PURE AND REFLECTED LIGHT 



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 



