of Eclinhurgh, Session 1883-84. 
657 
When the first glow was formed, and during the time it was 
descending as well as when it melted into the horizon colours, the 
glowing west no doubt was radiating a great amount of red light in 
all directions, but no red glow was visible overhead, on account of 
the great amount of white and blue light also reflected by the sky, 
by which the red effect was masked ; as we have seen, however, 
it was there all the time, but only became visible when the other 
light ceased to be reflected by the sky. Now, though during the 
second glow the sky seemed to get lit up with a red light, yet the 
brilliancy of its total illumination had not increased, it had rather 
decreased, as we can satisfy ourselves by observing that while all 
these changes are taking place the stars are becoming more and 
more numerous, showing that the daylight has been decreasing all 
the time. 
It has already been mentioned that immediately after sundown, 
a thin silvery film of cloud was always seen over the western sky 
before the first glow became visible. What appeared to be this film 
has also been detected overhead with the polariscope while the sun 
was shining on it, and its faintly indicated outlines traced, as it 
shone as a red filmy cloud when the polarised light was cut off. Now, 
as it was observed that the second glow took place in these same filmy 
clouds, and that this second glow was not visible for some time after 
the first glow, and not till the sunshine had almost left our atmo- 
sphere, it is evident that the filmy clouds from Avhich these glowing 
colours proceed are at no great elevation. Another thing which 
indicates that the second glow is but a reflection of the horizon colours, 
is that it was always possible to tell whether the second glow would 
be brilliant or not before it made its appearance. If the horizon 
colours were high up and brilliant, then there always followed a 
brilliant second glow ; but if they were low and dark, no second 
glow followed. There was thus a direct relation between the 
brilliancy of the sunset colour and the second glow. 
It may here be asked, Is the horizon glow sufficiently brilliant to 
light up the floating motes all over the sky, and account for the 
brightness of the second glow ? This doubt is again suggested by 
something which, from its very familiarity, we have almost ceased 
to notice. The amount of light that streams in from the western sky 
after sunset is much greater than Ave might imagine. It is not 
2 
VOL. XII. 
TJ 
