Because of the refractive effects of the atmosphere. Rays of light, passing obliquely from the sun through the air to the earth, are refracted three or four times by the varying density of the medium. Each refraction bends the rays towards the _perpendicular_; and hence we see the sun _before it rises_ and _after it sets_. [Illustration: Fig. 13.--DIAGRAM EXHIBITING THE REFRACTION OF THE SUN'S RAYS IN PASSING THROUGH THE ATMOSPHERE.] 483. _Why do figures, viewed through the hot air proceeding from furnaces, and from lime-kilns, appear distorted and tremulous?_ Because the ever varying density of the air which is flying away in hot currents, and succeeded by cold, _constantly changes the refractive power_ of the medium through which the figures are viewed. 484. _Why do the stars twinkle?_ Because their light reaches us through _variously heated and moving currents of air_. In this case the earth is the _kiln_, and the _stars_ the _object_ that is _viewed through the refractive medium_. 485. _Why does much twinkling of the stars foretell bad weather?_ Because it denotes that there are _various ærial currents_ of different temperatures and densities, producing _atmospheric disturbance_. [Verse: "And it shall come to pass, when I bring a cloud over the earth, that the bow shall be seen in the cloud."--GENESIS IX.] 486. _What causes the rainbow?_ The _refraction_ of the sun's rays by the _falling rain_. 487. _Why does the rainbow exhibit various colours?_ The colours belong to the _elementary rays of light_; and these rays having _different degrees of refrangibility_, some of them are bent more than others; they are therefore separated into _distinct rays of different colours_. 488. _Why are there sometimes two rainbows?_ Because the rays of _refracted_ light, reflected upon other drops of rain, are _again_ refracted, and then _reflected again_, forming a secondary bow. 489. _Why are the colours of the secondary bow arrayed in the reverse order of the primary bow?_ Because the secondary bow is _a reflection_ of the primary bow, and, like all reflections, is reversed. 490. _Why are reflections reversed?_ Because those rays which _first reach_ the reflecting surface are the _first returned_. If you hold your open hand towards the looking-glass, the light passing from the point of your finger will reach the reflector and be returned before the rays that pass from the back parts of the hand. Hence the image of the hand will present the reflection of the finger point towards the point of the finger. 491. _Why are the colours of the secondary rainbow fainter than those of the primary?_ Because they are derived from the _refraction and reflection_ of rays which have _already_ been refracted and reflected, and thereby _their intensity has been diminished_. 492. _What is a lunar rainbow?_ A _lunar rainbow_ is caused by the light of the _moon_, in the same manner as the _solar rainbow_ is caused by the light of the _sun_.