By this method of research man is expanding his universe beyond anything that could have been conceived in the pre-scientific days. Hour by hour, while we work and play and sleep, the mind of our race is discovering new worlds in which our posterity will dwell. For uncounted ages man walked upon the earth, surrounded by infinite swarms of bacterial life of whose existence he never dreamed. The invisible rays of the spectrum beat upon him, and he knew nothing of what they did to him, whether good or evil. He lifted his head and saw vast universes of suns, in comparison with which his world was a mere speck of dust; yet to him these universes were globes or lanterns which some divinity had hung in the sky. One of the most fascinating illustrations of how the mind runs ahead of the senses is the story of the planet Uranus, which, less than two hundred years ago, had never been beheld by the eye of man. A mathematician seated in his study, working over the observations of other planets, their motions in relation to their mass and distance, discovered that their behavior was not as it should be. At certain times none of them were in quite the right place, and he decided that this variation must be due to the existence of an unknown body. He worked out the problem of what must be the mass and the exact orbit of this body, in order for it to be responsible for the variations observed; and when he had completed these calculations, he announced to the astronomical world, "Turn your telescopes to a certain spot in the heavens at a certain minute of a certain night, and you will find a new planet of a certain size." And so for the first time the human senses became aware of a fact, which by themselves they might not have discovered in all eternity. Now, the importance of exact knowledge concerning a new planet may not be apparent to the ordinary man; but if the thing which is discovered is, for example, an unknown ray which will move an engine or destroy a cancer, then we realize the worthwhileness of research, and the masters of the world's commerce are willing to give here and there a pittance for the increase of such knowledge. But men of science, who have by this time come to a sense of their own dignity and importance, understand that there is no knowledge about reality which is useless, no research into nature which is wasted. You might say that to describe and classify the fleas which inhabit the bodies of rats and ground-squirrels, and to study under the microscope the bacteria which live in the blood of these fleas--that this would be an occupation hardly worthy of the divinity that is in man. But presently, as a result of this knowledge about fleas and flea diseases being in existence and available, a bacteriologist discovers the secret of the dread bubonic plague, which hundreds of times in past history has wiped out a great part of the population of Europe and Asia.