3 TYPES OF MICROSCOPES:

• Stereo
• Compound
• Inverted

STEREO MICROSCOPES — Low Power
Stereo microscopes are used to view more substantial specimens and are ideal for looking at transparent objects as well as opaque objects such as insects, bugs, leaves, rocks, gems, coins, stamps, fabric weaves, sand, soil and tiny electrical components.

Low power microscopes usually have both a top and bottom light (reflected and transmitted). Most stereo microscopes have two eyepieces and a separate objective lens for each eyepiece lens. This allows triangulation and provides a "stereo" or 3D image. When you look through a stereo microscope, the image will have depth. The most common magnifications used are 20X, 30X and 40X, but can range from 6.5X to 80X. They are available in one of two configurations:

• Dual-power: two magnification options (for example: 20X and 40X).
• Zoom: continuous zoom range from the lowest power to the highest power (for example: 20X to 40X).

Stereo microscopes come in:

• Binocular: Two eyepieces
• Trinocular: Two eyepieces for normal viewing plus a third phototube on which you can mount a camera without interfering with the normal operation of the microscope. While it is possible to mount a camera on a monocular or binocular microscope, it is far better to use a trinocular microscope designed for camera work.

Working Distance: the maximum distance the objective can be from the sample and still focus. This is an important feature in a stereo microscope. Models with a short working distance may not accommodate very large samples or leave enough room to work on the sample.

ADVANTAGES:

• No slides are required.
• Specimen can be placed directly on the stage.
• Samples may be examined in their natural state with little or no preparation.
• Ideal for educationalists, naturalists, collectors and appraisers with a wide range of applications including animal and insect dissection, natural history, botany, gems, coins, stamps, minerals and precision viewing of any sort.
• Often used to prepare specimens before further study on a compound microscope.

COMPOUND MICROSCOPES — High Power
Compound microscopes are used to view smaller specimens such as blood samples, bacteria, pond scum, water organisms, etc. These specimens require higher powers of magnification in order to see the detail. This type of microscope is the most common and is mainly used for studying materials prepared and placed as smears and sections between a slide and cover-slip.

A compound microscope has the light source and condenser underneath the stage, and the objective lenses on a revolving nosepiece that is above the stage, so that when you look through the microscope, you are looking down on the sample from the top. Typically, a compound microscope has 3 to 5 objective lenses that range from 4X to 100X oil-immersion magnification. With the eyepiece lens at 10X magnification, that gives a total magnification of 1000X. For most applications, a maximum magnification of 400X is sufficient.

Compound microscopes come in:

• Monocular: One eyepiece (one eye has to be kept closed while you look down the eyepiece holding a lens).
• Binocular: Two eyepieces
• Trinocular: Two eyepieces for normal viewing plus a third phototube on which you can mount a camera without interfering with the normal operation of the microscope. While it is possible to mount a camera on a monocular or binocular microscope, it is far better to use a trinocular microscope designed for camera work.

Compound microscopes have a light under the specimen that must pass through the sample in order for you to see an image. These microscopes also need a lens with a short focal distance, meaning the lens has to be placed near the specimen for it to be in focus. Compound microscopes have a more limited space and field of view than stereo microscopes. They are not suitable for viewing unprepared objects.

A compound microscope produces images that are upside down and reversed right to left, and the working space between the slide and the lens is very small. For this reason, it is not practical to work on specimens, nor can you fit large subjects under this type of microscope while observing the sample. If trying to work on specimens while observing or if you need larger sample containers, you should consider buying an inverted microscope.

INVERTED MICROSCOPES
An inverted microscope is upside down in comparison to a conventional compound microscope. The light source and condenser are above the stage, pointing down. The objective lenses and turret are below the stage, pointing up. The sample is placed on top of the stage the same as a compound microscope, except that you are actually viewing the bottom of the specimen instead of the top.

Inverted microscopes are used to view live specimens, either an entire culture or large sample in a relatively large container such as a Petri dish, without preparation, under more natural and less stressed conditions. Such a sample may sustain life over a much longer period of time in comparison to preparing microscope slides used with compound microscopes. Viewing samples in a Petri dish, for example, will have a much slower evaporation rate, higher gas exchange, slower rate of temperature change and no pressure from the coverslip vs. viewing live specimens on a microscope slide. Since inverted microscopes are often used for looking at living organisms and tissue that may be killed by staining, they often provide for "optical staining" through the use of phase contrast.

The maximum magnification available on an inverted microscope is more limited than compound microscopes, typically 40x is the highest powered objective (although some models have a 60x objective available). Oil immersion 100x objectives are often not available.

OPTICS:
All microscopes, regardless of type, have optics. The objective lens is the lens mounted on the nosepiece of the microscope. In stereo microscopes, the optic lenses are built into the eyepieces. The magnification can range from 4X to 1000X, but 4X, 10X and 40X are the most common. If you purchase a microscope that includes a 100X oil-immersion lens, you must use immersion oil for viewing objects when using this lens.

When light passes from the glass surface of the slide to the air above the slide, and then from the air above the slide into the objective lens, some image quality is lost. An oil-immersion objective uses a drop of oil between the slide and the objective which has the same refractive (light bending property) as glass, and thus no image quality is lost. This type of objective is an essential tool for serious, high magnification observing of very small microorganisms such as bacteria.

Achromatic lenses: Lenses have been color corrected so that they will show true specimen color. If a lens were not achromatic, you might not be able to view all colors. Achromatic also means 60% of the center of the lens surface is guaranteed to be focused and without aberrations or flaws.

Semi-plan objectives: Lenses that are 80% focused and aberration-free over the lens surface. Typically more expensive and are usually reserved for serious hobbiest or medical or lab professionals.

Plan objectives: Lenses that are 100% focused and without aberrations. Usually used in the laboratory or medical fields and are very expensive.

2 TYPES OF FOCUSING: