How to Buy School Microscopes
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 many others can be found in the MICRO booklists, on this website. You'll find another useful discussion of microscope selection at http://www.greatscopes.com/microscope.htm.
What kind of microscope should I buy?
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.
All rules have exceptions. A useful "simple" design for a classroom would be the inexpensive "The Private Eye 5x loupes." Classroom suggestions on using these loupes can be found at http://the-private-eye.com/index.html While much can be achieved with a simple loupe, the need for more magnification and image recording leads us to "compound" scopes for general classroom use.
But I only have $50-100 to spend; does that mean no microscopy?
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 about $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. Buy as many as you can afford; some local dealers may be willing to discount a bulk purchase for school use.
I want to equip my classroom with "real" microscopes; what will that cost?
Approximately $1000 (don't despair; see below). That will get you at least 10 good quality scopes in the $80-150 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.
Where do I find the $1000?
That may be easier than you think. Local corporations are often a good source of funding at this level. The MSA Local Affiliate Society (LAS) nearest you may be able to help. You'll find a list of LAS at http://www.microscopy.org/communities/local.cfm.
What type should I buy?
Two types, actually, in roughly equal numbers for middle school. Inspection/dissection scopes are used to look at surface details of 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 through 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.
A useful "simple" design for a classroom would be the inexpensive "The Private Eye 5x loupes." Classroom suggestions on using these loupes can be found at http://the-private-eye.com/index.html.
While much can be achieved with a simple loupe, the need for more magnification and image recording leads us to "compound" scopes.
What features should I look for?
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 subject at low power and immediately switch to higher magnifications of the same area.
I need compound scopes for my class, and I see a lot of features advertised; which ones are worthwhile?
"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". Magnification can be calculated by multiplying the power of the eyepiece lens by that of the objective lens. For example, using a 10x eyepiece and a 40x objective gives 400x. 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). True 1000x imaging requires a 4th objective (100x) in the turret, a multi-lens focusable condenser, plus the use of immersion oil. It should only be considered for advanced high school classes.
"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 is best accomplished with a lens turret rather than changeable screw-in lenses, which are easy to damage or lose.
Binocular eyepieces. NO. Although the information content of a stereo view will help an older student, two sets of optics cost a lot more, and if rough use knocks them out of line factory service is needed. Younger 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. MAYBE. Although the substage condenser, 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 NO. 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. Some manufacturers do have good, educationally useful video projection systems, but their cost makes it doubtful that they're "worthwhile" if the budget is limited. A separate digital camera connected to a computer is a better choice. The new digital microscopes that project onto a monitor allow the whole class to see the same thing at the same time. Avoid the "digital microscopes" that incorporate camera electronics into a glass-lens microscope itsef. That electronics will become obsolete rapidly, forcing you to abandon and replace the scope when it's optical and mechanical condition are still good.
Built-in illumination. YES. Scopes with mirrors are no longer sold. If you have some, 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 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 powered illuminators are offered on a few scopes, but they have short battery lives 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, no bulbs to replace, inexpensive ($150!).
Made in the U.S.A. NO. Brand names are no guarantee of American manufacture. Almost all American-brand microscopes are imported, and even scopes that are advertised as American-made will 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.
Mechanical stage. YES. Offered as an accessory on most compound scopes.
Can I check lens quality?
YES. You can tell a lot without test equipment. The rectangular engraved crosshatching around the portrait heads on U.S. currency 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 eyeglass lens cleaner. If it's within a multi-element lens, don't buy the scope.
Objective alignment. 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 the turret to the next higher power. The chosen object should remain close enough to center that you can still see it at the higher magnification, 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.
Where do I find these microscopes?
Major scientific supply catalogs and some of the school supply houses will have them; this web page has a dealer contact list. A microscopes-only dealer may provide both a presale quality-control check and in-house service. Shop carefully; prices may vary by 50% or more.
What about buying a used microscope? I see a lot of great deals on eBay.
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. Used research scopes will have extra controls that will be very confusing for both teachers and students.
For further information contact MSA's Project MICRO co-chair: Caroline Schooley.