SOLAR ECLIPSE EXPEDITIONS— MITCHELL 163 



A person having no knowledge of the theory underlying multiplet 

 groups would not advance very far in the practical operation of corre- 

 lating heights in the chromosphere with intensities either in sun or 

 chromosphere before the fact would be forced upon his attention that 

 generally the lines of greatest intensity reach the greatest heights, and 

 moreover the intensities and heights for any element are greatest for 

 the multiplets of lowest excitation potential. The best element for a 

 study of these correlations is neutral iron. Tliis element has been 

 assiduously observed in the laboratory and very exact wave lengths 

 are known. Fe is very rich in lines which have been grouped into 

 multiplets with a wide range of excitation potentials. 



From a study of 1,222 Fe lines in the flash spectrum the following 

 facts were noted: (1) The strongest lines in the sun belong to the 

 multiplets of lowest excitation potential; (2) with increase of excitation 

 potential, the maximum intensity of lines in the multiplets steadily 

 decreases; (3) for multiplets of any given excitation potential, there is 

 a close correlation between intensities and heights; (4) for any given 

 Rowland intensity, such as 6, the heights diminish as the excitation 

 potentials are increased. There are many important consequences 

 of these correlations. 



Fascinating results on sunspots were announced by Evershed in 

 1909. With the slit of his spectrograph placed across the spot, he 

 found that the wave lengths in the penumbra of the spots were differ- 

 ent from the values at the center of the sun. The displacements 

 which affected practically all lines were not constant but differed in 

 amount depending on the intensities of the lines investigated, the 

 shift being greater for the weaker than for the stronger lines. Ever- 

 slied's results were abundantly confirmed at Mount Wilson with the 

 result that conclusions are drawn that the displacements in wave 

 length are a Doppler effect caused by the actual flow of the solar gases 

 out of the spots at levels close to the sun's surface and into the spot 

 vortex at higher levels. The heights from the flash spectrum furnish 

 the explanation. As shown by St. John, the layers closest to the 

 sun's surface have a motion of translation out of the spot at the rate 

 of 2 kilometers per second. This motion becomes less and less in 

 amount at greater and greater heights. At a certain level the out- 

 ward motion ceases, wliile above this level the motion of translation 

 is into the spots with increasing speeds at greater and greater heights 

 above the photosphere. At the maximum heights reached by lines of 

 the chromosphere, 14,000 kms by H and K of ionized Ca, there is a 

 movement of the calcium vapor into the spot at the speed of 3.8 kilo- 

 meters per second corresponding to a displacement of 0.063 angstroms. 



The Evershed effect shows that there is a circulation of gaseous 

 material in the neighborhood of sunspots. Other lines of research 



