1889 - 90 .] 
Mr J. Aitken on Dust Particles. 
229 
not depend on the angle at which it is seen, but that it depends 
also on the size of the fog particle producing it. For instance, if we 
produce a cloudy condensation in a closed vessel placed between us 
and a light, the light is surrounded by a coloured halo, but the 
colour at any particular angle is not constant, but changes as we 
change the size of the fog particles ; so that if we have two 
quantities of air, each with different sized particles in it, and both 
of these seen at the same angular distance from the sun, we will see 
different colours at the same angle and close to each other. 
This mixture of colours producing the opalescent effect may be 
produced artificially. All that is necessary is to place a vessel of 
hot water nearly in a direct line between the observer and a bright 
light. As the condensed vapour rising from the hot water re-evapor- 
ates, the particles which were at first too large to give colour effects 
shine out in brilliant colours when vaporized to the size necessary to 
give the result. As the rising strata of air having different sized 
particles in them are close to each other, different colours are seen in 
close proximity, and as the size of the particles diminishes rapidly the 
colours change quickly. These colours are best seen when the steam 
is seen against a black surface, and the eye shaded from the direct 
light of the lamp. 
On this evening on the Rigi brilliantly coloured rings were also 
seen surrounding the sun when just setting. The rings changed too 
rapidly for anything definite to be noted. These rings did not appear 
when a foggy cloud passed over the sun, but when there was only a fine 
haze ; that is, the cloud particles were too large to produce colours, 
and it was only when they were nearly vaporized that the effects 
were produced. 
All the brilliant display of colour we were favoured with on the 
21st was gone by the following day. The morning of the 22nd 
was fine, the clouds had all settled down to a much lower level, 
to about 1000 feet below the top of the mountain, and the upper 
surface of the cloud stratum had now become more irregular, 
cumulus shapes tending to form at different places. During the 
rest of my visit to this station the clouds gradually cleared away 
and the weather was fine, and there were no other opportunities for 
seeing these interesting colour phenomena. 
When working at an elevated position such as the Rigi Kulm, 
