64 Prof. Pliillips on a Zone of Spots on the Sun. [Mar. 22, 



iminication*. The edges of tlie umbra and penumbra were much broken, 

 the former running out into a sharp point on the apparent left, and in- 

 cluding a lighter brown space. 



Except in this part, the dark umbra was uniformly and equally distant 

 from the penumbral border, so that it offered an excellent object, large 

 enough and distinct enough to be employed with confidence as a test of 

 the depth of the umbra beneath the luminous photosphere. 



In a former communication I assigned to an umbra of good figure, 

 carefully observed, a depth of only 300 milest» Since then M. Chacornacj: 

 has given the depth of 400 miles as an approach to average, though in 

 some cases 1000 miles might be nearer. That the spots are most frequently 

 sunk below the photosphere, as Wilson long since asserted, can no longer 

 be doubted, since Mr. De la Rue, Mr. Stewart, and Mr. Loewy have re- 

 corded, after many measures of photographs taken at Kew, as the general 

 result that the interior parts of the solar spot are sunk below the general 

 surface §. 



The spot now under consideration approached the edge of the disk on 

 the 15th of February, when the umbra seemed nearly bridged across by a 

 lighter part, and was perceptibly but very slightly nearer to the following, 

 or left-hand side. 



The reappearance of this spot was anxiously looked for. It came into 

 view on the morning of March 3, having when observed passed the limb 

 about 6° 16'. Its appearance was sketched on this occasion, and again on 

 successive days to the 14th of March. Its apparent magnitude was dimi- 

 nished, but the main features were less altered than is usual with sun- 

 spots. The umbra still appeared in such relation to the penumbral border 

 as to confirm the opinion of its being but very little depressed below the 

 photosphere. Remarkable changes happened between the 7th and 10th of 

 March. On the 8th two broad penumbral extensions appeared ; on the 9th 

 these were separated into two detached masses, and much altered in figure. 



Diagram No. 2. 





















\ ^ Pi'--' 







The path of spot A in February and March. 



* Proc. Roy. Soc. Nov. 23, 18(35. t Ibid. Jan. 26, 1865. 



J EuU. des Obs. 1865. § Researcbes in Solar Physics, 1865. 



