PURE AND REFLECTED LIGIIT 
Professor Langley observed the sun go down, 
not gorgeous in color, but coldly luminous, 
with the dark sky crowded close up to the disk, 
and the zenith deep violet-blue. Whenever or 
however the thickness of air between us and the 
sun is decreased, the coloring of light changes, 
growing from a yellow flame somewhat like 
candle-light to something kindred to the blue- 
violet flame of the electric arc-lamp. 
The atmosphere then is chiefly responsible 
for the quality of our light, and upon the clear- 
ness or thickness of the atmosphere depends 
also the quality of our coloring. If the air is 
comparatively clear, the light will be sharp and 
the prevailing notes of color in landscape will 
be blue and green, because the slightness of the 
interfering media allows the short color-waves 
of blue and green to come on down to earth in 
great quantities ; if the air is heavy with parti- 
cles, the light will be less intense and the notes 
of landscape will be yellow or red, because the 
density of the interfering media allows the 
stronger color-waves of yellow and red to pass 
through and down to earth, but obstructs the 
blues and greens. It is owing to density of at- 
mosphere that the heated portions of the globe, 
like Morocco, for instance, are less strong in 
Earth color- 
ing requ- 
lated by the 
density of 
air. 
