In 1887 Ramón y Cajal moved to Barcelona for a professorship. There he first learned about Golgi's method, a cell staining method which uses potassium dichromate and silver nitrate to (randomly) stain a few neurons a dark black color, while leaving the surrounding cells transparent. This method, which he improved, was central to his work, allowing him to turn his attention to the central nervous system (brain and spinal cord), in which neurons are so densely intertwined that standard microscopic inspection would be nearly impossible. During this period he made extensive detailed drawings of neural material, covering many species and most major regions of the brain.