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 experiments to
illustrate; consideration of these facts and refraction of light
striking crystals clearly leads to explanation of various complicated
halo phenomena such as recorded and such as seen by us on the Great
Barrier, and draws attention to the critical refraction angles of 32 deg.
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