Atmospheric Optical Illusions 91 
reddish, the inner border is a deep blue or violet. The second- 
ary bow is next brightest and appears surrounding the pri- 
mary bow with a radius about 8° greater than that of the 
primary. The order of colors of the secondary bow is re- 
versed. 
Sometimes there is visible a much fainter bow, the ter- 
tiary, which in all respects resembles the primary except that 
it is much fainter and is in the opposite part of the sky sur- 
rounding the sun. You will probably never see the tertiary 
bow. 
Rainbows differ considerably in appearance, probably due 
to the size of the drops involved. Sometimes the colors will 
be very brilliant, and at other times the bow will have but 
a dull whitish color—the fog bow. Rainbows are visible at 
night under a bright moon, or a bright electric light can pro- 
duce similar effects. They are not seen when the sun is di- 
rectly overhead except from airplanes and high points be- 
cause the primary bow is always formed opposite the sun, 
which in this case would be straight down. 
Sometimes extra rainbows appear as a result of the reflec- 
tion of sunlight on the surface of the ocean. In still other 
cases the surface may be slightly oily and very light rain- 
drops may rest on the surface for a while without mixing with 
the ocean water. Under such circumstances one may observe 
a horizontal rainbow. 
Rainbows are certainly beautiful and marvelous. But all 
the world’s scientists have never found a pot of gold at the 
end of one of them. 
ICE CRYSTALS IN THE SKY 
When water freezes into crystals it tends to do so in a hex- 
agonal pattern. The crystals may be flat flakes, or in long 
