242 DR. W. GEOFFEEY DUFFIELD ON THE 



If we include in our survey the readings made at 10 atmospheres more lines come 

 under observation ; among them are several that are nebulous, of these most are 

 weakened, e.g., 82, 135, 140 and 145, &c., but one, 133, has been classed as 

 strengthened. We also note that certain other lines which are nebulous under 

 pressure are included in Table IX., namely, those which are broadened towards the 

 violet, 232, 233, 239, &c. These nebulous lines all occur in Group L Group II. is 

 similarly extended by this process, and it now appears to include some lines which 

 become nebulous under pressure ; these differ from those assigned to Group I. in 

 appearing to be broadened to the red side, but there is a certain amount of doubt 

 about the accuracy of their measurements, and therefore of their position in Group II. 



Adopting the law that the displacement varies as the cube of the wave-length for 

 lines of the same group, we see from Diagram 6, fig. 4, that the maxima in the 

 distribution curves occur at 1075 and 3'0, which gives a ratio of 1 to 3'6 for the 

 maximum displacements of the two groups, a result very different from that found for 

 the groups in the iron spectrum. If, however, Group I. be sub-divided into Group IA. 

 and Group IB., we may take the similar nebulous lines 19, 21, 26, 31, 87, 165, 213, 217, 

 as forming the more displaced group for which the value of c/X 3 is about 5 x 10~ u . 

 The corresponding value for the remainder is not very different from 2'5 x 10~ u . 

 This would give a ratio of displacement for the three groups 2'5 : 5 : 10'75, which is 

 close to that found for the iron spectrum, namely, 1 : 2'2 : 4'5, and not far from 

 1:2:4, but it would only be justified if the division into three groups were an 

 established fact. 



It may be pointed out that the d/X 2 diagram (fig. 2), favours this relationship (the 

 ratios would be 1'3 : 27 : 5 '2), but, as has already been discussed, the diagram labours 

 under the disadvantage of including 247 in the intermediate group, dissociating it 

 from lines which it more strongly resembles, and it separates the similar lines 87, 165, 

 213, and 217. These difficulties are obviated in the d/\ 3 frequency curves. 



We are thus faced with the possibility of the following representations of the 

 groups : 



Group I. 



d = 3'OxlO- 14 X 3 . 

 Group II. 



d = 1075xlO- u X 3 , 



in which case the ratio is 1 : 3'5, or if there be three groups : 



Group IA., d= 2'5 xlQ- 14 X 3 or d = 1'3 x 10- 10 X 2 , 

 IB., d= 5'0 xlO- 14 X 3 d = 27xlO- 10 X 2 , 



II, d= 1075x10-" X 3 d = 5-2xlO- 10 X 2 . 



In either case the ratio is approximately 1:2:4. The writer inclines to the view that 

 this is the more probable ratio between the different groups, but the determination is 

 open to doubt. 



