Note:  Do not rely on this information. It is very old.

Galilean Telescope

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.