88 LIGHT SCIENCE FOR LEISURE HOURS. 



than most other astronomers, concedes a thickness of 

 from six to ten minutes ' that is, from a fifth to a 

 third of the solar diameter. 



This, as I have said, was written by Professor 

 Young in 1871, but before a certain most important 

 fact had come to his knowledge, which without at all 

 affecting what he here puts forward, renders it possible 

 to say much more as to the real extension of the 

 corona. 



We have seen that a certain object, surrounding 

 the sun on all sides to a distance of from 160,000 miles 

 to 290,000 miles from his surface, is demonstrably a 

 self-luminous envelope. It was to this envelope, or 

 perhaps rather to its brighter portion as seen from the 

 earth, that some proposed to assign the barbarous 

 name * leucosphere,' to distinguish it from the bright 

 layer of prominence matter close by the sun, which is 

 called the sierra, or chromatosphere. But the visible 

 extension of the corona is greater yet, and before the 

 eclipse of 1870 doubts still existed as to the actual 

 extent of that solar corona, which all had now begun 

 to recognise as a real entity. That some portion of 

 the light seen around the sun during total eclipse is in 

 reality only due to the illumination of our own atmo- 

 sphere is altogether beyond question. It is true, 

 indeed, as was pointed out by Professors Young and 

 Harkness, Dr. Curtis, and myself, that none of the 

 coronal light, for several degrees from the sun's place, 

 can be solar light reflected by our atmosphere, as had 

 been mistakenly supposed ; but it is no less certain 



