EFFECT OF PRESSURE UPON ARC SPECTRA. NICKEL. 209 



nebulous, and steeper in the case of those which are called sharp. The nebulous lines 

 are not so clean-cut as those described as sharp, indeed the former frequently stand 

 out against what looks very much like a fogged background. 



Many of the strong unsymmetrically broadened lines from 159 onwards are 

 accompanied by a haziness towards the red. It is doubtful whether it is continuous 

 with the intensity curve of the chief part of the line or whether the luminous patches 

 are superposed. It is possible that the different portions are due to light emitted by 

 different parts of the arc. 



A critical comparison between the broadening of the lines of various metals will be 

 given in a subsequent paper. 



(2) Continuous Spectrum. It has been shown that under great pressure the silver 

 arc spectrum becomes contimious, and that it is due, at any rate in the region of the 

 spectrum investigated, to the broadening of the lines of the first subordinate series.* 

 This phenomenon has been looked for in the nickel spectrum, but though there is a 

 certain amount of contimious spectrum upon certain photographs (e.g. Plates 3 and 4 

 at 155 atmospheres), it is generally caused by the hot metallic poles rather than by 

 the spreading of a line ; as evidence of this we note that the continuous background 

 is less pronounced at the higher pressure of 200 atmospheres. 



There is, however, a very great broadening of some of the lines upon Plate 1, the 

 spreading of lines 19 (X = 3566'50) and 31 (X = 3619'52) being particularly notice- 

 able (Plate l). Under 100 atmospheres the wings of 31 extend beyond line 36 on one 

 side and 26 on the other, and may be responsible for some of the continuous spectrum 

 which extends towards the region of longer wave-lengths. It is thus in some ways 

 analogous to the silver lines which belong to the 1st sub-series, and it may be a 

 member of this sub-series in the nickel spectrum. 



5. The Reversal of Lines. With increase of pi-essure there is at first a greater 

 tendency for lines to reverse, and these reversals are indicated in Table III., those 

 in italic (thus 3619'52, 47, 36, 148, &c.) representing a strong reversal, and those in 

 clarendon type (thus 3670'57, 39, 41, 102, &c.) a weak one. It will be observed that 

 more reversed lines are found in the region of small wave-length than in other parts 

 of the spectrum. 



In addition to the information conveyed in Table III., further details concerning 

 the reversal of lines are given in Table IX. , where the width of the absorption line is 

 indicated, and also the symmetry or dissymmetry of the position of the fine line 

 upon the broadened emission line. 



As in the case of broadening, the widths of the reversals are classified by s = slight, 

 m = moderate, c = considerable, g = great, vg = very great, G = very very great. 

 The term r s indicates that the absorption line is nearly symmetrically disposed upon 

 the emission line ; r v indicates that the absorption line is on the violet side of the 

 centre of the emission line. 



* W. G. DUFFIBLD, ' Phil. Trans.,' A, vol. 211, p. 33, 1910. 



