TEMPERATURE AND STRUCTURE OF THE SUN. 97 
resented in Fig. 7. The yellow light, the wave-length of 
which corresponds to those of the D lines, is not absorbed, 
but deflected in consequence of the anomalous dispersion. 
Becquerel’s and Julius’ observations had previously shown 
that the two D lines are anomalous to a different extent. 
From Wood’s experiments we now know that the anomalous 
absorption of the D lines is larger than that of other sodium 
lines. 
The same effect that is produced by the artificially^ ar- 
ranged prismatic mass of a gas is produced by its gradually 
variable density, as is the case in the spherical gaseous mass 
of the sun. In this case the observer with his spectroscope 
is always at a great distance from the “vapor prism,” which 
disperses anomalously, so that he cannot observe simul- 
taneously the whole phenomena. (In the laboratory we get 
the same effect by shortening and lengthening the slit of the 
spectroscope.) 
Let us first, imagine* an observer at such a great distance 
* In this discussion, we follow closely the treatment given by E. Prings- 
heim in his publication “Ueber Brechung und Dispersion des Lichtes 
auf der Sonne.” Archiv. fur Math, und Physik. 4 , 310-330, 1903. 
