790 Marvels of the Universe 
[By Theo. Carreras. 
MIRAGE. 
In Arctic regions Scoresby and others have related that images of ships have 
sometimes been seen in inverted position in the air at a time when the ships themselves 
were too far off to be seen. 
sides, in such a position as to 
be just hidden from the eye of 
an observer a short way off, it 
will be found that by pouring 
water into the vessel the coin 
will become visible. All rays 
of light passing from a rarer 
transparent medium to a denser 
one, as from air to water, will 
pass into the latter, but will 
suffer this change of direction, 
each being bent nearer towards 
the perpendicular to the com- 
mon surface of separation. 
Conversely, rays passing from 
the optically denser medium 
into the rarer one will be bent 
away from this perpendicular, 
and make a greater angle on 
emergence than they did before 
entry. When, however, the 
angle of incidence reaches a 
certain value called the critical 
angle, the emergent ray just 
grazes the surface in passing 
out, and no ray making a 
greater angle than this can 
emerge, but must remain within 
the denser medium. For air 
and water this critical angle is 
about 483°, and the phenome- 
non of total reflection occurs 
when a ray of light travelling 
in the denser of two media (or 
ina denser part of one and 
the same medium) arrives at 
the surface of separation, 
making an angle greater than 
the critical value with the 
perpendicular. The reflection 
is called total, because, in all ordinary cases of refraction, only a part of the light is refracted and 
passes out, a part being also reflected and another part scattered ; but in /ofa/ reflection the incident 
light is almost completely reflected (none of it being refracted) as though from the surface of a very 
perfect mirror. 
If a glass vessel containing water be held above the level of the eye, and the upper surface of 
the water be looked at from below, it will appear, when seen by total reflection, as a brilliant silvery 
reflecting surface. 
Most kinds of Mirage are examples of total reflection from layers of air. The sand of the desert 
is intensely heated by the tropical sun, and, in consequence, the layers of air nearest the ground 
