NICKEL UNDER MODERATE PRESSURES. 
363 
It will be seen that the constant difference, say D, referred to above, does not affect 
the final result. Thus if d x , d 2 , d 3 , c? 4 are the means of the differences between the 
settings for the normal and displaced lines in the four positions, the corresponding 
shifts are given by 
whence the mean shift 
Sj = d x — D T 
s 2 = d 2 D J 
s 3 — D d 3 ~1 
5 4 — D — d 4 J 
plate direct, 
plate reversed, 
s 4 + s 2 + s 3 + s 4 
4 
1 {{d 1 + d 2 ) — (d 3 +d i )}. 
The value of D need not therefore be determined. 
4. Changes in Character of the Lines. 
One of the most obvious results of the increase of pressure is the very striking 
alteration which takes place in the character of the lines. A great variety of 
behaviour is to be observed, and it has been found convenient to divide the lines into 
five groups following the classification of Gale and Adams # :— 
Class I.—Lines which reverse symmetrically. 
Class II.—-Lines which reverse unsymmetrically. 
Class III.—Lines which remain bright and fairly narrow. 
Class IV.—Lines which remain bright but are very much broadened sym¬ 
metrically. 
Class V.—-Lines which become very much broadened unsymmetrically towards 
the red. 
All the stronger arc lines of wave-length shorter than X 3900 are included in classes 
I. and II., the majority being unsymmetrically reversed. In all cases in which 
the reversal is unsymmetrical, the reversal lies on the violet side of the middle of the 
emission line, at least so far as nickel lines are concerned. The same conclusion has 
been reached by Duffield in the case of the iron arc. The rule does not appear to 
be universally true, however, for the K and H lines of calcium which appear on some 
plates as impurity lines, are unsymmetrically reversed in the opposite sense. The 
enhanced lines of nickel all belong to class IV. and exhibit no tendency to reverse. 
* ‘ Astrophysical Journal,’ XXXV. (1912), p. 15. 
3 A 2 
