PURE AND REFLECTED LIGHT 



17 



spring sometimes show wreaths and scarves of 

 yellow or red upon the clouds after sunset ; but 

 as a general rule these are not the seasons for 

 bright displays. 



The coming of the dawn and the passing of 

 the sunset doubtless occupy the same length of 

 time, but to us the latter often seems of shorter 

 duration. At the equator there is compara- 

 tively no glow on the sky after the sun disap- 

 pears. Almost immediately upon the vanish- 

 ing of the disk from view there is darkness. 

 Along the coast of Norway one may see the 

 after-glow upon the sky far into the night ; 

 and farther up the coast the sun itself may be 

 seen at midnight. The shape of the globe and 

 the inclination of its axis account for both these 

 appearances. In the temperate zones we have 

 something between the two extremes. The 

 sun for some time after its disappearance from 

 view keeps throwing light from below the 

 horizon upon the upper sky, and thus produces 

 the effect we call twilight. It used to be reck- 

 oned that when the sun had fallen eighteen or 

 nineteen degrees below the horizon the twilight 

 ceased entirely; but according to astronomers 

 it ceases whenever a star of the sixth magnitude 

 can be seen in the sky directly overhead. The 



