Robert Brown contributed to cell theory by showing the radical motion of molecules within a cell under the light of a microscope. The Brownian method was named after Brown's discovery of the way that the molecules moved.
In 1802, Franz Bauer first discovered and described the nucleus of a cell. He used an advanced microscopic system to achieve this. While Brown was not the first to discover the nucleus, he is often credited with it because he was the first scientist to give it a name while appropriately describing what it was and how it functioned within the cell.
Robert Brown was a botanist who studied every aspect of plant life, from the way that plants grow to the way that their cells work together. He used microscopes in a way that nobody had used them previously. He not only worked to aptly describe the nucleus, but he was able to provide a description of the cytoplasmic functions within cells. In unrelated studies, he was able to find out the difference between angiosperms and gymnosperms. These plants had previously been thought to be the same. Many of the plant families that are present in modern times were discovered, categorized and named by Brown.