PLATE XII. 



Ill this photograph tlie calcium vapor is much hettev showu than in Plate XI, 

 and the beginnings of eruptive phenomena have become more distinctly evident. 



It is to he understood that although the changes going on in the eruptive phe- 

 nomena of the spot gnmj) ])revent a ])erfect comparison of all the details of the 

 successive photographs in this and the subsecpieiit sei'ies, the large masses of 

 flocculi cliange so slowly in form that th(>y may be compared without danger of 

 sei'ious error. In general, the differences In'tween the successive i)i<'tures are 

 therefore due to differences in the extent and brightness oi the vapor at different 

 levels rather than to changes in form which have taken place between exposures. 

 In order to render possible a satisfactory comparison of the high and low level 

 flocculi surrounding this spot, the matched ])air of photographs, reproduced in 

 Plates XV, XVI, i« given for examination with the stereoscope. 



157 



