Progress and Problems of Solar Physics. 123 



currents by which they are formed, and Professor John- 

 stone Stoney and Sir Robert Ball state that undoubtedly 

 carbon is one of its main constituents. 



Above the photosphere is supposed to exist a thin 

 veil of "smoke" or relatively cooler gas by which the 

 disk is darkened; above this is the reversing stratum — 

 a layer of various incandescent metallic vapors which 

 absorb the various rays of the intensely heated and bril- 

 liant elements of the photosphere and produce the famil- 

 iar dark Praunhofer lines of the solar spectrum — still 

 higher up is the chromosphere, a region some five or six 

 thousand miles in depth, composed principallj^ of hydro- 

 gen, calcium and helium, and in which appears the beau- 

 tiful forms of the quiescent and eruptive prominences, 

 while above all and reaching at times to many millions 

 of miles is the marvelous Corona only visible in all its 

 beauty at a total solar eclipse. 



The application of the spectroscope to the problems 

 of solar chemistry has been productive of rapid and im- 

 portant advances in the physics of the sun in recent 

 years; the Fraunhofer lines in the spectrum have been 

 photographed and studied by elaborate appliances and 

 delicate instruments and by means of almost perfect 

 screws the late Prof. Rowland made possible the use of 

 plane and concave diffraction gratings ruled with equi- 

 distant lines, 20,000 to 40,000 of which were ruled on a 

 linear inch of polished speculum metal. With such pow- 

 erful means of light analysis the accurate position of 

 many thousands of dark lines in the solar spectrum has 

 been determined in terms of their wave lengths to the 

 tenth-millionth of a millimetre. With such diffraction 

 spectroscopes the study of the chromosphere and promi- 

 nences is carried on daily by scores of observers in all 

 parts of the world, who are accumulating and discussing 

 masses of data which are certain to bring forth new and 

 important discoveries relating to the great problem of 

 physical life on our earth. 



With the advent of the chemical interpretation of 

 the dark lines in the solar spectrum came more refined 

 methods of determining the positions of every line and 

 their delineation in maps, extending from the infra red 

 far into the ultra violet rays of the spectrum. 



