 to produce most of the image multiplication and a second to both magnify and focus the image resulting from the first. Some of these compound microscopes have only one eyepiece and are thus monocular; more often, they have two and are therefore called binocular.
Light microscopy can in turn be divided into brightfield and darkfield types. The former is the most common; if you've ever used a microscope in a school lab, chances are excellent that you engaged in some form of brightfield microscopy using a binocular compound microscope. These gadgets simply light up whatever is under study, and different structures in the visual field reflect different amounts and wavelengths of visible light based on their individual densities and other properties. In darkfield microscopy, a special component called a condenser is employed to force light to bounce off the item of interest at such an angle that the object is easy to visualize in the same general manner as a silhouette.
Parts of a Microscope
First, the flat, usually dark-colored slab on which rests your prepared slide (usually, viewed objects are placed on such slides) is called a stage. This is fitting, since, quite often, whatever is on the slide contains living matter that can move and is thus in a sense "performing" for the viewer. The stage contains a hole in the bottom called an aperture, situated within the diaphragm, and the specimen on the slide is placed over this opening, with the slide fixed in place using stage clips. Below the aperture is the illuminator, or light source. A condenser sits between the stage and the diaphragm.
In a compound microscope, the lens nearest the stage, which can be moved up and down for purposes of focusing the image, is called the objective lens, with a single microscope typically offering a range of these to pick from; the lens (or more often, lenses) you look through are called the eyepiece lenses. The objective lens can be moved up and down using two rotating knobs on the side of the microscope. The coarse adjustment knob is used to get in the right general visual range, whereas the fine adjustment knob is used to bring the image into maximally sharp focus. Finally, the nosepiece is used to change between objective lenses of different magnification powers; this is done by simply rotating the piece.
Mechanisms of Magnification
The total magnification power of a microscope is simply the product of the objective lens magnification and the eyepiece lens magnification. This might be 4x for the objective and 10x for the eyepiece for a total of 40, or it might be 10x for each type of lens for a total of 100x.
As noted, some objects have more than one objective lens available for use. A combination of 4x, 10x and 40x objective lens magnification levels is typical.
The function of the condenser is not to magnify light in any way, but to manipulate its direction and angles of reflection. The condenser controls how much light from the illuminator is permitted to pass up through the aperture, controlling the intensity of the light. It also, critically, regulates the contrast. In darkfield microscopy, it is the contrast between different, drab-colored objects in the visual field that is most important, not their appearance per se. They are used to tease out images that might not appear if the apparatus were simply used to bombard the slide with as much light as the eyes above it could tolerate, leaving the viewer to hope for the best results.
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.