449 
of Edinhurgli, Session 1883 - 84 . 
cease to be illuminated, the depth and fulness of the blue increases 
in a very marked degree. While the sky is deep blue overhead it 
will be observed that lower down the blue changes to blue-green, 
and in some cases to green, the wonderful greenness sometimes 
seen in a clear space in the sky being occasionally intensified by 
contrast with a rose-coloured cloud or haze alongside of it. 
These considerations seem to point to dust as the cause of the 
glowing colours of our late sunsets, as none of the colours are de- 
stroyed, but are simply sifted out and assorted, and the sunset colours 
seem to be produced in the following way : When we look into the 
clear blue sky overhead, we see the light selectively reflected from 
the small particles capable of scattering only the colours of short 
wave-lengths, and we see only blue. If in the evening we gradu- 
ally lower our gaze, and look into the clear sky in any direction not 
towards the sun, we will then see that the blue gradually changes 
to blue-green, and sometimes even to green, and lower down it 
passes into white or rose-colour near the horizon, according to the 
circumstances. This green would seem to be produced in the fol- 
lowing way : Suppose we are looking northwards, then the light 
which enters our atmosphere from the west has, before it arrives 
at the part of the sky into which we are looking, had much of its 
blue thrown out by reflection, and is therefore deficient in blue 
light ; and the particles at that elevation are not large enough to 
reflect the red, so only green is reflected by the sky, and the red 
passes on. When we look overhead, we also look through this green 
stratum, so to speak, but the green is overpowered by the greater 
brilliancy of the blue. And, further, when looking upwards at 
only a slight angle, we see the light reflected from a far greater 
amount of the green stratum than when looking through it towards 
the zenith. 
The fine particles of dust having thus scattered the blue and the 
green rays, only the red rays are allowed to pass on, and we see them 
reflected on the clouds far to the east of us, as well as to the 
south and north. Some of the most beautiful and delicate rose 
tints are formed by the air cooling and depositing its moisture 
on the dust, increasing the size of the particles till they are able to 
stop and reflect the rays of the red end of the spectrum, when the 
haze glows with a strange aurora-like light. 
