Woody Allen, quoted by B. A. Palevitz:

"...can the human soul be glimpsed through a microscope? Maybe, but you'd definitely need one of those very good ones with two eyepieces."

There are a lot of inexpensive microscopes available, of widely varying quality; some of the good ones cost no more than some of the nearly useless "toy" models. You don't have to be an optical expert to make a good choice, if you follow the simple evaluation criteria given here. Remember that "experts" don't always agree, and trust your own evaluation skills. [ The advice that follows is intended for teachers, parents, and school volunteers who are looking for good classroom equipment; an adult amateur will want a somewhat different microscope that can be upgraded with different lenses and accessories as the hobby progresses. Both will benefit from reading one of the several well-written introductory books that are available; Nachtigall, Exploring With the Microscope, for example. Full information on that book and the others cited here can be found in the MICRO bibliography.

The first choice is between "simple" and "compound" microscopes. A "simple" microscope (Leeuwenhoek used one) has just one lens and a "compound" scope has both an objective and an eyepiece. Don't buy a "simple" design! The working distances between eye and lens and lens and specimen are so small that they are very difficult to use. And a single powerful lens has so much aberration that the student who manages to get an image will be disappointed by its quality. Unfortunately, there are quite a few models offered in school supply catalogs.

Not at all! Many manufacturers offer a design that looks like a pocket flashlight. They usually magnify 30x, with two AA batteries providing illumination, and they sell for $5-10. You'll find them in electronics and "nature" stores, and many catalogs; quality varies, so it's wise to compare. They can be good enough to support extensive curriculum (see the Experiments with the Mini-Microscope book and video). Buy as many as you can afford; some local dealers may be willing to discount a bulk purchase for school use.

Approximately $1000 (don't despair; see below). That will get you at least 10 good quality scopes in the $70-150 price range. You can get scopes at that cost that will be durable and easy to use, with lenses that will deliver a sharp, bright image. In general, more expensive models will provide similar images but more convenience, and less expensive ones will have disappointing performance.

Two types, actually, in roughly equal numbers. Inspection/dissection scopes are used to look at large, opaque specimens at relatively low (20-30x) power. Illumination is usually from above, and the image is erect, as in the "real world". Compound microscopes are usually used with transmitted light to look at transparent specimens; the useful school magnification range is 10-400x. The image is inverted. It takes a bit of practice to follow a moving subject when it's upside-down; the CD-ROM Scopemaster provides helpful practice.

Both types should have metal bodies and metal rack-and-pinion focus, for durability and easy, precise focusing. That immediately eliminates the plastic "toy" scopes. Although a metal body is no guarantee of lens quality, metal focus gearing is more precise than twistable or plastic designs. Both types should have glass rather than plastic lenses and be able to focus on both thin specimens (slides) and the surface of larger objects at least an inch thick. Compound scopes should have a 3-lens turret and a substage diaphragm or series of "field stops" to control brightness. There are some good single-objective compound scopes available, but the three lens design is much more versatile; a student can locate a specimen at low power and immediately switch to higher magnifications.

"Magnifies 600-1200 times!" NO. When you see this claim in an advertisement, it's good reason to read no further. The wavelength(s) of visible light and the optical properties of glass lenses used in air (rather than the "immersion oil" used with research microscopes) limit the useful magnification of a compound school microscope to 400x; more is "empty magnification". Higher magnifications are achieved in "toy" microscopes by using an eyepiece of excessive power, which in turn makes the field of view very narrow, while emphasizing all the aberrations of the image produced by the objective lens. It's like enlarging a snapshot from a cheap camera to poster size; it's bigger, but there's no more detail. Most school microscopy needs 10-100x (bacteria require 400x).

"Zoom magnification!" NO. This is related to the preceding problem. A zoom eyepiece just makes things worse, because cheap zoom optics are full of aberrations. Magnification changes in a compound microscope should be accomplished by changing objective lenses, not by zooming the eyepiece. And that change is best accomplished with a lens turret rather than changeable screw-in lenses, which are easy to damage or lose.

Binocular eyepieces. NO. Two sets of optics cost a lot more, and if rough use knocks them out of line factory service is needed. Children's narrower interocular distance often won't fit adult eyepiece spacing. And fully 17% of children (5 or 6 in a class of 30) will have amblyopia, strabismus, or other binocular coordination problems.

Condenser. NO. Although the substage condenser lens, which focuses illumination on the specimen, is an essential part of a research microscope, it should be avoided in the $100 price range. If one is offered it will be a single lens which can't be focused, fixed in the stage. It will be easy to damage and difficult to clean.

"Projection microscopy!" MAYBE. Even if you can completely darken your room, illumination sufficient to project an image with one of the cheap direct projection scopes will also fry your specimen. One or two manufacturers do have good, educationally useful video projection systems, but their cost makes it doubtful that they're "worthwhile" if the budget is limited. If you want to show an image to a whole class, mount a video camera an inch or two above the eyepiece. Use a camera tripod or other separate stand to hold the camcorder, with a piece of black foam pipe insulation (it's vibration-free, adjustable, and cheap) as a connector. Try both close-focus and infinity camera focus settings. A digital feed to a computer is a better choice.

Built-in illumination. MAYBE. There's a lot of disagreement on this one. It's always more expensive than a mirror, and if every scope has an electric cord, the floor will look like spilled spaghetti. If you need illumination from above ("incident") for an opaque sample, a below-stage lamp won't provide it. Many built-ins require hard-to-get bulbs. Built-in battery illuminators are offered on a few scopes, but have short battery lives. If scopes have mirrors, you can use any kind of table lamp. A short fluorescent tube in the center of the table can illuminate several scopes; they're about $10 at hardware stores. BUT some children may have difficulty setting a mirror properly. If you do opt for built-in illumination, remember that wet samples can be a shock hazard and equip your electric outlets with ground fault interrupters, which automatically cut the power to a "short circuit". Recently built classrooms may have built-in GFIs. Microscopes with light-emitting diodes (LEDs) and rechargable batteries have entered the school market recently, and they're definitely worth considering. No wires, long battery life, inexpensive ($150!).

"Made in the U.S.A." NOT LIKELY. If this is important to you, check directly with the manufacturer, not the dealer. Most, perhaps all, American-brand microscopes are imported, and even scopes that are advertised as American-made probably have important imported components, such as lenses.

Widefield eyepieces. YES. These provide a large, bright image and are usually the best choice. They let you see more specimen area than a conventional eyepiece of the same power. This also means that more illumination is gathered and transmitted, providing a brighter image. They should be no more than 15x; 10x is preferred. In student scopes, they're often fixed in place, which protects against loss, damage, and internal dirt.

Fine focus. YES. Desirable, but scarce in the low price range.

Focus stop. YES. This will prevent slide or lens breakage. If the scope doesn't have fine focus this is particularly important. If you can't focus on a very thin specimen (paper, or a plastic slide) you may have to put the specimen on top of a glass slide.

You can tell a lot without test equipment. The rectangular engraved crosshatching around the portrait heads on U.S. currency (best on the $1 bill) is a useful specimen for a crude check of lens quality. Focus up and down and look for several things:

A flat focal plane. You can't expect inexpensive lenses to have a perfectly flat field; so-called "plan" lenses are costly. You will, however, find a lot of variation in quality; try to do a side-by-side comparison if you're choosing between two models. You're looking for an image that's really sharp from the center almost to the edge of the field of view, rather than one that must be refocused for each part of the circular field.

Achromatic lenses are highly desirable. This means that the lenses will focus one wavelength (usually mid-green, where our eyes are most sensitive) well. Uncorrected lenses will show color fringes around specimen detail. Apochromatic lenses, which are corrected for three wavelengths, are too expensive to consider here.

No distortion. The horizontal and vertical engraved lines should be straight.

No astigmatism. As you go thru the focal point (fuzzy-sharp-fuzzy), look for a "linear" image distortion that rotates 90 degrees as you go from above focus to below focus. This can be caused by an objective lens that isn't round or (more common in cheap optics) a lens that is tilted in its mount. Rotate the eyepiece to check for the same problem with that lens.

No internal dirt. Defocus the image and look carefully for lens dirt on both objective and eyepiece. Try removing it with gentle use of lens tissue and small amounts of alcohol or eyeglass lens cleaner (Sparkle brand window cleaner works well). If it's within a multi-element lens, don't buy the scope. Compound scope three-lens turrets should be reasonably well aligned and the cheaper ones often are not; compare the scopes that you're considering. Focus on a small centerfield object at the lowest magnification, and then rotate to the next higher power. The chosen object should be close enough to center at the new magnification that you can still see it, and it shouldn't be completely out-of-focus. Repeat for each objective, in sequence. Don't expect perfection; that is expensive and will only be found in research scopes. Try to do a scope comparison with two prepared slides. One should be a brightly-stained biological specimen and the other, something that's almost colorless. Use different magnifications and illumination (the "field stops" mentioned above). The better lenses should be obvious.

Major scientific supply catalogs and some of the school supply houses will have them; this web page has a dealer contact list. Shop carefully; prices may vary by 50% or more.

If you know enough to evaluate a used scope or repair a faulty one you probably won't be reading this basic advice. Microscopes don't "wear out"; they're often on the market because they aren't working as well as they should.

That may be easier than you think. Local corporations are often a good source of funding at this level. MSA is prepared to help you write a grant proposal, and the MSA Local Affiliate Society nearest you may be able to help.

Allen, W. See above. Quoted at the beginning of B.A. Palevitz et al. Protoplasma 109:23-55 (1981)

Insights Visual Productions 1991 Experiments With the Mini-Microscope: Teacher Handbook 52pp, paperback, 8.5x11", $79.00 with a 50 minute videotape. Order from Insights Visual Productions, Inc., P.O.Box 230644, Encinitas, CA 92024; 619-942-0528 or 800-942-0528. This manual deserves a separate listing; it's much more than a supplement to a videotape. Seven classroom sessions are fully supported with advice on lab preparation and procedure, background information, student worksheets, overhead masters, and quizzes. Topics include use of a 30x handheld microscope, measurement, and investigative observation of a variety of objects; see the tape description below. Tape plus manual provide enough material to get a teacher or volunteer started in classroom microscopy. Middle school. High school - adult. RECOMMENDED

Neuronware 1997 Scopemaster $69.00 from Neuronware, 1300 Bay St., Suite 0, Toronto, Ontario M5R 3K8, Canada; 888-371-4425. For Mac or Windows. An interactive microscope teaches the use of the controls of a compound microscope: The user can select three objectives, adjust the substage diaphragm, and use coarse and fine focus. Advice on microscope use appears if a mistake is made; if the advice is ignored, the high power objective even breaks with a resounding "crack" if the slide hits it! Slides must be centered on the stage in a realistic way that makes the inverted image of the compound microscope understandable; it will be nonthreatening, nondestructive practice for a beginner. Ten sets of nine slides each (mostly biological) are included, each with its own well-written reference book; the goal is specimen identification. The images are good color light micrographs; each can be viewed full-screen after the microscope is in focus, or all can be reviewed quickly in "teacher" mode. There is a self-test, and a printable test for class use. More information is available on the web at www.neuronware.com. Middle and high school. RECOMMENDED

Many more references may be found on this web site in the Project MICRO bibliography .

Caroline Schooley
Box 117, Caspar, CA 95420
Phone/FAX (707)964-9460
schooley@mcn.org

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