 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 i