of Edinhurgli, Session 1883-84. 
653 
was a similar glow to tlie south of us ; and seen by observers to the 
south-east of us ; and though we were looking into the sky where 
the glow was, and at the same elevation as they were, yet it was 
quite invisible to us. 
Two explanations suggest themselves to account for this 
peculiarity in the glow. One supposition is, that its brilliancy 
is dependent in some way on the brightness of the sky behind 
it ; the other supposition, and by far the more, probable one, is, 
that it is due to some peculiarity in the reflecting, particles. The 
glow being visible only on particles situated between us and the 
,sun, would seem to. indicate that the particles engaged in producing 
that glow are not ordinary dust particles, which absorb and radiate 
the light in all directions, but that this peculiar ef ect is produced 
by the regular reflection, of the sun’s rays. But it may be asked, 
Where are the necessary reflectors to come from? Now it is 
obvious that any small crystals floating in the air will, by the 
reflection from, their surfaces, produce this result. All examinations 
of the volcanic dust lately collected from the atmosphere show 
that a great quantity of it is composed of small glassy crystals. 
An abundance of such crystals would quite, account for the 
peculiarity in the visibility of the first glow, as these crystals would 
shine far more brilliantly when placed between the observer and 
the sun than in any other position, ft is now simply a question as 
to whether there is a. quantity of such small crystals sufficient to 
produce this result. The evidence seems to indicate that there is; 
if so, then the difficulty vanishes. Mixed with, these crystals there 
are also large quantities of ordinary kinds of dust, and it is the 
light reflected by the latter which principally causes the red glow 
seen in the south, north., and east Both kinds of dust are necessary 
fully to explain all the phenomena. 
The first glow looked as if it stood vertically in the western 
heavens ; this, however, is an effect of perspective. In reality we 
were looking into what was practically a horizontal, layer of mote-filled 
air; and as the.sun got lower and lower the part of tflis layer reflecting 
the red light, gradually moved westwards, which gave it the appear- 
ance of moving downwards. From the high angle at which this first 
glow was seen, it could not be far from us, at least when it was first 
visible— not more than ten or fifteen miles, perhaps much less. 
