112 
RADIATIVE EQUILIBRIUM 
At 10 million degrees the frequency RT/h corresponds to wave-length 
14-3 A, so that 69 per cent, of the weight is between 6 and 2 A. For 
elements of moderate atomic weight this corresponds to L radiation. It 
is this region that we must search particularly for sources of stellar 
opacity. 
When dealing with sources of continuous absorption which operate 
chiefly in the above stretch of spectrum, we shall often drop the distinction 
between absorption and opacity for approximate treatment. The correc 
tion necessary to reduce k x to k 2 will be referred to as Rosseland’s correction. 
Table 8. 
Weights for Calculating Opacity. 80 
x — hv/BT 
Weight at x 
Weight 0 to a; 
0 
•000 
•0000 
i 
•244 
•0016 
1 
•921 
•0121 
H 
1-872 
•038 
2 
2-897 
•084 
2i 
3-806 
•150 
3 
4-467 
•230 
4 
4-864 
•413 
5 
4-270 
•591 
6 
3-229 
•736 
7 
2-194 
•840 
8 
1-376 
•908 
9 
•810 
•949 
10 
•454 
•973 
oo 
•000 
1-000 
80. Consider two frequencies v' and v" , the first in a strong absorption 
line or band and the second in a region of little absorption. If we were 
considering the outer atmosphere of a star the absorption would cause the 
radiation to be lacking in the constituent v . But it is not so in the interior 
of the star, where thermodynamic equilibrium is nearly perfect. Strong 
absorption is compensated by strong emission, and v' and v" are present 
in the ordinary proportions indicated by Planck’s Law. The difference is 
that whereas the v" radiation is on the whole flowing outwards the v' 
radiation is practically brought to a standstill. 
If, for example, v" is on the average absorbed and re-emitted once in 
100 cm. path and v once in 1 cm. path, the difference of intensity of the 
outward and inward streams of v" corresponds to the difference of tempera 
ture of the points from which these streams were emitted, viz. the tempera 
ture drop in 200 cm. The difference of intensity of the outward and inward