655 
of Edinbicrghy Session 1883-84. 
account of the blue light from the sky overhead mixing with the 
red and making the sky look whitish. 
Now this second glow I hope to be able to prove is not caused, 
like the first, by the direct rays of the sun shining on the particles 
floating in the air, but by the particles engaged in the first glow being 
now illuminated, not directly by the sun's rays, but by the glowing 
colours on the western horizon, The clouds, dust, and floating 
matter to the west of us were so brilliantly illuminated, that they in 
turn became a source of illumination, reflecting their reddish light 
in all directions, and illaminating particles suspended in the air in 
every direction, during last winter, when these sunset effects were 
at their brightest, I observed one evening, after the sun had set, 
this red light streaming in from the glowing west with such 
brilliancy as to light up the smofie of a factory chimney, making 
it appear of a reddish colour; while all objects in the landscape of a 
reddish hue shone out with a brilliancy out of all proportion to 
what we are accustomed to see, 
This glowing light streaming in from tfie west falls on the 
particles suspended in the atmosphere, and illuminates them ; 
and the particles on which the sun shone directly and produced 
the first glow, are agaifl lit up with a reddish light, The effect 
of this rod illumination is not at first visible, as the sun is still 
shining on the upper strata of the atmosphere, and the very fine 
dust there reflects the blue light, which mi^es with the red and 
masks ft, As the sun sinks lower, it shines on less and less of the 
upper air, and soon the brilliancy of the blue is only equal to that 
of the red ; the heavens then appear whitish, and when at last the 
sun passes altogether out of the dusty air overhead, the red light 
becomes visible to the unassisted eye, and the heavens seem filled 
with a reddish glow. 
It will be observed that the explanation here offered puts the 
position of the illuminated particles producing the sunset effects at 
a much lower elevation than has generally been supposed. It will 
be necessary, therefore, for me to give my reasons for supposing 
that the phenomena are really produced by particles floating at com-' 
paratively low elevations. This explanation supposes that only 
the first glow is produced by the direct rays of the sun, and the 
second glow by reflection, or rather radiation from the illuminated 
