Shopping for a microscope is a lot like shopping for a car. Some folks want to learn all the details; others simply want to buy something they can use right now. The first time you went car shopping, you probably took a parent, sibling, or best friend along. But how many of us have someone to take along when shopping for a microscope? You do now!

Middle school and high school instructors will normally choose from a slide-viewing microscope or a stereo microscope that views three-dimensional objects.

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Slide-viewing microscopes, Drawing A - Standard, Inclined Tube and Drawing B - Student Size Straight Tube, are what most people think of when they hear the word 'microscope.' These microscopes have at least one eyepiece, one or more objective lenses, a platform (or stage) for holding microscope slides, and some means of illumination, either a mirror or a lamp.

A stereo microscope has dual eyepieces for three-dimensional stereo viewing, one objective lens, and a base that holds solid objects. Some stereo microscopes have illumination systems and others do not. (The stereo microscope shown in Drawing C has an illumination system.) If your students are going to examine rocks, minerals, insects, or other three-dimensional objects, you will need a stereo microscope.

The next question is one of size. Do you want a full-size standard microscope or a scaled-down student size? Student microscopes are approximately 75% the size of the standard size. The optics in a student microscope will not be quite as high in quality as in a standard microscope, but these smaller microscopes are easier for some students to handle.

A microscope contains a series of prisms and lenses that magnify an image. These lenses are found in both the eyepiece tube and in the objective lens. If an eyepiece has 10x magnification, then what you see through the eyepiece is magnified 10 times. If the eyepiece has 15x magnification, then everything is magnified 15 times.

A Huygenian eyepiece is simply a metal cylinder containing a series of lenses. A wide field eyepiece, ideal for beginning students, features a viewing field up to 50% larger than the standard eyepiece.

Many microscopes also feature a built-in eyepiece pointer. The pointer makes finding a specific part of the slide simple. Almost every microscope today has locked-in eyepieces, that is, eyepieces fastened to the eyepiece tube with tiny set screws. This reduces eyepiece theft.

The total magnification power of a microscope combines the eyepiece power and the objective power. If the objective is marked 4x and the eyepiece is marked 10x, total magnification is 40x. If the objective is 4x and the eyepiece is 15x, total magnification is 60x.

Typically, slide microscopes have either 10x or 15x eyepieces (some models offer both) with 4x, 10x, and 40x objectives. Low cost slide microscopes may have only two objectives, 4x and 10x. More complex microscopes may offer a fourth objective, 100x.

Routine classroom viewing of prepared slides will require at least a 4x and 10x objective. Students who know what to look for in a slide will be comfortable with the 40x objective. Advanced students, or those preparing their own microscope slides, will need the 100x.

The 100x objective is commonly called an oil immersion objective. For high power magnification, the space between the slide and the objective lens is filled with one of several oils. Because the refractive index of oil is similar to that of glass, more light is directed towards the lens. This provides sharper clarification of an image at high power.

Objectives are often color-coded for easy selection. Most manufacturers use red for 4x, yellow for 10x, blue for 40x, and white for 100x.

Good slide microscopes have both coarse and fine focus control knobs. The coarse focus knob raises and lowers either the objective or stage. In Drawing A, the coarse focus knob moves the microscope stage. In Drawing B, the coarse focus knob moves the objectives. The fine focus knob fine tunes the image. Coaxial means that both coarse and fine focusing are controlled by one knob.

The safety clutch, available on some microscopes, does the same job as your auto's safety clutch does for the car—the clutch disengages the gears. The microscope safety clutch disengages the focus knob and prevents the objective lens and the slide from touching. This is a valuable feature. Some microscopes also have retractable objectives that prevent the objective from touching the slide.

Better quality microscopes will feature parfocal objectives. Parfocal means that once the image is focused at a lower power the image will stay focused even when the objectives are rotated. It's easier to focus on an image at 4x then rotate to a higher powered objective; if the microscope has a parfocal feature, students won't be frustrated by having to refocus the image every time they rotate objectives.

Looking through a microscope is like looking at a mirror image. Some students are confused by reverse imaging and have trouble centering a slide. The mechanical stage permits easy manipulation of a slide. A graduated mechanical stage has a built-in scale that is useful for measuring. Some microscopes automatically parcenter slides. This keeps the viewing image centered underneath the objective being used, even when objectives are rotated.

Viewing distortions occur naturally during magnification because a single lens cannot focus all wavelengths of white light equally. A chromatic aberration causes colored auras around a specimen. Spherical aberrations cause 'ghosts' or double images. A microscope described as being achromatic combines two or more lenses with different densities so these distortions can be corrected. Every good quality microscope will have achromatic lenses.

Since we're discussing lens quality, let's review another basic term. The abbreviation DIN stands for Deutsche Industrial Normen. A DIN lens has met the strict lens standards first established by German lens manufacturers. This term can apply to either the eyepiece, the objective lens, or to both, and is a recognized symbol of lens quality.

Let's take a breath and review our choices so far.

1. Do you need a slide microscope or a stereo microscope?
2. Do you need a standard or student size microscope?
3. Do you want standard 10x eyepiece magnification or variable 10x/15x magnification?
4. Do you want a standard viewing field or a wide viewing field? Is a pointer, available in wide field eyepieces, an important aid for your students?
5. How many objectives do you need? Do you need the 100x for oil immersion?
6. Is it easier for your students to manipulate separate coarse and fine focus knobs, or do you prefer coaxial knobs?
7. Are retractable objectives and/or a safety clutch standard on your microscope?
8. Will class time be more productive if you have parfocal objectives?
9. Are your students coordinated enough to manipulate microscope slides, or would a mechanical stage or parcentering feature be important?
10. Do you need to have a microscope with achromatic lenses?

First, will it be easier for your students to use dual eyepieces for viewing, or will the students be able to comfortably use a single eyepiece? Monocular microscopes are usually less expensive than binocular models, but are not as easy to use for long periods of time. Also, dual eyepiece microscopes normally have a diopter correction in the left eyepiece. This special adaptation allows for perception differences between the left and right eyes. (This also means that if one eyepiece is lost or stolen you must order the correct replacement eyepiece.) If you can only afford one microscope for the classroom, make it binocular.

If the microscope features dual eyepieces, does it also offer adjustable interpupillary viewing distances? This phrase sounds impressive, but all it means is that the distance between the right and left eyepiece is adjustable. After all, the distance between your right and left is different from the distance between the right and left eyes of your students. Without this feature, you may have a binocular microscope but your students may be using it as a monocular model!

Since we're talking about comfort, you might want to consider an inclined tube microscope rather than a straight tube model. An inclined tube microscope has a 45° eyepiece tilt, making it more comfortable for viewing while seated. Straight tube models have a swivel arm that tilts, but liquids can run off a slide when the microscope is tilted.

For inclined tube microscopes, a rotating head is a nice feature, especially when students need to share a microscope. It's time-consuming for students to trade seats for a look-see, and it's not good for the microscope to be constantly moved. A rotating head that swivels in different directions solves the problem.

A teaching microscope will have an additional straight eyepiece. With this extra eyepiece, students and teachers can view the same image at the same time. Video cameras can also be attached to the straight eyepiece of a teaching microscope.

All slide microscopes should have platform clips to hold slides in place. Only lower cost microscopes have removable clips; locked-in clips are hard to lose.

With a slide microscope, it's necessary to shed a little light on the subject. Again, you have options that help you keep within your budget. Low-cost slide microscopes have an attached mirror. (In case of breakage, mirrors are usually replaceable.) Light must be focused on the mirror and then reflected towards the microscope slide. If you have a dimly lit classroom, or high ceiling lights, you may have to purchase a separate light source for the mirror.

A microscope with an attached substage illuminator (a light fastened underneath the stage) provides consistently better viewing of slides than does a microscope with a mirror. Bulbs vary from 15W to 30W, depending on the manufacturer. Low-power 15W bulbs may not provide enough light for some slides but a 30W bulb may be too hot for viewing live specimens. For general classroom use, a standard 20W bulb work best. Blue filters give more even light dispersal.

Halogen illuminators are another option to consider. Halogen illuminators can deliver intense light without heat build-up. This is a practical choice when microscopes are used during long viewing sessions.

A word of caution—make sure the illuminator has an on/off switch. A few very inexpensive models must be plugged and unplugged from wall outlets. Also, your classroom must have outlets near the microscopes. Tripping over dangling cords causes injuries and breaks microscopes!

Unlike the latest model car, microscopes come in fairly standard colors: black, tan, and grey. Some models have lightweight plastic bodies, while some have heavier metal bodies. An enamel coating makes both styles acid and stain resistant.

Before we tackle any other decisions, let's review this last group of options.

1. Will you be using a single (monocular) or dual (binocular) eyepiece?
2. If the microscope has a binocular eyepiece, is the interpupillary distance adjustable?
3. Will you select an inclined tube or straight tube microscope?
4. If this is an inclined tube microscope, does it have a rotating head?
5. Does the stage have locked-on clips?
6. Does the microscope use a mirror or an illuminator?
7. If the microscope has a mirror, will you also need to purchase an attachable light source?
8. If the microscope has a built-in substage illuminator, what type of bulb does it use?
9. If the microscope has a built-in substage illuminator, does it have an on/off switch?
10. Does the microscope have an enameled, stain-resistant finish?

Microscopes with built-in substage illuminators may feature different ways to control light intensity. These controls are known as condensers. The condenser may consist of a single lens mounted within the stage hole or it may be a series of focusable lenses, known as an ABBE condenser. Either way, the condenser maximizes light intensity.

A 40x objective may require a 0.65 condenser aperture (opening), while a 100x oil immersion objective needs a 1.25 condenser.

Diaphragms, openings that control the passage of light, fasten underneath the condenser. These can be either disc or iris diaphragms. A disc diaphragm has a series of openings (apertures) that gradually increase in size. Most discs have five or six apertures; a few models have only three.

An iris diaphragm looks exactly like the iris on a 35 mm camera. The fan-shaped blades slide smoothly open or close to control the amount of light passing through the aperture. An iris diaphragm is optional with lower powered objectives (4x to 40x) but is a must with the 100x oil immersion objective.

Condensers and diaphragms are unnecessary for microscopes with mirrors, but a disc diaphragm may be useful with substage illuminators. If you use a 100x objective, make sure your microscope has both an ABBE condenser and iris diaphragm.

Many of the slide microscope terms also apply to stereo microscopes. Stereo scopes will have dual eyepieces (necessary for stereo viewing), and one low power objective. The eyepiece should offer some interpupillary adjustment, and good quality stereo microscopes have a built-in diopter correction in the left eyepiece.

Eyepiece magnification may range from 10x to 40x. Some stereo scopes have zoom capabilities, with eyepiece magnification increasing smoothly. Others have fixed magnification powers. Again, the more options one chooses, the higher the price.

Objective magnification may range from 2x to 30x, depending on the manufacturer. Remember, for total magnification, multiply the eyepiece power by the objective power. Stereo microscopes will always have a lower total magnification range than a slide microscope has. Viewing of live specimens, such as insects, is best at the 60x to 120x total magnification range.

An important option to consider for your stereo scope is illumination. The lowest priced stereo scopes have no illumination system. The best offer both an upper (incident) light source and a lower (transmitted) light source. Mid-range stereo scopes may offer only upper lighting.

Although a stereo scope will work without an illumination system, incident lighting makes viewing much easier. Also, some rocks and minerals require either incidental or transmitted light for best viewing. And for safety's sake, make certain an illuminated stereo scope has an on/off switch.

A few stereo models offer a base plate color choice as an alternative to an illumination system. Light colored objects are viewed better against a dark base plate, and dark objects are viewed better against a light base plate.

Top quality stereo scopes offer a parfocal objective lens and a wide field eyepiece.

Finally, just like with a slide microscope, an inclined, rotating head is a comfort feature, useful when students sit and share microscopes.

Here's your checklist for choosing a stereo microscope:

1. What will I be viewing with this stereo microscope?
2. What total magnification range will be most useful?
3. Does the stereo scope offer feature interpupillary adjustment capabilities?
4. Does the stereo scope offer zoom magnification?
5. If I'm buying an illuminated stereo microscope, do I want upper (incident) lighting, lower (transmitted) lighting, or both?
6. If I'm buying a stereo microscope without an illumination system, does it have a light or dark base plate?
7. Does the stereo microscope have parfocal capabilities and wide field eyepieces?
8. Does this stereo microscope have an inclined, rotating head?

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