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Kinds of Microscopes

Stereoscopic Microscopes

Stereoscopic microscopes fill the magnification gap between the hand lens and the standard compound microscope. A dissecting microscope's magnification range usually runs between 10X and 60X. The objective of a standard microscope will often have a shorter focal length than the same power objective of a stereo microscope. This is because the objectives on the stereo microscope are designed to allow the maximum work space under the objectives. The image is erect, not inverted. The erected image may be caused by the ray paths not crossing (as they do in a standard microscope), or it may be caused by special prisms used to flip the image. The view is truly stereoscopic. Each eye sees a slightly different view of the specimen. Either a complete optical system is provided for each eye, or a beam splitting prism is set up so that each eye sees a slightly different view.

These microscopes are useful for microassembly, forensic science, entomology, and biological dissection. If you have none of these specialized interests, here is a quick project to create a useful accessory to the standard microscope.

The first figure shows a mount for a common hand lens, available in most drugstores, that will leave both hands free for dissecting and slide making. designs. The second figure shows a slightly more complex mount, with two hand lenses. The top lens is removable. Either of these designs can be built in a few minutes using two pieces of sheathed, solid core, copper electrical wire and a pair of wire cutting pliers. (A coat hanger can be used, but is much harder to work with.) Also consider a flip-up stereo magnifier such as jewelers wear. Many different low power magnifications can be achieved by using these.

Field Microscopes

Field microscopes are useful for identifying samples in the field to be taken back to the laboratory. These instruments are available in a very large price range. Some cost less than ten dollars and can be carried in a shirt pocket like a pen; others have the basic capabilities of a full size microscope, including a large range of magnifications and achromatic optics.

Field microscopes do not lend themselves to the specialized illumination setups that we will be discussing. Unless you have a specific interest that requires that most work be done in the field, plan to budget most of your funds for a standard microscope. Often a stand magnifier or loupe magnifier will substitute for a field microscope.

Research Microscopes

The most expensive light microscopes are extremely accurate and flexible instruments. They are usually referred to as research microscopes. These microscopes have a collection of specialized condensers, eyepieces, illuminators, and objectives. The objectives will be flat field, and will probably be fluorite or apochromatic. The objectives are also likely to be calibrated for infinite tube length, and the tube will have slots and sliders for special illuminations, including some special contrasting techniques such as fluorescence and interference contrast. Binocular eyepieces will be present, and they will have a high eyepoint to allow viewing while wearing eyeglasses. A rotating mechanical stage, built-in reflected and fluorescent illumination systems, and other amenities may be present. The price of such microscopes start at about ten thousand dollars and run to more than one hundred fifty thousand dollars.

The Standard Brightfield Microscope

We will be looking at some ways of using less expensive instruments in ways to better approximate the functionality of a research microscope. While these methods will change the way your microscope functions, they do not require modifications of the microscope that would void its warranty.

The minimum requirement for serious microscopy is an achromatic optical system. The prices of these microscopes begin at about $130 (1993 prices). For about twice this price you can purchase a microscope that meets DIN standards (which will be discussed presently) and has a fine focus control. Serious high school and college students should spend the extra money. For about a thousand dollars you can have a focusing condenser, a separate photo tube, plan optics, and halogen illumination. This is recommended for anyone who does a lot of photomicrography, although good photomicrographs can be made with cheaper microscopes if sufficient care is taken setting up illumination.

Unless you will be spending many hours every day peering into a microscope, binocular eyepieces are not needed.

Standards

The DIN (Deutche Industrie Norm) standard specifies sizes for several parts of the microscope. Objectives and eyepieces are available from different sources to fit DIN microscopes. DIN microscopes are par focal and par central. Par focal means that different objectives can be rotated into place without having the specimen go out of focus. Par central means that objects in the center of the visual field at one magnification will still be in the center when another objective is moved into place. These considerations alone make DIN microscopes worth the price difference.

Be aware, however, that the DIN standard does not cover everything. If you acquire a high quality objective as an upgrade to your present microscope, you may also have to buy an eyepiece that is made for that objective to get the objective's best performance. It is possible to acquire a DIN microscope with bad optics. The standard relates to the way components of the optical system fit and work together, not to how well they are made. New parts will fit the body tube, thread sizes will match, and one body tube will be the same diameter (23mm) and length (160mm) as another. The standard does not mean that you will be able to upgrade everything on the microscope. For instance, if you own a DIN instrument with a fixed condenser and want to upgrade to a focusing condenser, you may not be able to get one.

Japan also has a standard, known in this country as JIS. The thread sizes for objectives are identical to DIN, but the tube length is 170mm instead of the 160mm specified by DIN. The eyepiece width is 23mm, which is the same as the DIN eyepiece width. Even though the eyepieces and objectives of either standard can be used with the other, if an exchange is made the magnification stamped on the eyepiece and objective will no longer be accurate. Using the wrong size tube will also result in spherical and other aberrations, because optical corrections are made for a specific tube length.

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Care of Microscopes

Although microscopes are a mature technology, no one has found a way of making them really rugged. Used with care, however, a microscope will have a long life. Here are a few suggestions for keeping the delicate instrument working properly, and for keeping yourself healthy at the same time.

Many chemicals that can damage the finish and lens coatings of the microscope are used in slide making. Work with these chemicals away from the microscope, and do not store them in the same cabinet. Some chemicals can also be dangerous to you. Observe proper safety and health precautions. Do not expose your skin to chemicals. The fumes from many chemicals are also toxic. Wear eye protection. Young people may not understand the dangers of handling chemicals and some biological materials. An adult should provide supervision.

When observing a specimen, start with the lowest power objective and work through progressively higher powers. This helps to keep the area of interest on the specimen centered; you can adjust the slide position slightly between each change of objective. Be careful when rotating the high power dry objective (called the high dry objective) into position, even if you have par focal objectives. A very thin cover glass is usually placed over the specimen in a drop of resen, water, or salt solution. If the specimen is too thick and the cover glass is not laying flat on the slide, the objective can crash into it and be scratched as the cover glass breaks. Spring loaded objectives, which retract when they come into contact with the cover slip, can help with the problem to a limited extent.

The oil immersion objective has even less clearance than the high dry objective. Never move the oil objective into position by simply rotating the nosepiece after focusing the high dry objective. Special handling is required. Move the high dry objective well away from the specimen before rotating the oil objective into place. Place a folded white card behind the objective on the stage. Place a drop of immersion oil on the cover slip and another drop on the nose of the objective. (Oiling both helps to prevent entrapment of air bubbles, which degrades the image.) While shining a light at the card and looking through a magnifier, slowly lower the nose of the objective into the oil on the cover slip. While the card remains visible past the bottom of the objective, bring the objective as close as possible to the cover glass without actually touching it. (You can omit the magnifier and card with practice.) Finally, look into the eyepiece and use the fine focus adjuster to very slowly raise the objective (or lower the stage) until the specimen is in focus. Start with the lowest power eyepiece; this prevents moving through the focus position without realizing it and having to start over.

Set the focus stop screw on the first use of the oil objective. First, focus the objective. Loosen the locking nut at the base of the screw, gently turn the screw all the way down, unscrew one sixteenth of a turn, and re-tighten the locking nut. Even if the screw is set, do not assume that it will always prevent trouble with this objective. A slightly thicker cover glass or a bit of texture under the cover glass can still cause problems. Powerful objectives are unsuitable for thick specimens. If you have trouble focusing, try making a new slide from a small part of the original specimen.

The lenses should be cleaned any time the image is foggy or obscured. One way to check whether the objective needs cleaning is to remove the eyepiece and look directly down the body tube at the objective image. Smears on the objective will sometimes be visible. (Do not do this if there is dust in the air that could get into the body tube.) Wipe off the instrument with a soft cloth after use.

The next two figures demonstrate a method of cleaning the microscope optics. Begin by blowing loose dust from the lens with a puffer. These are available at camera shops. Then use a very soft lens brush to flick off any remaining dust. Wrap a piece of lens paper around a small stick as shown. A toothpick with an end clipped off will do. For larger lenses you may want to whittle the stick's end into a spade shape. Be sure the tip of the stick is well padded by the paper so the lens will not be scratched. Put a drop of lens cleaner on this newly made swab. Then use another piece of lens paper to shape the swab and blot excess cleaner from it. If the excess cleaner is not blotted, some of it may get between two elements of a compound lens -- an expensive mistake that will necessitate factory service. For the same reason, never apply cleaning fluid directly to the lens. Apply the swab to the lens, using as little pressure as possible. Use a dry swab to remove the liquid.

Special fluid for cleaning optics is available, or alcohol (ethyl, methyl, or isopropyl) can be used. A very small amount of xylene or toluene on a swab is sometimes useful for removing stubborn smudges that resist the usual fluids, but immediately remove these liquids from the lens with a dry swab; the cement in some lenses can be softened by these chemicals.

WARNING: Some old texts on microscope use may recommend benzene or benzine for cleaning optics. Benzene is associated with liver damage and leukemia; do not use it. Benzine (with an 'i') may or may not be intended to refer to a less dangerous fluid than benzene, like naphtha. Just be sure you know exactly what you're using and that you know how to handle it safely. Xylene and toluene are also toxic, and shoud be handled carefully in a ventilated area.

After being cleaned the optics should be inspected. There is no real harm in removing an objective to inspect it, but the more often you do so, the more likely dust will get into it. If this should happen, a compressed air canister can be used to blow it out. These are available at camera shops. Do not try to clean the inside of an objective with a damp swab. The next two figures show a method of inspecting the objective lenses without unscrewing them. Hold a card behind the objective while shining a light at the stage. Look at the objective lens with a magnifier while changing the angle of the card until a bright reflection can be seen in the lens. Any dirt or grease will be easy to see in the reflection.

Keep the instrument covered or put away in a cabinet when not in use. When moving the microscope, grasp the arm and support the base with your free hand. Do not leave slides on the stage when moving the microscope, and handle slides carefully. The condenser and objective are easily scratched.

Never use force on any part of the microscope. Do not try to disassemble the instrument, except when removing objectives and eyepieces to change them. Specialized tools and knowledge are required. If you must tinker, get the right tools and instructions before starting.

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