Robert Hooke contributed to the cell theory by discovering cells in 1665/05%3A_Cells/5.02%3A_Discovery_of_Cells_and_Cell_Theory). He was an English scientist who improved the design of the existing compound microscope and used it to observe organisms such as insects, sponges, bryozoans, foraminifera, and bird feathers. When he looked at a thin slice of cork under his microscope, he saw what looked like a honeycomb, which he illustrated and described as cells/05%3A_Cells/5.02%3A_Discovery_of_Cells_and_Cell_Theory). Hookes discovery of plant cells led to the understanding of cells as the smallest units of life, which is the foundation of the cell theory. He was also the first person to use the term "cell" to describe these tiny units of life/05%3A_Cells/5.02%3A_Discovery_of_Cells_and_Cell_Theory).