2$6 
SCOTT'S LAST EXPEDITION 
[May 
to grow darker, and later stratus cloud was undoubtedly 
spreading up from that direction — this at about 5 p.m. 
About 7 a moderate north wind sprang up. This seemed 
to indicate a southerly blow, and at about 9 the wind 
shifted to that quarter and blew gustily, 25 to 35 m.p.h. 
One cannot see the result on the Strait, but I fear it means 
that the ice has gone out again in places. The wind 
dropped as suddenly as it had arisen soon after midnight. 
In the evening Simpson gave us his first meteorological 
lecture — the subject, - Coronas, Halos, Rainbows, and 
Auroras.' He has a remarkable power of exposition and 
taught me more of these phenomena in the hour than 
I had learnt by all previous interested inquiries concerning 
them. 
I note one or two points concerning each phenomenon. 
Corona. — White to brown inside ring called Aureola — 
outside are sometimes seen two or three rings of prismatic 
light in addition. Caused by diffraction of light round 
drops of water or ice crystals ; diameter of rings inversely 
proportionate to size of drops or crystals — mixed sizes 
of ditto causes aureola without rings. 
Halos. — Caused by refraction and reflection through 
and from ice crystals. In this connection the hexagonal, 
tetrahedonal type of crystallisation is first to be noted ; then 
the infinite number of forms in which this can be modified 
together with result of fractures : two forms predominate, 
the plate and the needle; these forms falling through 
air assume definite position — the plate falls horizontally 
swaying to and fro, the needle turns rapidly about its longer 
axis, which remains horizontal. Simpson showed excellent 
