86 The Sky 
MIRAGES 
The air itself is not uniform over all regions of the sea. In 
some places it may be particularly heated and expanded; in 
others it may be saturated with water vapor; in others cold, 
dry, and dense. In general, the atmosphere is a rather hetero- 
geneous, nonuniform substance. As a result certain portions 
of the air have different optical densities from neighboring 
ones. When a ray of light passes through such regions of the 
atmosphere it is refracted, that is, has its path slightly 
changed when leaving one air mass and entering another. 
This bending of light due to refraction may bring a ship nor- 
ELEVATED IMAGE 
LIN OBJECT 
OBSERVER REFRACTED Lig) m 
| LOOMING 
mally beyond the horizon into view, or it may raise the image 
of distant coasts so high that they appear quite near. Some- 
times there is a kind of multiple refraction which makes 
several images appear one above the other, or it may appear 
to turn a distant ship upside down. Such illusions are called 
mirages and are usually due to abrupt changes in the density 
of air at different horizontal levels. A discontinuity in the 
vertical plane, which may change the apparent bearings of 
distant objects by several degrees, is more rarely met with. — 
: 
