Development of staining methods
* A Weigert, a German scientist contemporary with Koch, had observed the use by Cohn and other, of various dyes to make clear the details of cell structures in histological preparations.
* The natural dyes carmine and hematoxylin were widely being used.
* Ehrlich improved methods, discovered by William Perkin a brilliant British chemist, of preparing very fine dyes from coal-tar distillates.
* In 1875, Weigert found that the dye methyl violet could be used to reveal bacteria in histological preparations.
* This method of making bacteria visible was adopted by Koch and soon came into wide use.