7Q ,,| { . w. GEOFFREY DUFFIELD ON THE 



to be more probable. No relationship between the displacement and wave- 

 th was discovered in dealing with the iron arc, perhaps because a sufficiently 

 wide range of spectrum was not examined, but the resolution of the spectrum into 

 groups was, nevertheless, an outstanding feature. If we are correct m believing that 

 the equations representing the behaviour of the three groups are d == AX 3 , d ---- BX , 

 and d = CX 3 , it is clear that when lines belonging to different groups he close together 

 the individual readings of their displacements will also be approximately in the ratio 

 of A : B : C. Among those lines which have not appeared upon many of the plates 

 are d2 and kl, and these, possessing values for the quantity d/\ 3 of 815 and 598 

 respectively, are to be classified as members of Group III. to whose mean 653 they 

 more nearly approximate. 



The following is a list of the lines belonging to the different groups with their mean 



values for the quantity (//A. 3 : 



TABLE VIII. 



It is interesting to note that the lines that are thus grouped together have some 

 other properties in common. 



The members of Group I. are the two lines that broaden least, and that indeed 

 remain fairly fine throughout a considerable range of the pressure. They may both 

 be described as " hard" at low pressures, though jl is more clear cut than gl. 



Both are about equally, though very slightly, broadened to the red side under 

 pressure. 



The line /I, which is typical of Group II., is broadened symmetrically, and the two 

 lines *1 and t2, though very much weaker, are believed to be similarly broadened. 



Group III. is characterised by strong broadening to the red side, to which the 

 lines o2, pi, ql, q2, bear witness. /2 and r2 only differ from these in having a 

 bilateral broadening, but they are both more greatly extended on the red side. 



In addition to these groups, we may with advantage classify in Group IV. those 

 lines that are broadened towards the violet, and whose displacements have not been 

 definitely determined. 



