702 Professor Deivar [June 19, 



for a short time to be reversed, the other line remaining narrow and 

 quite unaltered. When the lithium had evaporated, and both lines 

 were again narrow, a small piece of Iceland spar was dropped into 

 the crucible, which immediately caused the expansion, and on one 

 occasion the reversal, of the more refrangible line, while now the 

 less refrangible line was unaffected. 



In this way we satisfied ourselves that the calcium line is the 

 more refrangible of the two, and is probably represented by the line 

 at wave-length 6101-9 in Angstrom's normal solar spectrum, while 

 the lithium line appears to be unrepresented. 



In the case of iron, which gives such a multitude of lines, it was 

 a priori probable that some lines would be coincident, or nearly so, 

 with lines of other elements ; and in fact we find that in five-sixths 

 of the supposed coincidences lines of iron are involved. We have, 

 therefore, chiefly directed our attention to iron lines. A complete 

 account of the separate resolutions will be found in the ' Proceedings 

 of the Eoyal Society,' May, 1881. 



PL II., Fig. 6, shows the appearance of the magnesium group of 

 the solar spectrum as observed in spectroscopes used by different 

 observers. The lines marked h^ and 6*, which appear to be single 

 lines in the maps of Angstrom and Kirchhoff, are resolved into double 

 lines by the greater dispersion employed by Thollon. The following 

 table shows the relative dispersion and number of lines seen by 

 different observers when powerful instruments are directed to the 

 same solar group : — 



Group E of Solar System. 



The indium line 4101*2 we found very difficult to separate from 

 the hydrogen line (Ji), as the latter had to be observed from a tube 

 with a spark, and it is both faint and diffuse; but several observa- 

 tions all led to the conclusion that the indium line is very slightly 

 less refrangible than that of hydrogen. 



We have also directly compared the iron line at 6316 '07 with 

 the solar spectrum, and found that the iron line corresponds with 

 the less refrangible of the two solar lines at this place, so that the 

 chromospheric line is in all probability the other line of the pair. 



There are still a few cases of supposed coincidences which we 

 have not examined. The results which we have recorded strongly 

 confirm Young's observations, and leave, we think, little doubt that 

 the few as yet unresolved coincidences either will yield to a higher 

 dispersion, or are merely accidental. It would indeed be strange if, 

 amongst all the variety of chemical elements and the still greater 

 variety of vibrations which some of them arc capable of taking up. 



