Who really invented the microscope?

Nobody knows. The earliest known compound microscopes, which pair an objective lens near the specimen with an eyepiece, appeared in Europe around 1620, and their inventor is unknown despite many claims over the years. Galileo Galilei is sometimes named: after 1610 he seems to have found that he could close-focus his telescope on small objects, and after seeing a compound microscope built by Cornelis Drebbel exhibited in Rome in 1624, he built his own improved version. Even the name came afterward: when Galileo submitted his instrument to the Accademia dei Lincei in 1625, Giovanni Faber coined the word "microscopio" for what Galileo had called the occhiolino, the "little eye."

Some of the most famous early discoveries came from much simpler instruments. Robert Hooke's Micrographia of 1665 made a huge impression with its illustrations, and Hooke made tiny lenses from small globules of glass. Antonie van Leeuwenhoek pushed the single lens furthest: he sandwiched a very small glass ball between holes in two riveted metal plates, with a screw-adjusted needle to hold the specimen, and reached up to 300 times magnification. With it he re-discovered red blood cells and spermatozoa, and on 9 October 1676 he reported the discovery of micro-organisms.

Compound microscopes needed more than lenses to live up to their promise. Their performance depends on a condenser that focuses light onto the specimen and an objective that captures light from it, and early instruments stayed limited until that principle was fully developed, from the late 19th to the very early 20th century. In 1893, August Köhler's method of even illumination became central to reaching the light microscope's theoretical resolution limits, and later, phase contrast and differential interference contrast made it possible to image unstained, transparent samples.