1881.] on the Oriij'ui and Llmfiti/ of Spectra. 693 



but rapidly contract and gradually become very faint or disappear 

 entirely. 



By examining the arc of a Siemens macliine, taken in a crucible of 

 dense magnesia under the dispersion of the sjiectrmn of the fourth 

 order, given by a Rutherford grating of 17,296 lines to the incli, wo 

 are able to separate the iron and magnesium lines which form the 

 very close pair h^ of the solar spectrum. Either of tlie two lines can 

 be rendered the more prominent of the pair at will, by introducing 

 iron or magnesium into the crucible. The less refrangible line of 

 the pair is thus seen to be due to iron, the more refrangible to mag- 

 nesium. Comparison of the solar line and tlie spark between 

 magnesium points confirms tliis conclusion, that the magnesium line 

 is the more refrangible of the two. 



In the ultra-violet part of the spectrum photographs show several 

 new lines. First, a trijjlet of lines above U at wave-lengths about 2942, 

 2938*5, 2937. These lines are a little below a pair of lines given 

 by the spark for which Cornu has found the wave-lengths 2934*9, 

 2926 * 7. The latter pair are not seen at all in photographs of the arc, 

 nor the former three in those of the spark. Tlie strong line, wave- 

 length about 2850, is always seen, very frequently reversed. Of the 

 quadruple group in the spark to which Cornu has assigned the wave- 

 lengths 2801*3, 2797*1, 2794-5, and 2789*9, the first and third are 

 strongly developed in the arc, the other two hardly at all. Next follows 

 a set of five nearly equidistant lines, well-defined and strong, but 

 much less strong than the two previously mentioned, wave-lengths 

 about 2782 * 2, 2780 * 7, 2779 • 5, 2778 • 2, 2776 * 9. The middle line is a 

 little stronger than the others. The same lines come out in the spark. 



Beyond these follow a series of pairs and triplets ; probably they 

 are trii)lcts in every case ; but the third, most refrangible, line of the 

 triplets is the weakest, and has not in every case been noticed as yet. 

 These succeed one another at decreasing intervals with diminishing 

 strength, and are alternately sharp and diffuse, the diffuse triplets being 

 the strongest. The positions are shown in PI. III., Fig. 2. The 

 series resembles in general character the sodium and the potassium 

 series described by us in a former communication, and we cannot resist 

 the inference that they must be harmonically related, tliough they do 

 not follow a simple harmonic law. The most refrangible line in the 

 figure at wave-length 2605 represents a faint diffuse band which is 

 not resolvable into lines ; it belongs, no doubt, to the diffuse members 

 of the series, and, to complete the series, there should be another 

 sharp group between it and the line at wave-length 2630. This 

 belonging to the weaker members of the series is too weak to be seen. 



It is worthy of remark that the line at wave-length 5710, described 

 by us in a previous communication,* is very nearly the octave of the 

 strong line at 2850. Moreover, the measures we have taken of 

 the wave-length of this last line, with a Rutherford grating of 17,296 



♦ ' Proc. Rov. Soc' No. 2(i0, p. 98. 



