1 78 SCOTT'S LAST EXPEDITION [Mav 



blow, and at about 9 the wind shifted to that quarter and blew 

 gustily, 25 to 35 ni.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 lec- 

 ture — 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 — out- 

 side 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 with- 

 out 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 num- 

 ber 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. Simp- 

 son 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° and 46°, the 

 radius of inner and outer rings, the position of mock suns, contra 

 suns, zenith circles, &c. 



Further measurements are needed; for Instance of streamers 

 from mock suns and examination of Ice crystals. (Record of 

 ice crystals seen on Barrier Surface.) 



Rainbows. — Caused by reflection and refraction from and 

 through drops of water — colours vary with size of drops, the 

 smaller the drop the lighter the colours and nearer to the violet 

 end of the spectrum — hence white rainbow as seen on the Bar- 

 rier, very small drops. 



