Human hippocampus stained with a method pioneered by Italian physician Camillo Gogli in 1873.Golgi discovered a chemical reaction that allowed him to examine nervous tissue in much greater detail than ever before. For some reason, hardening a piece of brain in potassium dichromate, and subsequently dousing it with silver nitrate, dyed only a few cell bodies and their respective projections in the tissue sample, revealing their complete structures and exact arrangement within the unstained tissue. If the reaction had stained all the neurons in a sample, Golgi would have been left with an unfathomable black blotch, as though someone had spilled a bottle of ink. Instead, his technique yielded neat black silhouettes against a translucent yellow background.
More in Scientific American’s Know Your Neurons series.