EFFECT OF PKESSURE UPON ARC SPECTRA. 221 







differences, separated from the rest of the spectrum three series of lines the frequencies 

 of whose members could be represented by three formulae ; these series have become 

 known as the Principal, First and Second Subordinate Series. 



The Zeeman effect affords another means for their classification, and investigations* 

 have shown that, though the pairs belonging to any particular series in the copper 

 spectrum behave similarly in the magnetic field, there are marked differences between 

 the behaviour of individual members of each pair. 



KiNGt has subjected the arc and spark spectra of copper to an exhaustive analysis, 

 and, from the behaviour of the lines in arcs of different current strengths and sparks 

 under various conditions, resolved them into three groups, which, however, do not 

 correspond to those found by KAYSER and RUNGE. 



From their behaviour under pressure we possess an additional means for separating 

 the lines of the copper spectrum into groups. HUMPHREYS^ first suggested this : 

 chronicling three series with " small," " medium," and " large " shifts respectively. 



Table III. shows that the displacements of the series lines d, I, o are greater than 

 those of the non-series lines, but the former lines are too broad for accurate measure- 

 ments, and with the copper spectrum there is not the same distinct division into groups 

 according to the amount of the displacement that is possible with the iron arc ; the 

 nature of the lines and their changes of relative intensity afford the best means for 

 classifying them. 



1. The nebulous and sharp lines retain their respective types throughout the range 



of pressure, 1 to 101 atmospheres. 



2. The nebulous non-series lines are strengthened under pressure relatively to the 



sharp lines. 



3. The four members of the recognised series (1st and 2nd sub-series) are weakened 



under pressure, and at the highest pressure are obliterated. 



4. The two members of the 1st sub-series pass through the stage of being faint 



hazy bands, and then as the pressure is increased are completely dissipated. 



5. The two members of the 2nd sub-series gradually diminish in intensity without 



abnormal widening, ultimately becoming obliterated. 



A comparison of these results with those of A. S. KING are of interest. KING has 

 investigated the spectra obtained from different parts of the copper arc and spark, 

 and has found that some lines occur more strongly near the poles and others more 

 strongly in the centre of the arc or spark. The writer, || in a subsequent investigation 

 of the iron arc, found some value in designating the former " polar" and the latter 



* RUNGE and PASCHEN, ' Astrophysical Journal,' XVI., p. 123 (1902). 

 t A. S. KING, ' Astrophysical Journal,' XX., p. 21 (1904). 

 I HUMPHREYS, 'Astrophysical Journal,' VI., p. 169 (1897). 

 DUFFIELD, 'Phil. Trans. Roy. Soc.,' 208, p. Ill (1908). 

 || DUFFIELD, ' Astrophysical Journal,' XXVII., 260 (1908). 



