AND ITS RELATION TO SOLAR SPECTRA. 55 



were indicated. The intensities of the solar lines, however, were often disproportionate 

 to those of corresponding lines in the arc spectrum. 



If we dealt only with the arc spectrum of the element in relation to the sun, the 

 selection of lines for representation in the Fraunhofer spectrum would certainly be 

 very remarkable. Of two lines in the same part of the spectrum one may be a 

 comparatively strong line in the sun, while the other, although at least equally strong 

 in the arc, may be weak or missing. For example, the strong line 5672 only occurs 

 with intensity in the sun, while 5684, which is much weaker in the arc, appears 

 with intensity 1 in the sun. 



These differences, however, become comprehensible when due attention is given to 

 the properties of the different lines. The lines of scandium which appear most 

 prominently among the Fraunhofer lines are, in fact, the enhanced lines, and within 

 the limits of error of estimation they appear with relative intensities which are 

 identical with those in the terrestrial spectrum. Lines other than those which are 

 enhanced, even though strong lines in the arc, occur only as very faint lines in the 

 solar spectrum. 



The coincidences between scandium and solar lines, within the limits of error, are 

 numerous, but many of them cannot reasonably be regarded as other than accidental. 

 Even a faint solar line may properly be attributed to scandium if coincident with an 

 enhanced line of corresponding intensity ; but if the scandium line be not an enhanced 

 one, the coincidence can only be accepted as significant when the solar line is faint 

 and the scandium line strong. 



The solar lines which may be regarded as true identifications with scandium, in 

 accordance with this conclusion, are indicated in Tables IV. and V., one showing 

 coincidences with arc-flame lines, and the other with enhanced lines. It will be seen 

 that, notwithstanding the occasional confusion caused by lines of other elements, the 

 stronger lines of the arc-flame are represented as consistently as can be expected when 

 dealing with very faint lines appearing in such a complex spectrum as that of the sun. 

 The identification of the enhanced lines, as shown in Table V., is much more definite, 

 in consequence of the greater intensities of the corresponding solar lines. 



The result of this comparison is of further interest in relation to the structure of 

 the reversing layer. It has already been suggested by JEWELL,* MrrcHELL,t and 

 others, that different Fraunhofer lines are produced at different levels, and the 

 discussion of the scandium lines tends to support this view. In the case of iron all 

 the arc lines, down to the faintest, are well represented by dark lines in the sun, and 

 there are no enhanced lines so strofigly shown as the stronger arc lines. With 

 scandium it is just the opposite, and the simplest supposition to make is that only a 

 small amount of scandium absorption originates at the level which produces the 

 majority of the Fraunhofer iron lines. Since direct observations show that enhanced 



* ' Astrophys. Jour.,' vol. 4, p. 138 (1896). 

 t 'Astrophys. Jour.,' vol. 22, p. 37 (1905). 



