651 
of Ecivtibargh, Session 1883-84. 
through which the rays have to pass, before arriving at our 
position, still goes on increasing ; and though the rays cannot now 
come direct to us on the surface of the earth, yet they pass through 
our atmosphere overhead, and illuminate any particles large enough 
that may be floating there. But not only do the sun’s rays pass 
through a greater extent of our atmosphere when the sun is 
below the horizon, but as the rays are then at a tangent to the 
earth’s surface, they have to pass through a greater proportion of 
air near the earth ; and in that region there are, in addition to the 
large particles, more of the very fine smalbwave-scattering particles, 
as well as more water vapour, than in the upper regions of our 
atmosphere. The rays, therefore, that pass have the colours of 
small wave length more perfectly sifted out of them, and the light 
tramsmitted is deeper red. 
When we look towards the north, south, and east soon after sun- 
set, we see that the sky near the horizon is of a dull reddish colour. 
This is due to the sun’s rays being deprived of all save their red, in 
their passage horizontally through so much of the atmosphere, and 
these red rays falling on the large particles low down in the 
atmosphere illuminate them with red light. This red light near the 
horizon would be much redder if it were not for the great amount of 
blue light reflected to the particles from the sky overhead. As the 
sun sinks, but before it ceases to shine on our atmosphere, the 
temperature of the air begins to fall, and its cooling is accompanied 
by an increase in the size of the particles floating in it, due to 
water vapour condensing upon them. The particles to the east 
lose the sun first, and are thus coolest ; and the rays in that 
direction being best sifted, the red colour is here more distinct than 
in the north or south. 
As the sun sinks lower, the particles overhead get larger and 
better able to reflect the red rays ; and the red colour at first visible 
in the east slowly rises, passes overhead, and descends in the west. 
We cannot, however, see it in the zenith with the unassisted eye, and 
it is not till it forms the first glow that it becomes visible. I have, 
however, been* able by means of a Mcol prism to detect the presence 
of the red glow overhead, tracing it from its first appearance 
in the east, and following it in its passage to the west, long 
before it became visible to the unassisted eye in the western glow. 
