If you have any plans purchasing a microscope, it is important to list a few things. The most important thing is to think about what you are going to observe with the microscope and which magnifications are required for this. In the microscopic world you have to deal with subjects that vary in size from roughly one micrometer to a millimeter, a 1000 fold difference. In a single drop of water from a ditch you can simultaneously find organisms that differ that much in size. Organisms with the above dimensions are observed with a biological microscope, also called transmitted light microscope.
For the novice hobbyist
Everyone knows the classic microscope, the so-called horseshoe stand. Many people who followed biology classes in high school are familiar with them. They are monocular microscopes with a straight body tube and a horseshoe shaped base. These relatively simple microscopes are good to start with. For a beginner, it is not recommended to spend a lot of money on a first microscope, especially since many people do not know whether it will be a long lasting hobby. For some people, the initial interest may disappear after a short while. In that case, someone would want to sell the microscope again and the more expensive it was, the more difficult this will be. If someone starts with a simple and cheap microscope, it is easy to sell and to upgrade to a more advanced instrument when the interest in the hobby stays.
With microscopes, buying an older model on the second-hand market pays off. Many microscopes from the last century were very solidly build. The color of a microscope indicates the age. Brass microscopes are very old (1920s and older) and are in general less suitable to work with. These very old microscopes are more for the collector and they serve better as an ornamental object. The most suitable microscopes are black (roughly 1940s-60s) or gray (roughly 1970s). In these time periods very little or no plastic was used in microscopes. These microscopes are often made entirely out of metal and are quite heavy, which makes them mechanically robust and stable instruments. Today's new microscopes are generally of inferior quality. I have a strong preference for black microscopes. You don't get them any better than that and they are made of a quality that surpasses everything that is being produced today. Experienced microscope users sometimes say: "if it's not black, send it back!". But also many gray microscopes from the 1970s are very good. Like other optical instruments, a microscope does not age.
The used market is thus an interesting place to look for a first microscope. Quite some people have a microscope for sale that they don’t use anymore. Quite often however, the owner cannot tell whether the microscope is in good shape or he/she has little experience with it. But a horseshoe stand is mechanically and optically not that complex and this makes the chance of a defect somewhat smaller. There are a few things to keep in mind when looking for a used microscope:
● Avoid toy microscopes and the like. The used market offers quite some toy microscopes that look a lot like a real microscope. Older toy microscopes are often completely made of metal and at first sight they seem decent. By looking at the specifications written on the objectives and the height of the stand, it is quite easy to distinguish such a toy microscope from a genuine microscope. A real microscope has achromatic objectives on which specifications are written like mechanical tube length (mostly 160 or 170 mm), cover glass thickness (0.17 mm) and aperture (this value depends on the objective). For example, on a 40x objective the following may be written: 40/0.65, 160/0.17. Sometimes not all of these values are mentioned, but the aperture should always be indicated. And that is the value 0.65 in the example above. The objectives of toy microscopes lack these specifications and they will only be labeled with for example ‘40x’. These are not achromatic lenses but single lenses that give poor images that contain a lot of chromatic artefacts. The height of a real microscope (horseshoe stand) is at least approximately 30 cm. Toy microscopes with a horseshoe stand are always significantly smaller than that.
Microscopes with a built-in LCD screen or so-called ‘digital’ microscopes are most of the time a bad (and too expensive!) choice. Here too, no optical specifications are given, the built-in camera is poor and the resolving power is very low if any at all. It sounds so modern and appealing, such a 'digital' microscope. But a microscope is by definition not a modern and certainly not a digital instrument. It is a timeless optical instrument that has hardly been changed since the early 20th century.
● Mechanics. All mechanical parts must run smoothly. Mechanical parts include revolving nose-piece, slide holder, mechanical stage, coarse and fine focusing and condenser adjustment. A simple educational microscope may not have all these parts, but there should always be a nose-piece and coarse/fine adjustment. With microscopes that have not been used for a long time, some parts may have become stiff. Fortunately, with some lubrication, the mechanical parts of decent microscopes can mostly be put back into working order.
● Coarse and fine adjustment. It is important that these run smoothly. You can do little with a microscope of which the focusing adjustment does not work properly. The fine focusing adjustment can be checked by viewing a slide. If it doesn’t work, it is better to cancel the purchase. There are also microscopes that only have coarse adjustment, but these are more difficult to use at higher magnifications. It is therefore advisable to buy a microscope that also has fine adjustment.
● Revolving nose-piece or turret. This is the revolving part that contains the objectives. Horseshoe stands usually have a nose-piece for 2, 3 or 4 objectives and sometimes there isn’t a revolving nose-piece at all and only 1 objective can be used. A microscope that can accommodate only 1 or 2 objectives is not very convenient to work with, so it is better to have a turret for 3 , 4 or more objectives.
● Optics. It is important that the image is sharp. If the image is blurry and cannot be brought into focus, there may be something wrong with the objective. If the image is sharp but has low contrast, than it is likely that the objective and/or eyepiece is dirty. The front lenses of eyepieces and objectives of used microscopes are quite frequently soiled, especially if you buy from someone who hasn’t have any knowledge about microscopes. Objectives and eyepieces can mostly easily be cleaned with distilled water that contains a few drops of dishwashing liquid.
● Objectives and eyepieces. For a beginner, a microscope with objectives 4/0.10, 10/0.25, 40/0.65 and eyepieces 5x (or 6x) and 10x will be sufficient. With this set of objectives and eyepieces, at least 90% of all microscopic subjects can be viewed in detail. A 100/1.25 oil immersion objective is not very useful for a beginner and even many experienced microscopists do not use this objective very often. On the other hand, an objective 20/0.40 or 25/0.45 is much more useful; this is a very convenient magnification which allows you to view many specimens in a pleasant way and with enough detail. Furthermore, a 20/0.40 or 25/0.45 objective has a pleasant working distance: there is a fair amount of space between objective and slide. For dark field microscopy, a 20x or 25x objective is ideal because it is very easy to obtain an impressive dark field image with this magnification. It is a very nice objective for observing pond life in brightfield or dark field illumination. Because it is not a standard objective, you usually won't find a microscope equipped with 20x-25x objective but it worth investing in it if you are really enjoying microscopy.
It is useful to have eyepieces with at least 2 different magnifications so that a number of final magnifications can be made. More magnification is not always better, on the contrary; it mostly gets worse. Also with microscopy, most of the time, less is more. It is all about resolving power (resolution), not about magnification. For example, a 40/0.65 objective gives a more crisp image with a 6x eyepiece than with a 10x eyepiece, while the resolving power remains the same. Some people like wide-field (WF) eyepieces with high eyepoint; you get a larger field of view and you don’t have to keep the eye very close to the eyepiece, which can be useful for spectacle wearers. Personally, I find a Huygens eyepiece more pleasant when looking with one eye. But when using both eyes, WF eyepieces are more comfortable.
● Monocular or binocular. Most horseshoe stands are monocular, only one eye is used. It has both advantages and disadvantages. For a beginner, however, I see more advantages than disadvantages because these microscopes are cheaper and optically less complex (so less chance of defects if you buy second-hand). In addition, some people have difficulties seeing through a binocular (with 2 eyes). Some people cannot merge the images from the two eyepieces into one field of view or they get a headache if the binocular head is not properly adjusted. It is also possible to view the image from a screen by using a good eyepiece camera, smartphone or DSLR camera so that you don’t have to look through the eyepiece. In that case, a monocular microscope is all you need. Viewing with one eye only can become tiring if the other eye is closed. The trick is to learn to observe with one eye while leaving the other eye open. Once you are used to this, you can conveniently look with one eye for longer times.
● Mirror or lamp. Both are possible, but with a horseshoe stand I prefer the good old mirror. With a mirror, a very good illumination can be achieved that is usually far better than a plug-in lamp that you often find on older horseshoe stands. With a mirror you can use any light source, from daylight to LED, it is all possible. And plug-in lamps that use a incandescent bulb can run very hot.
A microscope that often can be found on the used market in the Netherlands is the Olympus GB. This is a very good and solid black horseshoe microscope from the 1950s-60s. This microscope is also excellent for professional work. Both high-magnification dark field microscopy and phase contrast microscopy are possible with it. The special condensers that are required for this fit in this microscope. A somewhat simpler Olympus horseshoe stand is the HSA, (not to be confused with the Olympus ST which has no fine adjustment), a compact student microscope of excellent quality and also suitable for the younger researcher. These two microscopes are just an example, there are plenty of similar microscopes from other brands on the used market that are also suitable to start with.
Fig.1. Olympus GB (left) and HSA (right), two high quality microscopes that can be found on the used market for little money.
Another example of an excellent microscope is the Olympus E, a gray model from the 1970s. This is a very solid microscope that can be expanded with many accessories. It can often be found on the used market, in both monocular and binocular version.
Fig.2. Olympus E, monocular and binocular version. For this microscope, a good external Köhler lamp exists that can be mounted instead of the mirror.
For the more advanced user and professional purposes
Once someone has acquired a taste for microscopy and wants to take it the next level or use it for professional purposes, it is recommended to purchase a microscope for which many accessories are available. A microscope that is often for sale on the used market is the so called Zeiss Standard. This is an all-round microscope that can be expanded with many accessories that are also quite easy to find. From the late 1950s to the 1980s, Carl Zeiss produced the Zeiss Standard microscope, a microscope suitable for medical routine and research purposes. This microscope has become a household name in the medical world and can still often be found in current laboratories. The Zeiss Standard can be easily equipped with phase contrast. With phase contrast microscopy, transparent objects that are difficult to see become readily visible. The visibility of very small, colorless organisms such as bacteria, yeast and many protozoa can be greatly enhanced with phase contrast. In phase contrast, these transparent organisms are seen as black objects.
There are several different configurations of the Zeiss Standard. A few common types include the Standard GFL, Standard RA, Standard 14 and Standard 16. All these models are suitable for professional work and the advanced hobbyist and they can be expanded with the same accessories. The Zeiss GFL and RA microscopes were designed to be used with a incandescent lamp with external power supply. The later Standard 14 and 16 were also manufactured with a built-in halogen lighting, which is easier for routine use. A disadvantage of the built-in halogen lighting on the other hand is that conversion to LED illumination is a bit more difficult. But a Zeiss Standard 14 with halogen illumination is a great routine microscope for courses, laboratory and clinic.
If you are going to look for a used laboratory microscope it may be wise to take someone along who has some experience with these types of microscopes. Unlike the simple horseshoe stands, these microscopes are mechanically and optically much more complex, so the bigger chance that something is not well adjusted or misaligned.
Fig.3. Zeiss Standard 14 with a nose-piece for 4 objectives and built-in halogen lighting.