A Personal and Preliminary Note

I have been fascinated by microscopes for years. I have gradually acquired a varied collection of microscopes, mostly research instruments that were originally manufactured in the 1960s and 1970s. Most of these microscopes had missing parts or other serious problems when I acquired them. I have spent a lot of time over the years repairing and maintaining these instruments. Microscopes with problems generally sell for only a few percent of the original cost because they are basically useless without substantial repairs. Most of my microscopes had problems varying from minor to very severe when I acquired them.

When I retired from the university where I had been a faculty member for many years a few months ago, I brought all the microscopes that belonged to me home. Suddenly my house looked more like a microscope laboratory than my living quarters. Before that I had used many of them regularly in teaching and research activities.

In the course of repairing and maintaining these microscopes I learned many things about microscope maintenance and repair. Perhaps most importantly, I learned many things both to do and not to do.

A Few Words About Solvents

Microscopes and microscope parts frequently require cleaning with solvents. Inappropriate use of solvents can seriously damage microscope parts. Many oil immersion lenses are destroyed each year from this very cause.

All molecules and atoms are in motion from thermal agitation. In fact, temperature is a direct measure of molecular translational kinetic energy. If it were not for intermolecular or interatomic forces the entire Universe would be in the gaseous state! There are three very different kinds of forces that can occur.

• 1. "Van der Waals" forces. Electrons have negative charges, atomic nuclei have positive ones. When two molecules or atoms approach one another the electrons in the neighbouring structures tend to repel one another at any instant creating instantaneous partial charges in the structures that cause the structures to attract one another. This attractive force is present in all materials, the strength of this attraction depends on the surface area of the structure, and on how "polarisable" the electrons are in the two structures. Electrons are said to be polarisable if they can be moved around easily. Outer electrons in atoms with high atomic numbers are much more polarisable than the electrons in atoms with small ones.
• 2. "Polar" forces. The "electronegativity" of atoms varies. This is a measure of the tendency of an atom to pull electrons toward itself when it forms chemical bonds with other atoms. Generally when chemical bonds form the bonds have what is termed a "dipole moment" because one of the atoms making the bond interacts more strongly with the electron pair making the bond than the other does. This causes the resulting molecules to have permanent positive and negative ends that interact with neighbouring molecules. This force can be very strong in some molecules like sulphur dioxide.
• 3. "Hydrogen Bond" forces. When hydrogen is bonded to an electronegative atom it will carry a partial positive charge. This positive charge will interact strongly with non-bonding electrons in fluorine, oxygen, and nitrogen. This force can be extremely strong. It is the primary force holding water molecules together in liquid and in solid water.

Water is a very polar molecule that interacts strongly with other polar molecules both by its polar nature and by its extreme tendency to undergo hydrogen bonding. However, paint, most metallic surfaces, glass, and lens coatings tend to be highly insoluble in it. This is good! It makes water an appropriate material for cleaning most microscope parts.

Methanol, ethanol, and propanol are also rather polar, and also show substantial hydrogen bonding. They can, however, interact with some paints, and, surprisingly they can sometimes react rather rapidly with certain metals. Organic compounds dissolve MUCH better in these materials than in water. They may be appropriate for cleaning microscope parts when water does not work, provided they do not attack the surfaces. They are generally safe for lenses, however, one should be careful not to use so much of them that they interact with lens cements.

Benzene, toluene, and xylene are not polar at all. They also do not interact via hydrogen bonding as they contain no nitrogen, oxygen, nor fluorine. However, they contain aromatic rings that interact with many materials, especially materials that contain other aromatic rings, causing them to dissolve many organic materials very well. Above all, they will very often remove paint and they dissolve the cements used to cement the elements of lenses together. Many valuable lenses have been destroyed by inappropriate uses of these solvents. They should be used with GREAT caution. They, especially benzene, are highly chronically toxic.

Hexane, heptane, octane, and other alkanes, because of the nature of the carbon-hydrogen bond, are virtually non-polar in spite of their being some electro-negativity difference between carbon and hydrogen. Thus essentially the only forces holding these materials in the liquid state are the "van der Waals" forces. These materials will generally dissolve other non-polar and weakly polar materials quite well, but tend to show little interaction with most other materials. Hexane boils at only 63 degrees, heptane at 98, and octane 125. This makes them excellent solvents for cleaning lenses—and far safer than xylene or toluene!! Alkanes used for cleaning lenses need to be pure and free of high molecular weight materials. Mixtures of low boiling alkanes are sometimes sold as "petroleum ether" or "ligroine".

Chlorinated solvents are often rather toxic. Ones with unsymmetrical chemical structures are often quite polar. These materials tend aggressively to attack paint and other surfaces. Carbon tetrachloride is non-polar because it is completely symmetrical. It would be a good material for use around microscopes if it were not so toxic. Chloroform and methylene chloride are polar and they are particularly aggressive in attacking many types of painted surfaces. They also aggressively attack lens cements.

Other solvents such as acetone and ether might conceivably find use for cleaning microscope parts. Ether is a fire hazard because it ignites at only 190 degrees. Acetone is very polar and can attack some varieties of paints and lens cements. Commercial acetone also often contains non-volatile impurities.

It is important to remember that many solvents are highly flammable and that they can create a serious fire hazard.

Cleaning and Maintaining non-optical Parts

The best way to clean non-optical parts of microscopes is to use a soft, slightly water dampened cloth. Sometimes surfaces may be contaminated with sticky organic compounds. These will usually clean very well with a cloth dampened with alcohol. Should this fail a cloth dampened with hexane or heptane will likely solve the problem.

As implied earlier, it is usually an extremely bad idea to attempt to use xylene, toluene, chloroform, dichloromethane, acetone, or similar aggressive solvent because they tend to attack paint and some may attack metal. Serious damage can result from using such solvents. If in doubt test on a tiny area first!

Microscopes should have dust covers to protect them when not in use. Do NOT use commercial covers made from clear polyvinyl chloride. These contain plasticisers that are not only powerful estrogens, but also they tend to condense on microscope surfaces. It is best to make covers from cloth. Cotton often produces a lot of lint. Closely woven polyester cloth seems best. Simply cut two pieces that match the microscope's profile and sew one to each side of a rectangular strip that is approximately the microscope's maximum width. Over-lock sewing machines work best, though any sewing machine can be used. Some microscopes (like the Leitz Orthoplan) require oddly shaped dust covers, but most dust covers can be made in a few minutes. It is a good idea to wash a new cloth microscope cover before putting it into service to remove possible lint.

Occasionally it may be necessary to apply enamel to a microscope or a part of a microscope that has become scratched or that has had its surface protection wore away by years of use. It is best to sand the damaged area (carefully!) and carefully mask it with paper and masking tape, and then apply epoxy enamel. Older microscopes are usually black, so it is easy to match colour with them. Most other types of enamel are MUCH less durable. Spray on the enamel in very thin coats so that the coat is uniform and does not "run." Appearance can be dramatically improved by doing this provided great care is taken to do the procedure perfectly.

A little effort to protect microscope surfaces can result in their needing far less later maintenance.

Lubrication

Many microscope moving parts require careful lubrication. Faulty lubrication can result in serious damage or even destruction of microscope components.

Many common lubricants are highly unsuitable for use on microscopes. Some oils and greases contain molecules with double bonds. Double bonds are reactive and can polymerise and undergo other reactions over time that can convert them into fairly rigid solids that can effectively cement parts together. Other lubricants contain volatile low molecular weight components. Because these components have appreciable vapour pressures, they gradually become more viscous as the volatile constituents are lost. To make matters worse, the volatile components may condense on nearby optical surfaces.

There are special high quality lubricants available for optical instruments that minimise these problems. There seem to be few, if any, businesses specialising in providing lubricants for microscopes, but there are many Internet sites that sell lubricants for mechanical clocks and cameras that are highly suitable for use with microscopes. Although most of these lubricants are generally extremely expensive, the suppliers generally sell 10 and 25 mL bottles for very reasonable sums.

It is a standard rule NEVER EVER to lubricate objectives. The only exceptions to this might be to provide a VERY thin film of high quality oil to the outer shell of retractable lenses or to the adjusting collar of objectives with them. This should only be done, even then, if problems develop that could be cured by lubrication.

NEVER EVER apply an oil lubricant to a surface lubricated with graphite, and never apply graphite lubricant to a surface lubricated with oil. I once acquired an Olympus BHA with a trinocular head in which someone had used previously used both graphite and oil to lubricate the slide that switches between the camera port and the two visual ones. If Olympus had not made this dovetail fitting so that one side of it was held in place with screws the head would have to have been discarded, as it was absolutely impossible to move it at all!!! After removing the side of the dovetail fitting and cleaning with hexane the part worked fine again.

The moving parts of mechanical stage require fairly frequent lubrication. Depending on the design the lubricant may need to be anything from light oil to heavy grease. For the ones that require low viscosity oils it is particularly important to use high quality lubricants that do not increase in viscosity over time.

Focus mechanisms generally require heavy grease. Again high quality greases that have long term stability must be the only ones employed for this purpose.

Iris mechanisms and oil generally make a very bad combination. These generally are designed NOT to be lubricated with oils at all. When an iris is lubricated with oil, the oil will tend to become more viscous in time. Eventually it will become so viscous that when an attempt is made to close the iris, the blades will buckle. If one be lucky, one might be able to disassemble the iris mechanism, clean the blades with hexane, and reassemble the iris. It takes a bit of skill, however, to do this. If any of the blades should be damaged by this type of mishap, repair becomes virtually out of the question, because it is extraordinarily difficult to fabricate new iris leaves. A word to the wise. Do not oil iris mechanisms. (I recently successfully repaired a Zeiss microscope's substage illuminator's iris that had been coated with oil sometime in the past. Its blades buckled. I was able to disassemble the mechanism and clean the blades with hexane. There was so much lubricant on them that the hexane was bright yellow afterward! The buckled blades were only slightly bent so I was able to straighten them and reassemble the iris. It must be emphasised that this is a VERY difficult mechanical task that can be avoided by keeping oils away from irises!

Whenever grease or oil needs to be removed the best material to use for this purpose is almost always hexane. A reasonable substitute is liquid camping stove and lantern fuel. In either case, care must be taken to avoid catching the material on fire!

Cleaning Optical Parts

It is common to find old oculars and objectives that have been destroyed by careless cleaning. Careless cleaning can scratch optical surfaces. This type of damage can always be prevented by using only proper lens cleaning lens paper and brushes.

Careless cleaning with improperly chosen solvents can also damage lens cements. Oculars with damaged cements can sometimes be repaired, but objectives are generally rendered completely and irreparably useless by damage to their internal lens cements.

Many people have the erroneous idea that xylene is a good material to use for cleaning lenses. NO!!! Xylene is an aggressive solvent for lens cements. The same goes for acetone, methylene chloride, toluene, and many other common solvents.

Cotton swabs (like those sold under the commercial trade name "Q-tips" can be used very effectively for cleaning lenses. It is claimed, however, that they sometimes contain silica particles that can scratch lens surfaces. Thus it may be a better idea to use swabs like this but made from synthetic fabrics. Solvents may be necessary for cleaning lenses this way. There is little doubt that hexane is the BEST solvent for this purpose. Its boiling point is only 63C. Reagent grade hexane will leave no residue. It is totally non-polar, so it tends not to attack lens cements. Its high vapour pressure causes it to evaporate before it can get between lens elements anyway. The best way to use swabs to clean an optical surface is to moisten the swab with hexane, and then move it around the surface, starting at the centre, in a spiral motion. This is particularly useful for oil immersion lenses. If everyone would use hexane instead of xylene there would be FAR more old immersion lenses in perfect condition and still in service today! (Caution: One should check to be sure that swabs are not attacked by solvent before using them on lenses. One could end up depositing a thick layer of polymer on a lens if the solvent dissolve the swab material!!)

Never attempt to disassemble an objective. In general it is simply impossible to disassemble one and get it back together again. The only exception to this rule is very simple low power ones with few elements, which can sometimes be removed from their cells, cleaned and reassembled.

Sometimes, however, the inner shell from objectives with retractable elements can be removed for cleaning or service. Sometimes also phase rings from phase objectives are removable so that they can be removed or re-centred.

Objectives are expensive. Many apochromats come under the heading of extraordinarily expensive! Clean objectives with the greatest possible care! It is unfortunate the oil immersion lenses are so easy to damage because they frequently require solvents to remove immersion oil residues. This damage can be largely prevented by using hexane instead of aggressive solvents like xylene. Swabs are particularly useful for cleaning immersion lenses.

Dealing with Missing or Broken Parts

Many microscopes contain several thumb screws, usually for adjusting the alignment of optical parts in substage illumination systems and condensers. For some reason it seems very common for these to be lost or badly damaged.

There are three ways to deal with this problem:

• 1. Replace the thumb screw with an Allen head screw. The disadvantage of doing this is that after this it will be necessary to keep an appropriate tool with the microscope.
• 2. Place a suitably sized piece of metal rod stock in a turning lathe and cut down the stock to the size of the required thumb screw. Knurl one end, and turn the other end down to the diameter required for cutting threads for the diameter that is required for the missing piece. Use a suitable sized die to cut the threads. One can make an exact replacement of any thumb screw this way in a few minutes, but it requires the use of a metal lathe.
• 3. Obtain a piece of threaded rod with threads that match the missing thumb screw. Take a piece of metal rod stock of appropriate diameter, and drill a hole through its centre that can be threaded to match the threaded rod. Use a tap to cut threads like this. Screw this unto a piece of the threaded rod. If one use a lathe for this one can also knurl the rod stock and turn it down to exact diameters and also make it have shapes other than that of a simple cylinder. It is best to drill a hole into the side of a thumbscrew made in this manner with a small bit and thread the hole for a small set screw. This will prevent the threaded rod from coming out of the cylinder.

For some reason microscope manufacturers have often used very small square drive male head screws on phase rings centring mechanisms and on objectives centring mechanisms on polarising microscopes. The special tools required to turn these screws have an amazing tendency to get lost! Pocket watch winding keys are available in many sizes that can be used for replacing these, though one may need to fabricate handles for them.

When dealing with older microscopes there is a frequent problem that is likely to rear its ugly head, namely older metric threads have a different profile from the now standard 60 degree threads. When this problem arises it is generally best simply to run a modern tap with the same thread pitch into the fitting and cut the threads to the modern profile. Because the profile difference is slight, there is little resistance to the passage of the tap during the rethreading operation. (One should be very certain before doing this that the thread pitch is still the same! Failure to take this precaution can cause serious damage.)

A surprisingly large fraction of possible missing parts can be fabricated very easily with the use of lathes and milling machines. Small metal working machines generally work far better for making small parts than the large industrial ones used to fabricate parts for giant machinery. With sufficient skill in metal working practically any part can be fabricated without much difficulty, in fact.

Electrical Parts

The mechanical parts of well made microscopes are fabricated with such care and with such quality materials that they should easily last a century even with frequent use. Electrical parts are a VERY different matter.

Electrical cords are especially prone to problems. Fortunately it is generally a simple matter to purchase pieces of wire, remove the old wire, and replace it with the new. It is often necessary to make solder connexions, and that kind of repair is very simple to accomplish.

Older microscopes did not use solid state devices, and that means that electrical parts are generally restricted to transformers, wires, switches, and bulbs. All of these can be replaced.

Many used microscopes are sold without the original illuminating systems. Instead of trying to fabricate or locate replacements for these illumination systems, it generally makes far more sense to design a replacement illuminator with high power LEDs instead. Many articles on the use of these devices have appeared in Micscape in recent years. The K2 and 3 watt "Star" LEDS are particularly easy to mount in fabricated illuminators.

One should remember that high wattage LEDs need to be mounted to a conductive heat sink. Aluminium is an excellent conductor and relatively inexpensive, so LED holders are perhaps best fabricated from it. Copper is a far better conductor, but it is much more expensive and rather difficult to machine.

Some aluminium alloys are quite easy to machine, so, again illuminators can be fabricated that make precision fits into the illuminator socket of the microscope for which they are being designed. LEDs do require special power supplies. The most convenient thing to do is to obtain a constant current power supply. There are special ones available for LEDs, but other ones with appropriate voltage and current ratings also work adequately.

Because LEDs produce extremely concentrated essentially point source light, there is some evidence that they can cause eye damage if one stare directly at them because a lot of light can get concentrated on a tiny spot on one's retina. This is particularly a problem with LEDS that have strong emissions on the short wavelength end of the visible spectrum. Thus one should be careful to avoid looking at exposed illuminated LEDs. When LEDs are used for microscope illuminators, however, the light is spread out to cover the whole field, as it is in a conventional illuminator.

Another reason to fabricate LED holders for older microscopes is the simple fact that some older tungsten filament light bulbs are no longer be obtainable. One should remember that the LEDs are a lot better anyway! Unlike filament light bulbs their spectra do not change with current. They also are a closer approximation to daylight.

One final note: Always carefully plan all microscope repair and maintaining operations before beginning them. NEVER rush!

All comments to the author Robert Pavlis are welcomed.

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