EFFECT OF PEESSURE UPON ARC SPECTRA.-NICKEL. 



241 



For the purpose of comparing the characteristics of lines belonging to these two 

 groups Diagram 8 has been prepared. It closely resembles Diagram 6, fig. 4, but 

 instead of denoting the lines by means of numbers, their characteristics are quoted in 

 accordance with the abbreviations given below the diagram. It deals only with lines 

 whose displacements above 10 atmosphere have been determined. It is clear that 

 Group I. is by no means homogeneous, and that it may be divided into two Sub- 

 groups IA. and IB. The characteristics of the lines falling into the first part of 

 Group I. (Group IA.) are : 



1. Slight broadening. Nearly symmetrical. 



2. Sharp at atmospheric pressure. 



3. Reversed lines are very nearly symmetrical. 



4. Two lines are strengthened, none are weakened. 



Diagram 8. 



_ 

 laqaaa'b'banNN 



10 



14 



(J 5& 4 6 8 



a = slightly broadened. 



b = greater broadening (includes moderate, considerable, great, &c.). 



s = sharp at atmospheric pressure. 



N = nebulous at atmospheric pressure or under high pressures. 

 / = broadening or reversal is unsymmetrical. 

 Lines 8 and 31 not included in this diagram (ef. Diagram C, fig. 4). 



The characteristics of lines falling into the second part of Group I. (Group IB.) are : 



1. Greater broadening. [Many are described as moderately broadened, some as 



greatly broadened, though several are described as slightly broadened.] 



2. Nebulous at one atmosphere or nebulous at 10 atmospheres. 



3. The reversed lines are less symmetrical. 



4. Tendency to weaken under pressure. 



The characteristics of the lines falling into Group II. are : 



1. Great broadening. 



2. Their unsymmetrical appearance under pressure. They are all broadened 



greatly to the red side. 



3. None are reversed under pressure. 



