162 
ON THE EFFECT OF PRESSURE UPON ARC SPECTRA. 
III. Reversal :— 
1. As the pressure is increased, reversals at first become more numerous and 
broader. 
2. The tendency to reverse reaches a maximum in the neighbourhood of 20 to 25 
atmospheres, and a further increase in pressure reduces their number and width. 
3. Two types of reversal appear on the photographs, symmetrical and unsymmetrical. 
4. Within the range of pressures investigated, the reversals show no tendency to 
change their type. 
5. In the case of unsymmetrically reversed lines in the electric arc, the reversed 
portion does not in general correspond to the most intense part of the emission line, 
being usually on its more refrangible side. 
6. The displacements of the reversed parts of the unsymmetrically reversed lines of 
Group III. are about one-half the displacements of the corresponding emission lines. 
Indeed, the reversed parts of the lines of Group III. fall approximately in Group II. 
7. No relation between the order of reversal and the frequency of vibration, such 
as exists in the spark, has been observed in the iron arc for the ranges of wave-length 
and pressure examined. 
IV. Intensity :— 
1. The intensity of the light emitted by the iron arc is, under high pressures, much 
greater than at normal atmospheric pressure. 
2. Changes in the relative intensity of the lines are produced by pressure. Lists of 
enhanced and weakened lines are given. 
1 express with pleasure my appreciation of their services to Mr. T. Loyds, B.Sc., 
who assisted me in the preliminary operations, and in taking and measuring the first 
set of photographs, and to Messrs. Beeakley, Riley, Rossi, and West, who assisted 
in the measurement of the plates and in the taking of the second set of photographs. 
My indebtedness to Mr. Petayel, F.R.S., who designed the essential part of the 
pressure apparatus, has already been recorded. 
The research was suggested to me by Professor Schuster, F.R.S., and for placing 
the necessary apparatus at my disposal and for his advice and inspiring interest I am 
very grateful. 
In conclusion, I cannot resist the opportunity of expressing my admiration of the 
work of Humphreys and Mohler, the pioneers in this subject, whose results are 
here, in the main, confirmed. 
FESSEHTBD 
