A microscope is an instrument that enables the user to view objects smaller than can be seen with the naked eye. The word usually refers to an optical compound microscope.
Leeuwenhoek made one of the earliest practical microscopes and observed tiny "animalcules" under its high magnification (270X). No one knows why he did not realize that these were germs. Two centuries would pass before Pasteur's germ theory of disease.
Optical compound microscopes
Optical compound microscopes are somewhat similar to telescopes in that their basic design is a tube, with holding a lens or a system of lenses at each end of the tube. A microscope consists of an ocular or eyepiece that you look into, and an objective which is close to the object that is being observed.
Such microscopes can magnify from about 10 to 1000 times. They fall into two broad categories based on power. The higher the power, the closer the objective lens must be to the object and the shallower the depth of field.
Lower-powered microscopes, up to about 40X have enough clearance and depth-of-field to examine ordinary three-dimensional objects directly. They are often built as a binocular-like pair of microscopes; each of the two tubes sees the object at a slightly different angle, and the result is a dramatic three-dimensional view. They are used as dissecting microscopes in biology laboratories; in operating rooms for very exacting operations; and in industry for assembling and inspecting tiny objects.
Higher-powered microscopes cannot usually be used in this way. Normally, the object to be observed needs to be sliced with a microtome into a "thin section," "fixed" and "stained" with chemicals, and mounted on a flat glass slide with a cover slip cemented over it. At the higher powers, it is necessary to use oil-immersion objectives: a droplet of oil is placed on the slide and connects the objective to the cover slip, so that the light passes from glass into oil rather than air. When used in this way, the illuminator is a part of the optical system and just as important as the rest of the system.
Phase contrast microscopes
Many biological structures consist of thin, transparent, jellylike material, surrounded by water-like fluid. Even when magnified, such structures are difficult or impossible to see, because they do not affect light very much. Anyone who has ever dropped a contact lens into water should understand this. The traditional method for seeing these structures, developed in the 1900s, was to fix them (kill and harden them) with an agent like formadehyde or glutaraldehyde, and then treat them chemical dyes and stains. In the twentieth century, optical methods were developed that made these structures visible without staining. This was an important breakthrough, particularly in allowing the study of living cells.