Robert Brown was a Scottish biologist working in the beginning half of the 1800's. About 200 years before Robert Brown, Anton Van Leeuwenhoek invented the first microscope. Over the course of those 200 years many improvements happened to the microscope. Image magnification and image quality were both vastly improved. Those improvements greatly aided Brown's work. In 1833 Robert Brown noticed that all of the cells that he examined contained a tiny dark structure. Today we call this spot the nucleus. It was ground breaking work at the time.
The fact that Brown noticed that each cell that he examined had a nucleus means that Brown was always examining eukaryotic cells. That for sure covers a lot of living things. In fact, 4 of the 6 kingdoms of living things are made of eurkaryotic cells. It does mean that Robert Brown was not examining bacteria though, since bacteria is prokaryotic.