EFFECT OF PRESSURE UPON ARC SPECTRA. 
1.61 
21. Summary of Results. 
The spectrum of the iron arc in air has been examined in the region X = 4000 to 
X = 4500, under pressures varying from 1 to 101 atmospheres (absolute). 
I. Broadening :—- 
1. With increase of pressure all lines become broader. 
2. The amount of broadening is different for different lines, some almost becoming 
bands at high pressures and others remaining comparatively sharp. 
3. The broadening may be symmetrical or unsymmetrical; in the latter case the 
broadening is greater on the red side. 
II. Displacement :— 
1. Under pressure the most intense portion of every line is displaced from the 
position it occupies at a pressure of 1 atmosphere. 
2. Reversed as well as bright lines are displaced. 
3. With increase of pressure the displacement is towards the red side of the 
spectrum. 
4. The displacement is real and is not due to unsymmetrical broadening. 
5. The displacements are different for different lines. 
6. The lines of the iron arc can be grouped into series according to the amounts of 
their displacements. 
7. Three groups can in this way be distinguished from one another; the displace¬ 
ments of Groups L, II., III. bear to one another the approximate ratio 1:2:4. 
(The existence of a fourth group is suggested by the behaviour of two lines, but 
further evidence is needed upon this point. 1 : 2 : 4 : 8 would be the relations 
existing between the four groups.) 
8. Though all the lines, with two exceptions, fall definitely into one or other of 
these groups, the lines belonging to any one group differ to an appreciable extent 
amongst themselves in the amounts of their displacements. 
9. The relation between the pressure and the displacement is, in general, a linear 
one, but some photographs taken at 15, 20, and 25 atmospheres pressure give readings 
incompatible with this relation. Other photographs at 15 and 25 atmospheres present 
values which are compatible with it. 
10. The abnormal readings are approximately twice those required by the displace¬ 
ments at other pressures for the displacement to be a continuous and linear function 
of the pressure. 
11. On the photographs showing abnormal displacements, the reversals are more 
numerous and broader than they are on plates giving normal values, and there is 
some evidence in favour of a connection between the occurrence of abnormal displace¬ 
ments and the tendency of the lines to reverse. 
VOL. CCVIII.-A. Y 
