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Basic Microscope Recommendations

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Decent bright-field microscopes are not very expensive. They range from about $200 - $300 (and up). Really cheap 'scopes may not be suitable for study or video. Microscopes last for decades if well-maintained.

Microscope Parts

Microscope Magnification

It would be nice if we could see microbes clearly at outrageously high power. Leave that to the Electron Scanning Microscopes. Most of the diagnostic features of microscopic organisms are available with bright-field illumination at total magnifications of up to 1000x.

1000x viewing power can be interesting but it is rarely used unless fixed bacteria is under study. 1000x requires a special oil between the objective lens and cover glass. The oil follows the lens while scanning the slide.

Useful and Useless (or False) Magnification - Thumbs Up for the 10x Eyepiece

This is important to know:

Microscope objectives display their magnification power followed by the very important Numerical Aperture (abbreviation: N.A.) on the side of the lens tube, i.e., 10x/N.A. 0.25. This N.A. number determines the highest useful magnification by multiplying it by 1000.

For instance, a 10x eyepiece and a 40x objective with a N.A. value of 0.65 would yield a total magnification of 400x. The N.A. value of 0.65 X 1000 = 650. Since the magnification of 400x does not exceed 650, it is useful magnification and will be sharp.

Useless magnification is when things get bigger through the eyepiece but not any sharper. No new detail comes into focus.

if a 20x eyepiece is used with a 40x objective with a N.A. value of 0.65, the total magnification is 800x. This exceeds the total useful magnification of 0.65 times 1000 = 650.

Microscopes often advertise magnifications of up to 2500x. The are mostly false magnifications given the objectives included with the 'scope.

Standard microscope objectives usually include the following objectives:

(A 10x eyepiece works best for clear viewing)
4x/N.A. 0.17
10x/N.A. 0.25
40x/N.A. 0.65 and
100x/N.A. 1.25 (oil immersion)
Each increase in magnification will yield sharp images with new detail with a 10x eyepiece.

Bill Porter, who posts on the facebook page "Amateur Microscopy," suggests that the 100x objective is so rarely used, it's best to replace it with a 20x objective and arange them on the turret thusly:

Viewing powers are displayed using the 10x eyepiece:
4x/N.A. 0.17   = 40x
10x/N.A. 0.25  =100x
20x /N.A. 0.40 = 200x
40x/N.A. 0.65 = 400x
Again, the 10x eyepiece is best for viewing clear images.

Bill also suggested a useful combination by eliminating the 4x objective and adding a 60x objective lens to the turret:

10x/N.A. 0.25  =100x
20x /N.A. 0.40 = 200x
40x/N.A. 0.65 = 400x
60x/N.A. 0.85 = 600x

Experimenting with diffent power eyepieces is recommended keeping in mind the highest useful magnification given the N.A. number of the objective.

Basic Recommended Microscope Needs

Compound microscope (or trinocular for camera setup) with a sturdy frame
Quality 10x eyepiece and objectives as mentioned above.
Focusing knob with fine adjustment
X-Y stage control
LED illumination with brightness control
Condenser for focusing the illumination
Iris for light exposure control
Dust cover
Laptop or computer for the camera feed if used

Recommended Supplies

Glass microscope slides - 1" x 3" (76.2 x 25.4 mm), 1.0-1.2 mm thick
Glass cover slips - #1 thickness, 22mm x 22mm,  0.13-0.17 mm thick
Small dish to hold a sample from the culture
Vacuum sucking pen with broad suction header
Long 10ml plastic pipette
Paper towels

What You Don't Really Need

You don't need 20x or 25x eyepieces. At higher powers they don't provide any greater detail (see above).

There are several illumination schemes available for professional transmitted light microscopy, such as bright filed, dark-field, phase contrast, polarized, differential interference contrast, modulation contrast, to name a few. These techniques are helpful for viewing specific parts of cells.

The common bright-field technique, where light shines up from beneath, is all that is needed by the amateur to study and identify microscopic organisms.

Adjusting the light brightness, condenser iris, and iris exposure (if equipped) help in viewing bacteria, tiny see-through amoebas, cilia, testate structures and other hard to see details.

At lower viewing powers such as 100x, adjust lighting to be darker to prevent eye fatigue and better see clear cells. As the viewing power increases, adjust the lighting to suit.

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Copyright 2017 Steve Cunningham, Baltimore, MD