AND ITS RELATION TO SOLAR SPECTRA. 57 



some of the Wolf-Eayet stars, where the less refrangible lines of hydrogen are bright 

 and the more refrangible ones dark.* 



A general comparison of the unenhanced lines with the spot spectrum shows that 

 all the more prominent lines are considerably strengthened in passing from the sun to 

 the spot. Seeing that the spot spectrum is far more crowded with lines and flutings 

 than the solar spectrum, it is not surprising that there are several- apparent coin- 

 cidences of spot lines with the fainter lines of scandium, but, with our present 

 knowledge, it would be unphilosophical to regard sucli occasional coincidences as 

 significant. The only scandium lines which can confidently be regarded as intensified 

 in the spot spectrum are, in fact, those which are brightest in the arc-flame spectrum 

 previously described. The strongest line of all is 6305 '88, and other prominent lines 

 are 6210'90, 5700'40, 5687'05, and 5()72'05; in the blue end, the strengthening of 

 arc-flame lines, like the weakening of enhanced lines, is less marked in the photographs 

 at present available. 



The principal lines of the arc-flame spectrum, including all the unenhanced lines of 

 intensity 6 or more from Table I., are brought together in Table IV. and compared 

 with the sun and sunspots. There is a certain amount of confusion with lines of other 

 elements in spots, but it may be reasonably concluded that the intensities of the lines 

 in spots correspond closely with their intensities in the arc-flame. The line 5514'44 

 does not appear in the spot spectrum as might have been expected, but it is at the 

 lower limit of intensity, and it would be unwise to conclude from the behaviour of a 

 single line that the conditions in spots are very different from those of the arc-flame. 

 It may be supposed that while there is a general increase in the scandium flame 

 absorption, the increase does not suffice to bring the fainter lines to an intensity 

 within the range of observation. 



It is quite certain, therefore, that while the enhanced lines of scandium are 

 weakened in spots, the remaining lines are strengthened more or less in proportion to 

 their intensities in the arc-flame, at least in the less refrangible parts of the spectrum. 

 As the relative intensities of the two sets of lines in spots approximate to those found 

 in the arc spectrum, it is possible that there is a descent of scandium vapour from the 

 upper region, where the conditions are such as to produce enhanced lines, to a lower 

 level where the prevailing conditions approximate to those of the arc. Ixeduced 

 temperature of the spot vapour, accompanying this change of level, perhaps provides 

 the readiest explanation of the difference between the Fraunhofer and spot spectrum, 

 but much further investigation in several directions is necessary before the precise 

 nature of the action in spots can be ascertained. 



Comparison with the Chromosphere. 

 Two principal sources of data relating to the chromosphere are available for 



* CAMPBELL, ' Ast. and Astrophysics,' vol. 13, p. 457 (1894). 

 VOL. CCIX. A, I 



