EFFECT OF PRESSURE UPON ARC SPECTRA. 
151 
16. The Anomalous Displacement of Unsymmetrically Reversed Lines. 
Tables I. and II., pp. 136 and 137, show that several lines reverse for a small range of 
pressure only. 
Diagram VII. illustrates the behaviour of lines of this type belonging to Set A, 
Groups I. and II., the dotted parts of the curve relating to the reversed line. It is 
apparent that for Group I. the atom absorbing the vibration follows the same course 
under pressure as the one producing the radiation. But this is not the case for the 
reversed lines of Group III., whose displacement curves may be compared with the 
displacement curves for unreversed lines belonging to this group, and also with their 
own displacement curves for that range of pressures during which reversal does not 
take place. 
The remarkable fact is brought to light that the lines of Group III. present two 
rates of increase of the displacement with pressure—one for the lines when unreversed, 
and the other, very much lower, for the line in its reversed state. 
Corroborative evidence of this phenomenon has been obtained from the second 
series of photographs, Set B, for which similar curves are shown in Diagram A III. 
