THE KEASON WHY. 



123 



1 There is no darkness nor shadow of death, where the workers of iniquity may 

 hide themselves." JOB xxxiv. 



the meridian. (See Fig. 13.) A straight line passed upward through the air, 

 would not be so long as that which passes to S. Consequently, as the air is 

 generally impregnated with moisture, at the time when these effects are 

 observed, the rays of light are caused to diverge more, and the disc of the sun or 

 moon appears magnified. Probably both of these reasons contribute to the effect. 

 This latter reason also explains why the disc of the sun or moon may some- 

 times appear oval in shape, the lower stratum of air being more loaded with 

 moisture than that through which we view the upper part of the disc. 



526. Why do our shadows lengthen as the sun goes down ? 

 Because light travels only in straight lines, and as the sun 



descends, the direction of his rays becomes more oblique, thereby 

 causing longer shadows. 



527. What is the cause of the optical illusions frequently 

 observed in nature ? 



There are various kinds of natural optical illusions : 



The mirage, in which landscapes are seen reflected in buniiiig 1 

 sands. 



The fata morgana, in which two or three reflections of objects 

 occur at the same time. 



The aerial spectra, or aerial reflections, &c. 



Fig. 16. ILI/TSTRATINCt THE APPEARANCE OF PHANTOM SHIPS. 



The optical illusions itbc.ve cr.umoratcd owe their origin to various 



