322 ANNUAL REPOKT SMITHSONIAN INSTITUTION, 1908. 



mixture, but the heavier elements sink toward lower levels in general. 

 Such partial separations in level and the vigorous motions which 

 take place in the sun are effectually hidden to the telescopic observer, 

 partly because he can not tell one transparent gas from another and 

 partly owing to the circumstance that the motions at different levels 

 are mixed beyond recognition on account of the great depth of the 

 field of view. The separate currents are as undistinguishable as the 

 separate motions of the motes of a wide sunbeam when viewed from 

 a distance. 



All this is changed for the spectroscope. Iron vapor strongly ab- 

 sorbs the rays of certain special wave lengths, and vapors of the other 

 elements — hydrogen, calcium, etc. — do the like for still other rays. 

 Hence instead of the brilliance due to the extremely hot interior lay- 

 ers of the sun there is found in the Fraunhofer lines chiefly the less 

 intense emission of the cooler vapors of the elements which lie in the 

 outer surface layers. If the sun is viewed in the red spectral line 

 C (now usually called Ha), there is seen chiefly the hydrogen and not 

 the iron, sodium, or other elements. Hydrogen has several other 

 strong lines in the solar spectrum. Among the most conspicuous are 

 Hj8 (also called F) in the blue, Hy and H8 in the violet. 



It is well known that when light is produced by heating a bar of 

 iron or other substance to incandescence the light is at first red, and 

 becomes white and, finally, in the electric arc, even of a violet tinge 

 with increasing temperature. In short, the violet end of the spectrum 

 requires a higher temperature for its copious emission than does the 

 red. This holds in a general way for gases in their emission of line 

 spectra as well as for solids with their continuous spectrum. Accord- 

 ingly the red Ha line will be stronger with respect to the violet H8 

 line when emitted from hydrogen gas at a lower temperature than at 

 a higher one. Hence we may expect that if a mass of hydrogen gas 

 extends for a very considerable thickness in the outer layers of the 

 sun the spectrum of the higher and therefore cooler parts will be 

 relatively stronger at Ha than at H8. Therefore if the sun is viewed 

 only by Ha light, the aspect will be more that of the highest levels 

 than of the lower ones. If viewed on the other hand through H8, or 

 still more if one of the lines of iron is chosen, it will be on the whole 

 more of the aspect of a lower section which is presented. 



Readers w^ill recall that the Smithsonian Eeport for 1904 con- 

 tained an abstract of the account, by Hale and Ellerman, of the 

 Eumford spectroheliograph of the Yerkes Observatory. Referring 

 for details of the instrument to that publication, it is enough to say 

 here that the spectroheliograph, invented by Hale about 1890, is to 

 all intents and purposes a screen which limits the observer to rays 

 of a single shade of color, and this may be at any part of the spec- 

 trum. By the aid of the spectroheliograph the sun may be photo- 



