Anton Van Leeuwenhoek, the famous Dutch microscopist, using a good quality simple lens (mag × 200) observed nuclei and unicellular organisms including bacteria. He was the first man to discover the bacteria and protozoan.
After the discovery of nucleus, two German biologists, named M.J. Schledien (Botanist) and T. Schwann (Zoologist) postulated a theory known as “Cell Theory” (1838-39). According to this theory, all living bodies are essentially cellular in nature i.e. the cells are the basic unit of structure and function in living organism.
Ordinary compound microscope