Robert Hooke discovered cells by looking at a thin slice of cork through a microscope. Hooke saw small empty spaces which he then named cells.
Hooke was employed by King Charles II of England, who wanted some studies done with the use of microscopes. Using his mechanical and technical talents, Hooke was able to refine the microscopes to get a clearer and close look at the objects he observed, which allowed him to get the first glimpses of cells. He also called the spaces he saw pores, but the term cells was more appreciated. The spaces reminded Hooke of monks' cells, from which he borrowed the name.
Even using his primitive technology, Hooke was able to predict that there were over 1.2 million cells per cubic inch, showing that he did have some idea of how small and common the objects are. To show his discoveries, he drew what he saw. These drawings were collected and published, along with his findings, in 1665, in his book titled "Microphagia: or Some Physiological Descriptions of Miniature Bodies Made by Magnifying Glasses." The book included information about microscopes and cells, and put him on track to becoming one of the founding fathers of cell theory.