OPTICS 



the few days preceding the departure and the arrival of 

 the sun respectively before and after the winter night. 



Especially at the intermediate seasons of the year the 

 advent and departure of the sun each day was accompanied 

 by prismatic sunset and sunrise effects. Mount Erebus 

 was often bathed in a delicate pink light. 



Purple lights are apt to be produced on snow surfaces 

 when obliquely lighted. 



Cavities in snow formations appear of a wonderful 

 azure blue colour. Those in ice, on the other hand, appear 

 bluish-green, or greenish. 



Earth Shadows 



The earth shadows, or dark shadow bands crossing the 

 sky, seen when the sun is very low on the horizon, were 

 observed in a variety of forms. Some of these certainly 

 bore a relation to the relative positions of Mount Erebus 

 and the sun. When on top of Mount Erebus we remarked 

 the great conical shadow it threw at sunrise over McMurdo 

 Sound and even as far as the western mountains. It was 

 noted later on that a relationship existed between some 

 of the earth shadows seen in the sky from Cape Royds 

 and this conical shadow of Erebus. 



Other forms of the shadows are not so easily explained. 

 On one occasion when the sun was low on the northern 

 horizon near noon, just after its return, we observed the 

 sky overhead crossed by six parallel earth shadow beams, 

 directed from the sun supplying a Noah's Ark appearance. 



Vol.II.-24 



