Telescope

an optical instrument. It consists essentially of a tube in which lenses are mounted. The lenses collect the rays of light from a distant object and bring them to a focus in such a way that they seem to proceed from a large object near at hand. The telescope is said, therefore, to magnify objects and bring them nearer.

Telescope is a Greek word, meaning to see afar. The first astronomical telescope was manufactured at Florence by Galileo in 1609. Milton, who visited Galileo in that city, called his instrument the optik tube. Galileo's instrument was a simple refracting telescope. The field glass, the spyglass, and the opera glass are constructed on the same principle. Sir Isaac Newton invented a telescope in which the light travels the length of the tube, falls on a reflecting surface or mirror set at an angle, turns at right angles and passes out through the side of the tube to the eye of the observer. Such an instrument is known as a reflecting telescope.

The large astronomical instruments of the present day are reflecting telescopes. The largest reflecting telescope in the world is that of the Earl of Rosse. He mounted it at Birr Castle, Parsonstown, Ireland, in 1845. The tube is fifty-four feet in length and is seven feet in diameter.

The largest refractors in America were made at the laboratory of Clark and Sons, Cambridge, Massachusetts. Of these the Yerkes telescope, owned by the University of Chicago, is sixty-five feet long. The aperture of the tube is forty inches in diameter. The telescope of the Lick Observatory, next in size, belonging to the University of California, has an aperture of thirty-six inches and a focal length of fifty-six feet. There are said to be but three firms in the world, one in England, one in Germany and one in France, capable of making suitable glass for lenses.

See Galileo; Microscope; Astronomy; Lick; Clark; Lens; Stars