354 ANNUAL OF SCrENTIFIC DISCOVERT. 



sun betrins to .iscend, and after a time his return is heralderl by a 

 fuint d.iwn, wliich circles slowly around the horizon, completing 

 its circuit every twenty-i'our liours. This dawn grows graiiindly 

 brighter; and on the 20th of March the peaks are gilded witii the 

 lirst level rays of the six months' day. The bringer of this long 

 day continues to wind his spiral way up\vards till lie reaches his 

 highest place on the 21st of June, and his annual course is com- 

 pleted. 



Power of Stars in Overcoming Twilight. — The following are 

 extracts from a letter from M. Jiabinet to Admiral Smyth, jnib- 

 lished in the "Astronomical Ivcgislcr " : — 



"Another sidereal quality I have observed with great interest 

 is, that some stars have more power of overcoming twiliglit than 

 others of the same magnitude. The fact was published some j^ears 

 ago. Now as to the cause : Let us suppose a star just sufficiently 

 bright to be perceived in twilight; its light, then, must be equal to 

 at least one-sixtieth of the uniform light of the sky. But if we 

 imagine tiiat tiiis star, preserving the same brightness, is smaller, 

 and therein' oc-cupies only onc-lourth of the space it did before, 

 tiien its light will be one-tilteentli of the general ligiit of the sky, 

 and will consequently be ver}- perccptil)ie. One of the stars of 

 Cassiojieia possesses this power of conquering twiliglit; so doea 

 y Draconis, f Pegasi, and others. It is easy to prove tliis by ar 

 optical cxperLmcnt. 





