“I will try to make these threads multiply in something that is as near as possible like the stuff an animal’s body is made of--it must be just like living stuff,” Koch muttered, and he put a wee pin-point piece of spleen from a dead mouse--spleen that was packed with the tangled threads, into a little drop of the watery liquid from the eye of an ox. “That ought to be good food for them,” he grumbled. “But maybe, too, the threads have got to have the temperature of a mouse’s body to grow,” he said, and he built with his own hands a clumsy incubator, heated by an oil lamp. In this uncertain machine he deposited the two flat pieces of glass between which he had put the drop of liquid from the ox-eye. Then, in the middle of the night, after he had gone to bed, but not to sleep, he got up to turn the wick of his smoky incubator lamp down a little, and instead of going back to rest, again and again he slid the thin strips of glass with their imprisoned infinitely little sticks before his microscope. Sometimes he thought he could see them growing--but he could not be sure, because other microbes, swimming and cavorting ones, had an abominable way of getting in between these strips of glass, over-growing, choking out the slender dangerous rods of anthrax. “I must grow my rods pure, absolutely pure, without any other microbes around,” he muttered. And he kept flounderingly trying ways to do this, and his perplexity pushed up huge wrinkles over the bridge of his nose, and built crow’s-feet round his eyes.... Then one day a perfectly easy, a foolishly simple way to watch his rods grow flashed into Koch’s head. “I’ll put them in a _hanging-drop_, where no other bugs can get in among them,” he muttered. On a flat, clear piece of glass, very thin, which he had heated thoroughly to destroy all chance microbes, Koch placed a drop of the watery fluid of an eye from a just-butchered healthy ox; into this drop he delicately inserted the wee-est fragment of spleen, fresh out of a mouse that had a moment before died miserably of anthrax. Over the drop he put a thick oblong piece of glass with a concave well scooped out of it so that the drop would not be touched. Around this well he had smeared some vaseline to make the thin glass stick to the thick one. Then, dextrously, he turned this simple apparatus upside down, and presto!--here was his hanging-drop, his ox-eye fluid with its rod-swarming spleen, imprisoned in the well--away from all other microbes. [Illustration: (line drawing of hanging-drop)] Koch did not know it, perhaps, but this--apart from that day when Leeuwenhoek first saw little animals in rain water--was a most important moment in microbe hunting, and in the fight of mankind against death.