EFFECT OF PRESSURE UPON ARC SPECTRA. 
157 
Attention was drawn to this phenomenon by what was at first thought to be the 
displacement of a line (a2) towards the violet. Under pressure this line appeared to 
have a reversal on the violet side of the comparison line, but the phenomenon has 
now been traced to the enhancing of a very faint line occurring close to a2 on its 
more refrangible side, which gave to the space between them the appearance of a 
reversal. It was also found that the only other apparent displacement of a line (bO) 
to the violet was due to a similar cause. 
(The measurements given for two other lines are possibly vitiated by the 
appearance in close proximity to them, in this case on their red side, of two enhanced 
lines. The displacements tabulated for g6 and g7 are larger than those for any other 
lines at the same pressures [Table II.], and the range of pressures at which they are 
reversed, 3-5 atmospheres, does not coincide with the region of reversal for 
other lines. Their behaviour may possibly be explained by the enhancing of two 
faint lines on their red side. This consideration has prevented them from being 
definitely assigned to a Group IY. possessing a displacement about four times as large 
as that of Group II.) 
An examination of Photograph 1, Plate 5, led to the discovery of several lines 
that are enhanced under pressure. The spectra were photographed under 1, +25, 
+ 50, +75, +100 atmospheres, and although the spectrum at 1 atmosphere is not 
exposed as much as the others, the relative intensities of the lines in it can be 
readily compared with their relative intensities in the other spectra. Pairs of lines 
to be compared are marked on the plate, and the member with the x will be seen to 
have increased in intensity under pressure. A list of such lines occurring in the 
region 4000-4500 A.U. is given in Table VIII. 
Letter. 
Table YIII. 
Wave-length. 
Links Enhanced under Pressure. 
ala* 
4454-50* 
a3 
4443-30 
* Marked x on Plate 5, fig. 1. 
60 
4430-74 
62* 
4422-67* 
All enhanced lines are marked E on the 
r,2* 
4369-89* 
same Plate. 
f/0 
4219-47 
Lines Weakened under Pressure. 
4484-361 
f The weakening effect may be to some 
4469-531 
extent due to widening of the lines. 
e2 
f 0 
4282-58 
4250-28 
4247-60 
Weakened lines are marked W on 
Plate 5, fig. 1, and on Plate 8. 
4238-951 
1 Marked O on Plate 5, fig. 1. 
Note. —The relative intensities also seem to vary in the different photographs at normal atmospheric 
pressure, but different amounts of exposure possibly account for this, 
