82 The Sky 
cance. There are many other equally fascinating and more 
- frequent effects. 
THE MECHANICS OF AN ILLUSION 
We shall discuss here the strange optical effects that are 
observable in the earth’s atmosphere such as rainbows, sun- 
dogs, halos, mirages, twilight glow, etc. Most of these illu- 
sions have at one time or another been the subject of rigor- 
ous scientific study and you will find mathematical theories ~ 
us En in uaOons s aE TU aN Opes thes are = Sue 
MEDIUM 
REFLECTION 
REFRACTION 
to a variety of causes—the many tricks that the air of our 
atmosphere can play upon a light beam before it reaches our 
eyes. At the risk of being tedious, we should begin by list- 
ing the principal means by which the atmosphere interferes 
with the simple straight line course that a light beam in- 
variably follows when undisturbed. 
Reflection occurs when a ray of light strikes a mirror. The 
light is bent back abruptly in its path just as a rubber ball 
