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circles darkened under those gray eyes that still, but not so often, danced with that owlish humor.

So this compound six hundred and six, saving its thousands from death, from insanity, from the ostracism worse than death that came to those sufferers whose bodies the pale spirochete gnawed until they were things for loathing, this 606 began killing its tens. Paul Ehrlich wore his too feeble body to a shadow, trying to explain a mystery too deep for explanation. There is no light on that mystery now, ten years after Ehrlich smoked the last of his black cigars. So it was that this triumph of Paul Ehrlich was at the same time the last disproof of his theories, which were so often wrong. "Compound six hundred and six unites chemically with the spirochetes and kills them—it does not unite chemically with the human body and so can do no damage!" That had been his theory. . . .

But alas! What is the chemistry of what this subtle 606 does to the still more subtle—and unknown—machine that is the human body? Nothing is known about it even now. Paul Ehrlich paid the penalty for his fault—which may be forgiven him seeing the blessings he has brought to men—his fault of not foreseeing that once in every so many thousands of bodies a magic bullet may shoot two ways. But then, the microbe hunters of the great line have always been gamblers: let us think of the good brave adventurer Paul Ehrlich was and the thousands he has saved.

Let us remember him, trail-breaker who turned a corner for microbe hunters and started them looking for magic bullets. Already (though it is too soon to tell the whole story) certain obscure searchers, some of them old slaves of Paul Ehrlich, sweating in the great dye factories of Elberfeld, have hit upon a most fantastical drug. Its chemistry is kept a secret. It is called "Bayer 205." It is a mild mysterious powder that cures the hitherto always fatal sleeping sickness of Rhodesia and Nyassaland. That was the ill, you remember, that the hard man, David Bruce, fought his last fight, in vain, to prevent.