But for this property we should have no twilight. The sun, instead of sending up his beams while 18° below the visible horizon, would come upon us out of an intense darkness, pass over our sky a brazen inglorious orb, and set in an instant amid unwelcome night.
Reflection is the rebound of the rays of light or heat from an opposing surface at the same angle as that at which they fall upon it. These are called angles of incidence and reflection, and are equal.
Refraction is the bending of a ray passing obliquely from a rarer into a denser medium. This may be observed when a rod is placed slantingly in a vessel of clear water; the part immersed will appear bent or broken. This is ordinary refraction. Terrestrial refraction is the same thing, occurring whenever there is a difference of density in the aerial strata.
The atmosphere absorbs some portion of the light which it receives. It is not all reflected or refracted or even penetrative.
Objects seen under various degrees of light, either convected or retarded by different media, appear near or distant, distinct or confused. Thus, we are often surprised at the apparent nearness and brightness of an opposite shore or neighboring island, in some conditions of the air, while at other times they seem distant and lie in shadowy obscurity.
The looming up of a vessel on the water is another common instance of the principle of refraction.
It has been noticed by almost every one, that, during the warm and moist nights of summer, the moon, as she rises above the horizon, appears much larger than when at the zenith. So the setting sun is seen of apparently increased size. Sir John Herschel asserts that the appearance is an illusion, and so do some others. Professor Carey says, that, if we look through a paper tube at the moon when on the horizon, the paper being folded so as to make the aperture of its exact size, and then look again at it when it reaches the zenith, we shall find there is no difference.
On the other hand, an experiment is offered by a German Professor, of the name of Milo, of this kind: If we look through a tube so constructed as to have one side filled with spirits of wine and the other with common air, the half of the object seen through the former will be found to appear much larger to the eye than the other half seen through the latter.
It is laid down, that, where extraordinary refraction takes place laterally or vertically, the visual angle of the spectator is singularly enlarged, and objects are magnified, as if seen through a telescope. Dr. Scoresby, a celebrated meteorologist and navigator, mentions some curious instances of the effects of refraction seen by him in the Arctic Ocean.
Many remarkable phenomena attend this state of the atmosphere, known as the Fata Morgana of Sicily, the Mirage of the Desert, the Spectre of the Brocken, and the more common exhibitions of halos, coronæ, and mock suns. The Mountain House at Catskill has repeatedly been seen brightly pictured on the clouds below. Rainbows are also due to this condition of the atmosphere.