SCIENCE OF COMMON THINGS. 269 



Why the sun and moon seem larger on the horizon than overhead. 



17*41 Why do the sun and moon seem larger at their rising and setting 

 than at any other time f 



The appearance is an illusion, in consequence of ter- 

 restrial objects being placed in close comparison with 

 them at one time, and not at the other. 



1*74:3 Is this illusion an optical one, or a mental illusion t 



A mental one, since the organs of vision do not pre- 

 sent to us a larger image of the moon or sun in the 

 horizon than in the zenith. 



17*43 What do we mean by the horizon ? 



The circle or line where the earth and sky appear to 

 meet. 



1744 What do we mean by the zenith f 



The point or part of the heavens immediately over- 

 head. 



17*45 Is the moon nearer or farther from us when upon the horizon f 



When the moon is on the horizon, it is about four 

 thousand miles farther from us than when in the zenith ; 

 its apparent diameter, therefore, instead of appearing 

 larger, ought to appear about a sixtieth part less. 



17*46 Why are we so often mistaken in respect to the actual distance 

 of a conflagration at night ? 



Light radiating from a centre rapidly weakens as the 

 distance from the centre increases, being, for instance, 

 only one-fourth part as intense at double the distance. 

 The eye learns to make these allowances, and by the 

 clearness and intensity of the light proceeding from the 

 object, judges with considerable accuracy of the com- 

 parative distance. But a fire at night appears uncom- 

 monly brilliant, and therefore seems near. 



17*47* Why does the evening star rising over a hill-top appear as if 

 situated directly over the top of the eminence ? 



Because we make brightness and clearness to depend 

 on contiguity, as it ordinarily "does ; and as the star is 

 bright, we unconsciously think it near us. 



1748 What is the cause of colors t 



The action of light. 



1749 How is thit proved ? 



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