Janssen created the first microscope. It had a 3x magnification but when extended, could reach up to 10x ("History of the Microscope", 2010)
Robert Hooke, studying thin slices of cork, discovered box-like structures he called "cells". They were called cells because the structure reminded him of the cells of a monastery("History of the Microscope", 2010).
Anton van Leeuwenhoek created a powerful microscope, and with it, discovered the existence of unicellular organisms.
Matthias Schleiden states that all plants are made of cells.
Theodore Schwann states that all animals are made of cells.
Rudolf Virchow states that all cells come from preexisting cells.
Carl Zeiss, Ernst Abbe, and Otto Schott revolutionized the manufacture of optical instruments and invented the modern compound microscope.
Max Knoll and Ernst Ruska invented the electron microscope. With it, they were able to see even atoms.