Galilean Telescope is the simplest form of telescope, invented by Galileo, and employed most usefully by him in his observations on the heavens. It consists simply of two lenses - an object or field-glass and a concave eye-piece. The object-glass would of itself give an inverted image of the object, but the eye-piece is placed so as to prevent the formation of a real image from the field-glass, and the result is that an enlarged erect image is obtained. Opera-glasses are constructed on this principle; tbey have the advantage of showing objects in their right position, an advantage which is not shared with ordinary telescopes. Possessing only two lenses, it absorbs but little light; but because of the divergence of the waves of light, as they emerge from the concave eye-piece, it is necessary that the eye shall be placed very near the eye-piece. The invention is strictly due to a Dutchman, Jacques Metius, who discovered it by accident in 1609. By its means Galileo discovered the mountains of the moon, Jupiter's satellites, and the spots on the sun.