The device that led to the discovery of cells was the microscope.

Here's why:

* Magnification: Microscopes allow us to see objects that are too small to be seen with the naked eye.

* Robert Hooke: In 1665, Robert Hooke used a microscope to examine a thin slice of cork. He observed small, box-like compartments that he named "cells" because they reminded him of the cells of a monastery.

* Anton van Leeuwenhoek: Around the same time, Anton van Leeuwenhoek, using a more powerful microscope, observed living cells, including bacteria, red blood cells, and sperm.

So, the microscope's ability to magnify tiny objects enabled scientists to see cells for the first time, leading to the groundbreaking discovery of the fundamental unit of life.