Rainbows are arches of prismatic colors, formed by the reflection of
rays of light from within drops of water. The rays, which are refracted
in entering the drops, are reflected from their posterior surfaces, and
again refracted as they re-enter the air, the colors being separated by
their unequal refrangibility.
Halos and coronas are circles of prismatic colors which, in certain
states of the atmosphere, surround the Sun and the Moon.
Halos are supposed to be occasioned by the presence, in the atmosphere,
of small ice crystals which act as minute prisms, decomposing and
refracting the light which passes through them.
Coronas are seen when a light mist is floating in the air, and are
supposed to be formed by reflection from the external surface of the
globules of vapor.