The first microscope was invented by Zacharias and Hans Janssen. It was a tube with a lens at both ends. Zacharias also invented the telescope.
Robert Hooke invented a better microscope with an oil lamp for lighting. His microscope had a lens that focused the light from the flame onto the specimen. He then looked at a cork under the microscope and saw little rectangle rooms. He thought they looked like the cells that monks lived in, so he decided to call the little rooms cells.
Anton Van Leeuwenhoek invented a microscope that had a very tiny lens, but could magnify the object up to 266x. He discovered bacteria, free-living and parasitic microscopic protists, sperm cells, blood cells, microscopic nematodes and rotifers, and much more.
In 1838, Matthias Schleiden proposed the idea that all plants are made of cells. He found that out by looking at all of he plants under the microscope and saw that they all had something in common, they are all made of cells!
A year later, Theodor Schwann concluded that all animals are made of cells. Then, Schleiden and Schwann concluded that all living things are made of cells.
Although Schleiden and Schwann made a very useful discovery about living things, they did not say where the cells came from. In 1855, Rudolph Virchow proposed that all cells come from preexisting cells.
After all of these discoveries, the cell theory was made. It states that all living things are made of cells, cells are the basic units of structure and function in living things, and all cells are produced from other cells.