This gelatinous little world also hosted other organisms, in this case a tiny nematode and a branching black mold.
The Boney article, though dated and verbose, contains much thought-provoking information on algal mucins:
There is much interest in algal mucins in the health and biotech field because they apparently have medicinal and nutritional properties:http://news.algaeworld.org/2015/08/5-functional-properties-of-marine-algae-for-animal-health/#more-18160
Moss, Rain, and Rocks: Pond Life Outside the Pond