EFFECT OF PRESSURE UPON ARC SPECTRA. 
135 
Set A. 
Set B. 
Pressure 
in atmospheres. 
Number of photographs. 
Pressure 
in atmospheres. 
Number of 
photographs. 
Pressure spectrum 
inside. 
Pressure spectrum 
outside. 
Pressure spectrum 
outside. 
5 
3 
3 
1 
10 
2 
— 
4 
1 
15 
— 
2 
5 
1 
20 
i 
2 
10 
1 
25 
— 
2 
15 
2 
30 
i 
2 
20 
i 
40 
2 
— 
25 
2 
60 
i 
1 
30 
1 
80 
— 
1 
40 
3 
95 
— 
1 
50 
2 
100 
— 
1 
60 
i 
70 
i 
80 
i 
100 
* 
Two photographs 
were usually taken upon each plate. 
The first set are numbered D1 to D19. 
The second set are numbered 1)21 to D33. 
10. Description of the Tables of the Displacements. 
The results of the measurements of the two sets of plates are given in Tables I. and II. 
The first columns contain a list of arbitrary letters and numbers assigned to the 
different lines to facilitate reference to them. Beginning at the red end they run 
1, 2, 3, 4, 5, 6, a0, al, a2, aB, «4, 60, hi, 62, 63, 64, cO, cl, c2, &c. Plate 4 contains 
photographs showing lines beginning with 2 and ending with j 0. 
The second columns give the wave-lengths of the lines according to Ivayser and 
Bunge’s tables. 
In the subsequent columns the displacements of the lines at different pressures are 
set forth in thousandths of an Angstrom unit. The use of black figures, e.g ., 223, for 
displacements indicates that the line is reversed at the corresponding pressure, italic 
figures, e.g., 223, denoting a faint reversal; the displacements are those of the centre 
of the reversal. An asterisk # indicates that the line is reversed, but that the 
measurements of the displacements were not consistent. Numbers in brackets have 
not been plotted in the diagrams. 
When two values for the displacement are given, one is for the line when reversed 
and the other for the unreversed line (measured on different plates), the upper number 
referring to the latter. 
In all cases the shifts are towards the side of greater wave-length. 
The pressures are the excess above one atmosphere. 
