Four hundred years ago, the world of the microscope was unexplored. That means the structure of things like plants and the tissues of animals were a mystery, and there were thousands of other plants and animals that we didn't even know existed! The causes of the diseases could only be hypothesized about and medical science was limited. Antonie van Leeuwenhook's invention of the microscope in the 17th century brought about a revolution in scientific knowledge.
The word microscope comes from the word micro, meaning very small, and scope, meaning an instrument for looking at objects. Anything which is too small to be seen with the naked eye is microscopic.
While it had been known for more than 2,000 years that glass bends light, the fist accurate lenses were not made until around the year 1300. It wasn't until 1600, though, that it was discovered that optical instruments could be made by combining lenses.
Antonie van Leeuwenhook was a Dutch scientist and one of the pioneers of microscopy in the late 17th century. He made his own simple microscopes which had a single lens and were handheld. He made several drawings of what he observed and discovered bacteria, although he didn't know what they were at the time. In the middle of the 17th century, Robert Hooke drew pictures of cork seen through the microscope. Just like van Leeuwenhook, Hooke wasn't sure of exactly what he had seen.
Because of the low glass quality and imperfect shape of the lenses, many early microscopists saw very distorted images. Throughout the 19th century, huge improvements were made to lenses and the microscope as we know it today was gradually developed into a better instrument.