Progress and Problems of Solar Physics. 1 25 



graphic plate tliis work is proceeding slowly and in 

 course of time it will have given up all its secrets. 

 Professor Rowland was actively engaged upon this great 

 task of photographing the spectra of all known elements 

 for comparison with the solar record when death cut 

 short his brilliant and busy life in 1901. 



Not all the dark lines are due to solar origin, many 

 hundreds being purely telluric, part being the result of 

 dry air absorption, while others are caused by the se- 

 lective absorption of aqueous vapor. The method of 

 detecting telluric from solar lines is interesting and 

 depends upon their not shifting position with the rota- 

 tion of the sun on its axis; lines which are purely of solar 

 origin will by their displacement towards the blue end 

 indicate a motion towards us of the sun's eastern limb, 

 and a recession of its western limb being accompanied 

 by a shifting towards the red. Many other lines in the 

 solar spectrum seem to be variable. Sidgreaves, Cortie, 

 Lockyer, Rowland and others have called attention to 

 some examples of lines which at times are very faint 

 and missing, and afterwards gain in intensity. No 

 explanation as yet satisfactorily accounts for this phe- 

 nomenon; it may be connected in some manner with the 

 periodicity of the solar spots. 



Many other changes have been observed in the 

 Fraunhofer dark lines. Professor Jewell has observed 

 displacements of lines which were not due to motion, but 

 to the condition of the elements ]3roducing the dark line 

 displacement, increased density or pressure affecting the 

 wave lengths of the lines. This discovery will have an 

 important bearing in future investigations not alone 

 upon the sun, but also upon other bodies, such as comets, 

 nebulae and stars. 



During the solar eclipse of December, 1870, Prof. 

 O. A. Young made the interesting discovery that at the 

 moment of second contact of the sun's and moon's limbs 

 the dark Fraunhofer lines visible in his spectroscope 

 were suddenly reversed, and the field of view presented 

 a series of bright lines upon a dark background. This 

 result was not unlooked for, but the question as to 

 whether it was a complete reversal of the dark lines 

 could not be decided in a second or two of precious time 



