MOUNT WILSON SOLAR OBSERVATORY. 



253 



same time lines of equal intensity as dark lines in the solar spectrum 

 of such elements as Fe and Ni remain as dark lines, or are faintly 

 doubly reversed. It seems necessary to conclude that the level of the 

 elements of high atomic weight is decidedly below that of Fe and ele- 

 ments of similar behavior, even for hues of the same solar intensity, 

 an inference amply confirmed on other grounds by the work of St. John, 

 Adams, and others. The hypothesis put forward by St. John that 

 the great intensity in the flash spectrum of the lines of elements of 

 high atomic weight is due to the intense radiation of thin strata of 

 gases at a very high temperature at the base of the reversing layer 

 seems to be in excellent accord with the observations. 



The double reversals of the enhanced lines are considerably narrower 

 on the average than those of arc lines of the same solar intensity. 



There is no certain indication of any systematic displacement of the 

 bright lines relative to the dark lines of the spectrum of the sun's limb. 

 If any such displacement exists it can not exceed a very few thousandths 

 of an Angstrom unit. It would appear necessary, therefore, if anoma- 

 lous dispersion is assumed to exist in the sun, to conclude that the 

 regular radial density gradient is completely balanced by irregular 

 gradients of opposite direction, a conclusion opposed to many facts of 

 observation. 



As a by-product of the chromospheric investigation, some remark- 

 able differences have been observed in the dark-line spectrum of the 

 sun's extreme limb as compared with that of the center of the sun, 

 which are so great as to render the spectrum unrecognizable in certain 

 regions. The two most important features are: 



(a) The great weakening or strengthening of some of the solar lines, 

 and in a few cases the appearance of new lines. Among other cases 

 attention may be called to the following : 



(h) The presence of what appears to be a band spectrum in the region 

 between X 5050 and X 5150. This may be identical with the spectrum 

 of magnesium hydride observed by Fowler in the spectrum of sun- 

 spots. • 



