Progress and Problems of Solar Phys'ws. 127 



reaching far out into the tenuous gases of the Corona. 

 It was first detected and its position marked by Prof. 

 C. A. Young in 1869. In the eclipse which was visible 

 in the southeastern states in May, 1900 (and of which the 

 writer was an interested observer) this veteran and hon- 

 ored astronomer again endeavored to secure the accurate 

 wave-length of this green line in the spectrum, and 

 although supplied with a powerful spectrograph and a 

 visual integrating spectroscope he was unable to see or 

 photograph the line owing to its faintness at that 

 eclipse, although in preceding eclipses he had no diffi- 

 culty in observing it. 



Of the precise nature of the photosphere the spectro- 

 scope as yet offers us no positive information. Young 

 believes it to be a sheet of luminous clouds floating in 

 permanent gases of the solar atmosphere, but no one can 

 yet positively state the actual constituents of this cloud 

 mass. 



It is supposed that the substances revealed hj the 

 spectroscope which produce the Fraunhofer lines, 

 namely, the gases, metals and perhaps carbon, are the 

 constituents, and this is probably true, but many other 

 theories, some of which are of an electrical nature, have 

 been advanced, and the whole question is still one of 

 great interest to students of solar physics. 



Kegarding a well formed group of sun spots. Pro- 

 fessor Young says that "the umbra, with its central 

 nuclei, and overlying bridges, veils and clouds; the pe- 

 numbra, with its delicate structure of filaments and 

 plumes; the surrounding faculae and the agitated sur- 

 face of the photosphere in the whole neighborhood of the 

 disturbance; above all, the continual change and 

 progress of phenomena, — combine to make a fine sun 

 spot one of the most beautiful and intensely interesting 

 of telescopic objects." ^^^^ Observations of these objects 

 have been made ever since the days of Galileo in 1610, 

 but it is onh^ within the last thir-ty-flve or forty years 

 that they have been observed and studied with any 

 accuracy and regularity. As it is principally from a 

 study of the sunspots that we are likely to gain any 

 knowledge of the constitution of the sun's interior, a 

 short account of their structure, appearance and phe- 



