152 
MB. W. GEOFFBEY DUFFIELD ON THE 
These agree well with those obtained from Set A, and testify to the fact that the lines 
of Group III., when reversed, fall approximately in Group II. 
The ratio of the displacements of the unreversed to those of the reversed lines of 
Group III. is for Set A 1‘6, and for Set B 17, if the general directions of the curves 
be considered. From individual values at the pressures at which actual readings 
have been made, the ratio is very much higher, but the uncertain region, 15 to 30 
atmospheres, has then an undue influence. 
Diagram VIII. emphasizes the possibility of subdividing Group I. 
17. The Reversal of Lines under Pressure. 
It has already been pointed out (p. 133) that the immediate effect of pressure is to 
increase the number and intensity of the reversals, and that there is a pressure at 
which the reversals reach a maximum in these respects. The varying tendencies of 
the different lines towards reversal may be studied in the two tables of displacements, 
in which the number indicated by different type signifies that at the corresponding 
pressure the spectral line is reversed. 
It will be seen that 
Lines Originally Not Reversed either 
(1) remain without reversal throughout the whole range of pressures investigated, 
e.g., 3, 4, 5, al, «3, c2, c3, &c. ; or 
(2) suffer reversal for a certain range of pressure, and then, as the pressure is 
increased, return to their initial condition. For Set A the range is 15 to 25 atmospheres. 
For Set B the greatest tendency to reverse occurs at 20 atmospheres. Lines belonging 
to this class are : hi, cl, d 1 , &c. (Diagram VIII.). 
Two lines appear to suffer reversal in the neighbourhood of 3 to 5 atmospheres in 
Set B, and then to return to their unreversed condition. These lines are remarkable 
also for their large displacements under pressure. They are g6, g7, and are discussed 
under the heading of “ Lines Enhanced under Pressure,” p. 156. 
Lines Originally Reversed 
Continue as reversed lines up to the highest pressure. These are almost invariably 
strong lines, and their reversals are broad, cO, d 0 , d2, &c. The width, however, 
decreases in the neighbourhood of 100 atmospheres, and two of these lines in Set A 
do not show reversal above 80 atmospheres. 
For Set A the reversals become more numerous and stronger up to 25 atmospheres, 
but at higher pressures they decrease in number and intensity. For Set B the 
maximum occurs at 20 atmospheres, though at 10 atmospheres they are also numerous, 
but not so broad. It may be that the paucity of reversals at the highest pressures is 
due entirely to the short duration of the are in those regions, but though this 
