“You see, I can measure exactly--it is always the same!--the amount of poison to kill in forty-eight hours a mouse weighing ten grams.... You know, I can now plot a curve of the way the immunity of my mice increases--it is as exact as experiments in the science of physics.... You understand, I have found how it is this poison kills my mice; it clots his blood corpuscles inside his arteries! That is the whole explanation of it ...” and Paul Ehrlich waved test-tubes filled with brick-red clotted clumps of mouse blood at his famous chief, proving to him that the amount of poison to clot that blood was just the amount that would kill the mouse that the blood came from. Torrents of figures and experiment Paul Ehrlich poured over Robert Koch---- “But wait a moment, my dear Ehrlich! I can’t follow you--please explain more clearly!” “Certainly, Herr Doktor! That I can do right off!” Never for a moment does Ehrlich stop talking, but grabs a piece of chalk, gets down on his knees, and scrawls huge diagrams of his ideas over the laboratory floor--“Now, do you see, is that clear?” There was no dignity about Paul Ehrlich! Neither about his attitudes, for he would draw pictures of his theories anywhere, with no more sense of propriety than an annoying little boy, on his cuffs and the bottoms of shoes, on his own shirt front to the distress of his wife, and on the shirt fronts of his colleagues if they did not dodge fast enough. Nor could you properly say Paul Ehrlich was dignified about his thoughts, because, twenty-four hours a day he was having the most outrageous thoughts of why we are immune or how to measure immunity or how a dye could be turned into a magic bullet. He left a trail of fantastic pictures of those thoughts behind him everywhere! Just the same he was the most exact of men in his experiments. He was the first to cry out against the messy ways of microbe hunters, who searched for truth by pouring a little of this into some of that, and in that laboratory of Robert Koch he murdered fifty white mice where one was killed before, trying to dig up simple laws, to be expressed in numbers, that he felt lay beneath the enigmas of immunity and life and death. And that exactness, though it did nothing to answer those riddles, helped him at last to make the magic bullet. III