The 16th century invention that helped to broaden the study of cells was the microscope.

While not technically invented in the 16th century (the first rudimentary microscopes appeared earlier), it was during this period that the compound microscope was developed, allowing for much higher magnification and revealing the hidden world of microscopic structures. This was crucial in the development of cell theory, as it allowed scientists like Robert Hooke to observe and describe the cellular structure of plants and other materials for the first time.