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Are there worlds all around me that I can't see?" asked my nine-year-old son. He had just finished reading Dr. Seuss's Horton Hears a Who, in which the big-eared elephant Horton hears the cries of a civilization in peril -- a civilization no bigger than a speck of dust on a flower. Nobody believes Horton until it is almost too late -- they're too quick to disbelieve in things they cannot see. So the boy was curious. I took his curiosity as a prompt to investigate the purchase of a good used microscope. Kids are too fickle to risk the purchase of something brand-new, I decided. A used microscope would at least provide an introduction. My search led me to Optical Service Company in Poway (858-679-0522), where I met with owner Karen Podvin. Besides servicing microscopes for professionals -- veterinarians, research labs, biotech firms -- Podvin sells a range of new and used microscopes. Among the used scopes, she said, prices vary according to features and condition. But, she said, while a basic microscope from Swift might cost $300 new, "I sell it used for about $100. It does everything a new one does. If you take care of a microscope, it'll last pretty much forever."

The first step in taking care of a scope is to keep it clean. "Microscopes hate to be dirty. Dust is bad news. You want to cover it with a plastic cover -- if you have nothing else, use a plastic grocery bag."

I had forgotten what I learned in high school biology, except for the dissection of the fetal pig, so Podvin gave me a refresher tour of a scope's components. "This is a bright-field compound microscope," she said, producing one that she thought would suit my needs. "It's called 'compound' because it has many degrees of magnification. This one has three degrees. You look through the eyepiece, which magnifies the image ten times. The eyepiece is the thing that gets the dirtiest. When we service microscopes in clinical labs, I find things like mascara on the eyepiece. Keep it clean -- any glass cleaner or lens cleaner will work, even alcohol. Put the cleaner on a cloth. Never wipe the eyepiece without liquid; dust will scratch the antireflective coating."

At the base of the viewing tube, you arrive at the objectives. "To get the total magnification, you multiply the eyepiece by the objective. The first objective is four times, so it's a 40x magnification. The second is 10, and the third is 40, which produce 100x and 400x, respectively." You might use 40x for an amoeba, but 400x for red blood cells.

The objective hovers just above the slide containing whatever you want to look at, and the slide rests on the stage. "If you're smart, you'll always use coverslips on your slide -- a coverslip is a thin piece of glass that goes over your sample. That will keep your objectives clean." A scratched objective will result in a muddy image, she said.

The microscope Podvin showed me was called a "bright-field microscope, because the stage is illuminated. There's a bulb down below, and it shines through a condenser and then up through the slide. That's called incident illumination. Because the light is coming from below, whatever you put on your slide has to be basically transparent." (Replacement bulbs vary widely in price; Podvin recommends keeping a supply.)

Obtaining transparent specimens wouldn't be difficult, she said. "Oftentimes, we have parents or grandparents come in before Christmas, wanting microscopes for the kids. One thing they do is, after the rains, they go out and find ponds of water. They're teeming with very small, biological things. They take a drop of pond water and put it on a slide. The light shines through the little organisms -- most of them are transparent. It's really fascinating."

For more precise viewing, said Podvin, I could look at microscopes with mechanical stages. "The mechanism moves the stage to the left or right -- you have a lot of control. A lot of veterinarians use these; if they're looking for, say, ear mites, they can scan a whole slide. If you're moving the slide with your fingers, it's easy to lose your place and forget where you've been on the slide. But because they have more moving parts, microscopes with mechanical stages aren't usually given to students."

High-end microscopes -- things with mechanical stages and 1000x magnification -- are great for microbiologists, but Podvin noted that they can start at $500 instead of $100 -- "and that's used." Better to buy only as much power as you need -- you can see an amoeba just fine with a basic scope. However, she said, "if you're going to spend a lot of time looking through a microscope, you might want to get a binocular scope, which has two eyepieces." They start at $500 (used), but they're easier on the eyes.

If your son is a rock collector -- and my oldest is definitely that -- you might consider a metallurgical microscope, one outfitted to examine nontransparent objects. "It's usually used for looking at printed circuit boards. Instead of coming up from below, the light comes from the top, goes through a special objective, and then bounces back. Geologists use them to look for certain elements embedded in rocks."

A better investment might be a stereo-microscope ($500-$700 used). "It allows you to look at three-dimensional things with great detail. You can tell depth as well as width and length. In industry, it's used for assembling circuit boards." They're also used in biology. "They're good for dissecting. If you're looking at a small biological organism, it enables you to see detail on the eye or tail."

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