16 
NATURE FOR ITS OWN SAKE 
Spectrum 
colers on the 
sky. 
Sunset sky 
effects. 
twilight hardly admit of description. All hues, 
all tints are possible, and nothing is of long 
duration. The appearance is almost as tran- 
sient as the aurora, for it is shifting in position, 
shifting in light and color continually. When 
there are no clouds, the normal evening sky 
shows a continuous spectrum, and the order of 
colors begins with red at the horizon and ex- 
tends in successive bands through orange, yel- 
low, green, and finally shades into the blue of 
the upper sky. These colors are intensified 
or depressed by atmospheric conditions, and 
they are complicated by the appearance of 
clouds, though the order of their appearance 
even with clouds is usually maintained, the 
reds being the lowest down and the succession 
rising through the intermediate colors to blue. 
The most splendid evening effects are, gen- 
erally speaking, in the autumn, when with 
Indian summer there is much heat and dust in 
the air. Scarlets, carmines, rubies, and burn- 
ing golds are then apparent. After several 
days of rain have left a damp, thick atmos- 
phere, a clearing western sky with fleecy clouds 
will often show very brilliant yellows in bands, 
and in between these bands small spaces of 
malachite green. The winter and the early 
