Visible Objects make no impression on the Sense, but by the Images or Species which they send to the Eye: these Images are nothing but Rays of Light, issuing from the Contours, and other parts of the Object, which passing through the Air, meet on the Pupil of the Eye, and there form an Angle: Now as there are always Vapours in the Air, which surrounds us, it happens that several of the Rays never reach the Eye, being broken and intercepted in their Passage; insomuch that at a great distance, so many of these Rays are lost, that the Image comes maim'd and imperfect; and the Object in consequence thereof, appears confused and obscure. Add to this that the Organs of sight are fre quently indisposed; so that the Rays of Light failing of their ordinary impression, the Object appears dim and indistinct.