OPTICS 



Flagstaff Point, Cape Royds, near which the observer 

 stood. 



While the broad ends of these curved " shadows " are 

 appreciably darker than the sky, the central portion is 

 different. It is an exceedingly fine line, neither hghter 

 nor darker than the sky above and below, yet perfectly 

 well defined. In some instances the darker ends have a 

 bright line outside, and a blue zone inside, which broadens 

 at the ends. Usuallj^ the observer was in the shadow 

 of Erebus when these effects were seen, but similar 

 shadows could be detected from the outside, as on 

 September 29, when a curved shadow crossed the Ferrar 

 Glacier, low down, the sun being in the south-east. On 

 the same day the curved shadow from Erebus was 

 measured, 30° above the horizon (to the north). On this 

 occasion it did not reach to the mountains, but ended on 

 the sea ice. 



The two previous figures show shadows from Erebus. 

 The following one shows three peaks of the western 

 mountains, with shadows projected from them, the sun 

 being behind them. 



Almost as puzzling as the curved " shadows " are those 

 instances when several of the shadows diverge from one 

 point as shown in the figure below. On this occasion the 

 sun was behind the observer. 



