448 
Proceedings of the Roycd Society 
the blackness of the escaping smoke. The supply of electricity of 
sufficiently high potential is, however, a difficulty for the present. 
3. The Eemarkable Sunsets. By Mr John Aitken. 
The very remarkable and beautiful sunsets which have been so 
frequent of late, in which the sky has been lit up with a wondrous 
wealth of colouring, and with a splendour more than earthly, has 
given rise to much interest and speculation as to the cause of the 
brilliant colouring. According to one explanation, the effect is pro- 
duced by the light becoming coloured in its passage through the 
atmosphere by an excess of water vapour, or other absorbing 
medium, at present in the air. The other explanation is, that 
the effects are the result of a superabundance of atmospheric dust, 
probably due to the late eruptions of Krakatoa and other volcanic 
mountains. 
There seems to be a possibility of determining by observations 
which of these theories is the more probable. In all the descrip- 
tions of the sunsets the point which is most generally remarked on 
is the immense wealth of the various shades and tints of red. 
Now, if dust is the cause of these glowing sunset colours, then 
there must be somewhere a display of the colours complementary to 
the reds ; because the dust acts, not by the selective absorption, and 
destruction of the colours, but by a selective dispersion of them. 
The very small particles of dust in the atmosphere stop the direct 
course of the rays and reflect them in all directions ; but the dust 
particles are so very small, especially in the upper regions, that they 
are only capable of stopping, and reflecting, or scattering the rays of 
the blue end of the spectrum, while the red rays pass on unchecked. 
There therefore ought to be somewhere in the sky a display of the 
colours of the blue end of the spectrum. From the observations I 
have been able to make since this suggestion presented itself, I find 
that the display of blue and green colours is quite as prominent a 
feature of the late sunsets as the reds. 
Overhead the display of blue is fuller than I have ever seen it 
before; and as the sun passes below the horizon, and the lower 
stratum of air with its larger particles, which reflect white light. 
