1115 
a few circlets high. Indeed, taking any dispersion line for itself, the 
amount of its displacement towards the red will be determined by 
the shape of the dispersion curve; this, however, does not depend 
exclusively on the positions and intensities of the nearest companion 
lines, but also on the value nm, which the index of refraction of the 
medium would have for light of the small spectral region under 
consideration, if this ‘where free from lines. Since n, varies along 
the spectrum, there will be a corresponding fluctuation of the values 
of the displacements, on which the influence of close neighbouring 
lines superposes itself. Thus we conceive that the two swarms of 
black dots and circlets must partially penetrate each other. 
Let us next consider numerically, to which extent the displacement 
of a line towards the red is modified, on the average, by the presence 
of close companions. 
For that purpose we make use of the two curves shown in the 
diagram, which represent the general increase of the displacement 
with wave-length. They are derived for Mount Wilson *) from 450, 
for Kodaikanal from 392 measured displacements. (The second curve 
lies sensibly lower than the first; this may be due to the accidental 
fact that in the Kodaikanal material a greater number of very weak 
lines, showing small displacements, have been included); These curves 
define for every region in the spectrum an average or normal displa- 
cement d. Now we have calculated for each influenced line the 
value of the expression 
which may be denominated “relative departure”. 
With lines having a companion on the violet side, these depar- 
tures are for the greater part positive, with lines having a com- 
panion on the red side they are mainly negative, so that the first 
group gives a positive, the second group a negative “sum of departures”’. 
From the Mount Wilson data we derive (after applying a correction 
explained in the original paper): 
= D,= + 7,09 (25 lines) and SD, = — 7,09 (23 lines) . (1) 
and from the Kodaikanal data 
= D, = + 19,15 (36 lines) and =D, = — 19,16 (44 lines) . (2) 
Hence the mean value of a relative departure (positive or negative) 
is: 
7,09 + 7,09 + 19,15 + 19,16 
D= = 0,410, 
128 
1) Cf. Junius and van Cirtert, l.c. 
72% 
