of Edinburgh^ Session 1883-84, 
647 
2. On Knots, Part II, By Professor Tait. 
[This Paper will appear in the Transactions. ] 
3. Second Kote on the Eemarkable Sunsets^ By Mt John 
Aitken. 
When I communicated my first note on this subject on the 21st 
of January last, the only definite conclusion I had then arrived at 
was that the very remarkable and brilliant colour effects, lately seen 
in the heavens at sunrise and sunset^ were due to the presence of an 
unusual amount of dust at the time floating in our' atmosphere. 
This atmospheric dust acts as a Sifting medium, breaks up the White 
light into its components, and reflects all the different colours 
to us from different parts of the shy. 
Owing to the cloudy state of our northern Skies, and to the ever- 
varying condition of our atmosphere, it was found very difficult to 
follow the successive Colour changes in such a way as to enable 
me to form anything like a satisfactory explanation of all the 
phenomena. Since coming, however, to the south of France, and 
seeing all the different sunset effects repeat themselves evening after 
evening in cloudless skies, I have been enabled to form a clearer 
idea of how the different effects are produced ;■ and from the observa- 
tions made under these more favourable conditions, 1 shall now 
attempt to give what appears to be the explanation of some of the 
principal phenomena of these very remarkable and brilliant sunsets. 
Though it was the month of March before I saw these sunsets 
under the more favourable conditions,' and the brilliancy of the 
display had very greatly diminished, yet sufficient remained to 
enable the different colour changes to be followed, and 1 was still 
able to detect marked differences in the brilliancy of the effects on 
different evenings. Briefly stated, the following is something like 
the order in which the different phenomena followed each other, 
evening after evening,- in these cloudless skies, the only difference 
being in the brilliancy of the colours on the different evenings. 
During the day the sUn was sUrroUnded by an unusual amount of 
white light or glare. As it descended, this glare gradually in- 
creased in brilliancy and extent ; but while it was still an hour 
from setting, no colours, save blue, were anywhere visible in the 
heavens, and the horizon all round was white. As the sun was 
