Anton van Leeuwenhoek
In the 1600's Leeuwenhoek developed the first microscope using a single magnifying lens. He was able to describe blood cells and bacteria by viewing scrapings from his teeth.
Robert Hooke
Around 1655 Hooke used Leeuwenhoek's ideas and made the first compound microscope, which used more than one lens to magnify an object. He took some thin samples of tree bark and observed some tiny hollow boxes.