Related article

A microscope is an instrument that allows us to see a large image of a tiny object. It does this by focusing light, or some other form of radiation, through one or more lenses and thus magnifying the object. The capacity to see objects invisible to the naked eye revolutionized biology and medicine, revealing the cells that make up organisms and the very existence of microorganisms.
So how does it work? Most microscopes have two lens systems. You look through an eyepiece lens (or lenses) down to a second lens, or set of lenses, called the objective lens. Directly below the objective lens is the specimen on a glass slide. Below that a mirror reflects light up through the slide into your eye. As the light passes through the lenses, the image of the specimen is magnified. The combination of lenses increases the magnification. Focus is adjusted by moving the objective lens closer to or farther away from the specimen. Using multiple lenses (called compound lenses) in either the eyepiece or—almost always—the objective lens corrects distortion (called chronic aberration) in any one lens.
That lenses magnify has been known since ancient times. That two lenses together magnify further was noticed sometime around the turn of the 17th century and applied in both the microscope and the telescope. The invention of the microscope has been credited to three different Dutch eyeglass makers: Hans Lippershey (circa 1570-circa 1619), who may also have invented the first practical telescope; and Hans Janssen (dates unknown) and his son, Zacharias Janssen (1580-circa1638). Whoever did it first, what they produced was a handheld sliding tube with a concave lens at one end and a convex lens at the other.
The English natural philosopher Robert Hooke (1635-1703) made significant improvement in the design, creating an instrument with most of the basic features of modern optical microscopes. It was mounted on a ball-and-socket adjustable frame and included a specimen stage (the part that holds the slide), lens-focusing adjustments, and an eyecup. Although Hooke introduced an external light source and a condenser system to focus light on the specimen, his images were blurry. The problem: A bad combination of lenses.
Dutch biologist Antoni van Leeuwenhoek (1632-1723) was the person who really moved the optics forward. His instrument employed only one lens at a time, but van Leeuwenhoek ground the lenses himself. These lenses were so fine that he could magnify objects as much as 275 times. Thus van Leeuwenhoek became the first person to observe single-celled animals (protozoa), red blood cells, and sperm. For this he is called the father of microbiology.
Later advances created compound microscopes with magnifications of up to 2,000 times. There is a finite limit, however, to the resolution possible with visible light, whose long wavelengths are subject to bending, or diffraction. This led to the development of microscopes that use electrons and X-rays instead of light. Some electron microscopes have achieved magnifications of up to 2 million times. Such instruments have made it possible to view matter at the atomic level.