The point of heat at which the eye begins to discover luminosity has been estimated at 1,000 deg. 546. _What is the velocity of artificial light?_ The light of a fire, or of a candle, or gas, travels with the same velocity as the light of the sun,--a velocity which would convey light eight times round the world while a person could count "one." 547. _At what rate of velocity does the light of the stars travel?_ At the same velocity as all other light. And yet there are stars so distant that, although the light of the sun reaches the earth in eight minutes and a half, it requires _hundreds of years_ to bring their light to us. [Verse: "What is man, that thou art mindful of him? and the son of man that thou visitest him?"--PSALM VIII.] 548. _What is the relative intensity of primary and reflected light?_ The intensity of a reflection depends upon the power of the reflecting surface. But, taking the sun and moon as the great examples of primary and reflected light, the intensity of the _sun's light_ is 801,072 times _greater than that of the moon_. 549. _What is polarized light?_ _Polarized light_ is light which has been subjected to _compound refraction_, and which, after polarization, exhibits a new series of phenomena, differing materially from those that pertain to the primary conditions of light. 550. _What are the chief deductions from the phenomena observed under the polarization of light?_ The polarization of light appears to confirm in a high degree the vibratory _theory of light_; and to show that the vibrations of light have two planes or directions of motion. The mast of a ship, for instance, has two motions: it progresses _vertically_ as the ship is impelled forward, and it rolls _laterally_ through the motion of the billows. Something like this occurs in the vibrations of light, only the _vertical vibration_ is the condition of _one ray_, and the _lateral vibration_ is the condition of another ray, and the vibrations of these two rays intersect each other in the solar ray. When these vibrations occur together, the ray has certain properties and powers. But by polarization the rays may be _separated_, and the result is two distinct rays, having _different vibrations_. It then appears that various bodies are transparent to these polarized rays _only in certain directions_. And this fact is supposed to show that bodies are made up of their atoms arranged in certain planes, through or between which the _lateral_ or the _vertical_ waves of light, together or singly, can or cannot pass; and that the transparency or the opacity of a body is determined by the _relation of its atomic planes_ to _the planes of the vibrations of light_. _Ordinary light_, passing through transparent media, produces no very remarkable effect in its course; but _polarized light_ appears to illuminate every atom of the permeated substance, and by surrounding it with a prismatic clothing, to afford an illustration of its _molecular arrangement_.