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The Basics: What is a compound microscope?

                   
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The compound microscope is a tool that can be used for many purposes. From medical research to a day at the beach, your curiosity is limited only by your imagination. As with all disciplines, it is best to learn the basics and to apply them each time you use the microscope. By knowing and following a few simple rules, your scientific experience will be enhanced.

A light source illuminates the object to be seen. That source can be a mirror, or the instrument can be self-illuminating. As light passes through the object, the lens nearest the object, called the objective lens, produces an enlarged image of the object in the primary image angle. The lens that you look into, the eyepiece, acts as a magnifier and produces an enlarged image of the image produced by the objective lens. To ascertain magnification, simply multiply the eyepiece magnification, usually 10X, by the magnification of the objective lens, usually 4x, 10X, 40X and 100X. For example, a 10x eyepiece in conjunction with a 40X objective, will give you a magnification factor of 400. The object will be magnified 400 times larger than you can view it with the naked eye. The question is often asked why can you not have, say, a 100X eyepiece, and a 1000X objective. Would that not produce a 100,000 magnification? It would, but the problem is resolution, the way your eyes see the image. A compound light microscope is limited to about 2000X magnification. Beyond that limit you could indeed magnify it, but neither your eyes nor your brain would be able to recognize the image. With a limit of around 2000X magnification you can view bacteria, algae, protozoa and a variety of human/animal cells. Viruses, molecules and atoms are beyond the capabilities of today’s compound microscopes and can be viewed only with an electron microscope.

A microscope is a delicate precision instrument and care must always be used when transporting it. It is best to hold it by the arm, the part that connects the nosepiece to the base, with one hand, your other hand supporting the base. When not in use, cover your microscope, and for long periods of storage we suggest that it be kept in the original Styrofoam packaging. It is best to never allow anything, especially not your hands, to come in contact with any lens surfaces. The oil from your hands can smudge or cause scratches. Always clean the lenses only with the proper lens cleaner and lens paper, being sure to follow the directions given. To clean the surfaces of the instrument, we suggest only a damp cloth. Periodically, a competent technician should professionally service your microscope.

Place your microscope on a secure table, free from vibration, to begin. Try to have the microscope at least one foot away from any edge to avoid an accidental fall. Turn on your illumination, if so equipped, or, if mirror equipped, turn the microscope toward the best available light source and tilt the mirror, as you look through the eyepiece, until the brightest possible light shows through.

You should now begin to learn an important skill that will significantly increase your enjoyment of the instrument. You must learn to view through the eyepiece(s) with both eyes open! Whether you have a monocular microscope (one eyepiece,) or a binocular microscope (two eyepieces,) start from the beginning to use both eyes. Try this experiment. Close one eye and squint (as you usually would while looking through the microscope) as long as you can. I can assure you it will not be very long. It is tiring and uncomfortable, to say the least. And how long do you think you will enjoy looking through your microscope with this posture? You get the picture. Now, try this. Open both eyes, but place your hand over one eye. Isn’t that more comfortable? You could probably do this indefinitely. If you were spending hours each day looking through a microscope, which would you prefer? Every professional microscopist has mastered this skill. Just train yourself to always keep both eyes open. It may seem difficult, but your eye will automatically shut out the image from the eye not used for viewing through the monocular microscope, and with the binocular microscope, both eyes will focus on the image. Like riding a bicycle, once you learn it, you never forget.

Select the proper objective for the magnification you desire by rotating the turret or nosepiece such that the objective is in alignment with the eyepiece. You will hear a "click" signaling you are in the proper position. Place your slide on the stage and secure it with the stage clips, or with a mechanical stage, if so equipped. Try to align the specimen as near to the center of where the objective will come down. This will make it easier to locate when viewing through the eyepiece. Before looking through the eyepiece, turn the coarse focus knob, which will lower the objective, until the objective is almost in contact with the slide. The purpose for this is simple. If you are looking through the eyepiece and you move the objective downward, you run a very high risk of contacting the slide with the objective and damaging one or both of them. Slides are cheap, objectives aren’t. By starting in the lowered position, your focus will bring the objective up, away from any potential damage. As you turn the coarse focus knob, your specimen will come into focus. If so equipped, once you have the best focus possible with the coarse, use the fine focus for minute adjustments.

Most microscopes are equipped with a diaphragm of some sort. This is located directly below the stage, but above the light source. Usually diaphragms are of two types: a disc diaphragm (a disc pre-drilled with holes, from smaller to larger, that you dial in the appropriate light) or a better choice, the iris diaphragm (which, like a camera lens, offer unlimited adjustment). Depending upon your specimen, more light or less light may be required for the best viewing. In conjunction with the diaphragm is the Abbe condenser that further focuses the light through another lens before it reaches the objective. An additional option is the adjustable Abbe condenser, which allows movement of the condenser below the stage of about one half inch. Again, it depends on your specimen as to how much or how little light is required.

Once you have focused with a particular magnification, you will want to view the specimen under different magnifications. Simply rotate the nosepiece until the objective you require is in line with the eyepiece. Listen for the "click." Most good microscopes are parfocal and parcentric, meaning when you switch from one objective to another, the specimen will remain in focus and centered in your field of view.

These are the basics. Starting out, try easy specimens. Probably the best place to begin is with newsprint. Newsprint is very easy to focus, and through the different magnifications, you will see a variety of details. As with all things, practice makes perfect. Be easy on yourself at first and do not become frustrated with the initial difficulty of the finding and focusing of your intended specimen. Your local library is a wonderful resource for information and experiments. Never hesitate to ask a science teacher, or other professional, for tips. Learn the skills that will enhance your micro-exploration of the incredible world we live in.
 
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