The optical microscope is a type of microscope, which uses visible light and a system of lenses to magnify images of small samples. Optical microscopes are the oldest and simplest of the microscopes. There are two basic configurations of optical microscope in use, the simple and compound.

A simple microscope is a microscope that uses only one lens for magnification, and is the original light microscope. Van Leeuwenhoek’s microscopes consisted of a single, small, convex lens mounted on a plate with a mechanism to hold the material to be examined. Demonstrations by British microscopist Brian J. Ford have produced surprisingly detailed images from such basic instruments.

The use of a single, convex lens to magnify objects for viewing is found today only in the magnifying glass, the hand-lens, and the loupe. The compound microscope uses a set of many lenses in order to maximize magnification. The diagram below shows a compound microscope. In its simplest form - as used by Robert Hooke, for example, the compound microscope would have a single glass lens of short focal length for the objective, and another single glass lens for the eyepiece or ocular lens.

Modern microscopes of this kind are usually more complex, with multiple lens components in both objective and eyepiece assemblies. These multi-component lenses are designed to reduce aberrations, particularly chromatic aberration and spherical aberration. In modern microscopes the mirror is replaced by a lamp unit providing stable, controllable illumination. It is impossible to say who invented the compound microscope. Dutch spectacle-makers Hans Janssen and his son Zacharias Janssen are often said to have invented the first compound microscope in 1590, but this was a declaration by Zacharias Janssen himself halfway through the 17th century. The date is certainly not likely, as it has been shown that Zacharias Janssen actually was born around 1590. Another favorite for the title of ‘inventor of the microscope’ was Galileo Galilei. He developed an occhiolino or compound microscope with a convex and a concave lens in 1609. Galilei’s microscope was celebrated in the ´Lynx academy´ founded by Federico Cesi in 1603. Francesco Stelluti’s drawings of three bees were part of pope Urban VIII´s seal, and count as the first microscopic figure published.

Christian Huygens, another Dutchman, developed a simple 2-lens ocular system in the late 1600s that was achromatically corrected and therefore a huge step forward in microscope development. The Huygens ocular is still being produced to this day, but suffers from a small field size, and the eye relief is uncomfortably close compared to modern wide field oculars. Anton van Leeuwenhoek is generally credited with bringing the microscope to the attention of biologists, even though simple magnifying lenses were already being produced in the 1500s, and the magnifying principle of water-filled glass bowls had been described by the Romans. Van Leeuwenhoek’s home-made microscopes were actually very small simple instruments with a single very strong lens.

They were awkward in use but enabled van Leeuwenhoek to see highly detailed images, mainly because a single lens does not suffer the lens faults that are doubled or even multiplied when using several lenses in combination as in a compound microscope. It actually took about 150 years of optical development before the compound microscope was able to provide the same quality image as van Leeuwenhoek’s simple microscopes.