OPTICS 



Several of these (the prismatic sunsets and sunrises, 

 the curved earth-shadows, and the arched ice-bhnk of the 

 Barrier) are characterised by their arched form, their 

 upper boundary being an arc of a great circle. 



Prismatic Sunrises and Sunsets 



The phenomenon to which we gave this name consisted 

 in a high arch, coloured in bands like the solar spectrum, 

 which appeared in the sky opposite to the sun, before sun- 

 rise and after sunset. It was of daily occurrence in clear 



Diagram of Prismatic Arch, March 13 



weather, during the whole of the two periods when the sun 

 was rising and setting. Even during the long night it 

 was common at noon for nearly a month after the sun 

 had set, and it recommenced a month before the sun 

 reappeared. 



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