Galileo knew nothing of what we call the reflecting telescope. He found that by using a convex-lens as an object-glass he could bring the rays of light from any distant object to a focus, and it did not apparently occur to him that he could achieve the same end by the use of a concave mirror. James Gregory, a Scotchman, designed the first reflector in 1663, and described it in a book, but he was too poor to construct it. Nine years later Sir Isaac Newton, having studied Gregory’s plans, built the first reflecting telescope, which is now to be seen in the hall of the Royal Society in London. But invention has gone yet farther in perfecting these instruments with which to study the skies, and the great telescopes of modern times have in most instances discarded Newton’s reflector for the refracting instrument. And these are built on a tremendous scale. The Yerkes telescope at Williams Bay, Wisconsin, has a refractor of forty inches, and the one built for the Paris Exposition of 1900, one of fifty inches. In numerous other details they have changed, and yet each is chiefly indebted to that simple spy-glass of Galileo, by which he was able to show the nobles and senators of Venice full-rigged ships, which without it were barely distant specks on the horizon. Or, going still farther back, the men who make our present telescopes are following the trail that was first blazed on the day when the Dutch apprentice of Middleburg chanced to pick up two spectacle lenses and look through the two of them at once. Galileo made many great discoveries and inventions; there was hardly a field of science that he did not enter and explore; but his greatest work was to open a new world to men’s attention. It was this that brought him before the Inquisition and that branded him as a dangerous heretic, and it was this that placed him in the forefront of the world’s discoverers. Men might say that the earth stood still, because it suited them best to believe so, but Galileo gave the world an instrument by which it could study the matter for itself, and the world has gone on using that instrument and that method ever since. IV WATT AND THE STEAM-ENGINE 1736-1819 It was no pressing need that drove John Gutenberg to the invention of his printing press, nor was it necessity that led to Galileo’s discovery of the telescope, but it was a very urgent demand that led to the building of a steam-engine by James Watt. England and Scotland found that men and women, even with the aid of horses, could not work the coal mines as they must be worked if the countries were to be kept supplied with fuel. The small mines were used up, the larger ones must be deepened, and in that event it would be too long and arduous a task for men and women to raise the coal in small baskets, or for horses to draw it out by the windlass. A machine must be constructed that would do the work more quickly, more easily, and more cheaply.