DIFFUSE MATTER IN SPACE 
381 
because if the star s light traverses a diffuse nebula (with density perhaps 
10,000 times that of the interstellar cloud) the calculation is altogether 
upset. 
In some binaries the calcium lines show a radial velocity variable in 
the same period as the other lines but with smaller amplitude. It seems 
obvious that this is the result of a blend of the fixed calcium lines with the 
calcium lines of the star itself. Some writers, however, have attributed 
it to motion of a calcium envelope, supposed to surround the whole 
system, which follows with reduced amplitude the motion of the bigger 
component towards and away from us. The suggestion disregards alto 
gether the dynamics of the problem; obviously the motion of the principal 
star could not communicate displacement to a distant rare medium without 
great lag of phase. 
A word may be added as to the method of production of the lines. 
We have already seen that the atoms which have performed the line 
absorption cannot get rid of their energy except by radiating it again. 
But the absorbed radiation is taken out of the ray travelling from the star 
to the earth and the emitted radiation is sent out indiscriminately in all 
directions; if any of it meets the eye of the observer it is not coming from 
the direction of the star but is part of the general light of the sky. As 
there is no thermodynamic equilibrium it is not necessary that the emission 
should be qualitatively the exact inverse of the absorption—the atom 
may be excited in one step and return to its normal state by several 
intermediate steps. In fact the emission will usually be in lower frequencies 
than the absorption owing to the relative deficiency of the field of radiation 
in low frequencies. 
260. According to Milne’s theory of the chromosphere (254-83) the 
monochromatic absorption coefficient for H and K light is of the order 
10 9 , so that moderately dark lines will be produced by 10~ 9 gm. or 1-5.10 13 
atoms of Ca + per sq. cm. Assuming that fixed lines of this intensity occur 
in stars distant 500 parsecs (1-5.10 21 cm.), there must be 1 atom of Ca + 
in 10 8 cu. cm. The un-ionised atoms are presumably much rarer since their 
principal lines do not appear, but there may be any number of doubly 
ionised atoms. Judging by terrestrial abundance calcium might be ex 
pected to form rather more than 1 per cent, of the whole material. 
The fixed calcium lines may thus be considered to give a lower limit 
of about 10~ 6 atoms per cu. cm. for the density of interstellar matter. 
There is rather a wide gap between this and the upper limit of 1 atom per 
cu. cm. fixed by dynamical considerations; but this will be closed up if 
most of the calcium is doubly ionised. Another clue to the density is 
obtained by considering the general scale of the local condensations oc 
curring in it—the diffuse and dark nebulae. Since the investigation of the