Maybe you know a trustworthy second hand dealer, in this case I would also take second hand microscopes into consideration. There are simply too many things that can go wrong, even if the quality of the second hand microscope is otherwise quite good. Be aware that a quality second hand microscope from well-known manufacturers can be obtained for a fairly low price. If well maintained, these microscopes may provide greater value (and more spare parts) than new no-name devices, which may be more expensive. Still, without advice you run the risk of buying a microscope with objectives that are not appropriate for you, or microscopes that are not operating reliably. There is no way to see from a picture if the objectives are intact, if there is no stage drift and if the gear operate smoothly.
Hospitals and research institutions sometimes sell useful used microscopes, but these may be equipped with specialized optics, which can push the price up. Unlike consumer products, which come out of the box, microscopes are commonly assembled according to the research needs and second hand microscopes may have a combination of (expensive) parts that are required for a specific research or medical task, but which may not be necessary or suitable for enthusiast microscopists or for educational use.
Also do not buy microscopes from people who do not know much about them. Non experts are not able to assess the quality of a microscope and are not able to give a reliable account on the operation of the device. I have seen horribly over-priced used microscopes on Online auction websites. Some of them were in a very bad condition and it was clear that the seller did not know how to assess the quality of the microscope (or its true value).
If you do choose to buy second hand microscopes, keep the following check list in mind. Again, not all of these points can be seen by looking only at a picture.
Here are some problems that can be expected from older microscopes:
- Inappropriate objectives: Some research microscopes are equipped with special purpose objectives, which can be expensive, but which may not be suitable for general-purpose microscopy work.
- Smoothness of operation: Are all mechanical parts operating lightly and smoothly? Knobs which are difficult to turn are a clear sign that the oil in the gearing system has become solid and that the microscope needs to be cleaned and re-oiled. Over the years the lubrication oil in the gears accumulates dust, which causes it to solidify. Some natural lubrication oils also solidify over time. Do not force turn the knobs, as this might damage the gears.
- Stage drift: Older microscopes may show stage drift. In this case the stage does not stay where it is, but is pulled down by gravity. it is then impossible to focus the microscope, because the focus is lost immediately again. The gears need to be tightened in this case (servicing costs). If not repaired, microscopes that show a stage drift are completely useless.
- Delamination of objectives: In older objectives, some of the lens elements may become unstuck from each other. The kit holding gluing together different lens elements becomes loose. Air bubbles start to appear between the lenses of the objective. these can only be seen when looking through the objective from the inside out (the objective has to be unscrewed from the revolving nose piece). Delamination will reduce contrast and image quality, and is a sign that the microscope is older. The is nothing that you can do about this. Delaminated objectives may still be usable, but there is some reduction in resolution.
- Fungal growth: Microscopes which have been used in tropical areas may show fungal growth on the optical surfaces. Some of these may be in parts which are not directly reachable. This is the reason, why some manufacturers produce optics with an anti-fungal coating on the glass surfaces.
- Loss of coatings: Te optical surfaces of the microscope are coated to prevent light reflection and to increase contrast. The use of improper cleaning solvents might remove the coating.
- Dirty objectives: I have seen objectives which were covered with solidified mounting medium.
- Loss of lens coatings: This is due to improper cleaning technique and the use of the wrong cleaning solutions.
- Bad general appearance: microscopes which shows sign of external damage could have been dropped. There is the possibility that there are other problems as well.
- Cracks and other mechanical defects: This can include broken off teeth of the gearing, cracks due to drops, etc.
- Cracked objectives: Take care that the objectives are clean and not cracked. Unscrew the objectives and look through the objective from the back side. Cracks then become visible.