The microscope counts as one of the more remarkable inventions in the scientific world. Not only has it helped satisfy a great deal of basic human curiosity about things that are too small to see with the unaided eye, but it also has helped save countless lives. For example, a host of modern-day diagnostic procedures would be impossible without microscopes, which are absolutely vital in the microbiology world in visualizing bacteria, certain parasites, protozoans, fungi and viruses. And without being able to look at human and other animals cells and understand how they divide, the problem of deciding how to simply approach the various manifestations of cancer would remain a complete mystery. Life-giving advances such as in vitro fertilization ultimately owe their existence to the wonders of microscopy.
Like everything else in the world of medical and other technology, the microscopes of not so many years ago look like blunders and quaint relics when pitted against the best of the second decade of the 21st century – machines that one day will be snickered at in their own right for their obsolescence. The major players in microscopes are their lenses, for it is these, after all, that magnify images. It is therefore useful to know how the different kinds of lenses interact to form the often-surreal images that make their way into biology textbooks and onto the World Wide Web. Some of these images would be impossible to see without a special knickknack called a condenser.
History of the Microscope
The first known optical instrument that merits the designation of "microscope" was probably the device created by the Dutch youngster Zacharias Janssen, whose 1595 invention likely had considerable input from the lad's father. This microscope's magnifying power was anywhere from 3x to 9x. (With microscopes, "3x" simply means that the magnification achieved allows for visualization of the object at three times its actual size, and correspondingly for other numerical coefficients.) This was accomplished by essentially placing lenses at both ends of a hollow tube. As low-tech as this may seem, lenses themselves were not easy to come by in the 16th century.
In 1660, Robert Hooke, who is perhaps best known for his contribution to physics (in particular the physical properties of springs), produced a compound microscope sufficiently powerful to visualize what we now call cells, examining the cork in the bark of oak trees. In fact, Hooke is credited with coming up with the term "cell" in a biological context. Hooke later clarified how oxygen participates in human respiration and also dabbled in astrophysics; for such a true renaissance person, he is curiously underappreciated today compared to the likes of, say, Isaac Newton.
Anton van Leeuwenhoek, a contemporary of Hooke, made use of a simple microscope (that is, one with a single lens) rather than a compound microscope (a device with more than one lens). This was largely because he came from an unprivileged background and had to work at a humdrum job between making major contributions to science. Leeuwenhoek was the first human to describe bacteria and protozoans, and his findings helped prove that the circulation of blood throughout living tissues is a core process of life.
Types of Microscopes
First, microscopes can be classified based on the type of electromagnetic energy they use to visualize objects. The microscopes used in most settings, including middle and high school as well as most medical offices and hospitals, are light microscopes. These are exactly what they sound like and make use of ordinary light to view objects. More sophisticated instruments use beams of electrons to "illuminate" objects of interest. These electron microscopes use magnetic fields rather than glass lenses to focus the electromagnetic energy on the subjects under examination.
Light microscopes come in simple and compound varieties. A simple microscope has only one lens, and today such devices have very limited applications. The far more common type is the compound microscope, which uses one kind of lens 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.