COMPOSITION OF THE SUN RUSSELL 211 



important lines which are absorbed by the atom in at least one of 

 its energy states (whether normal or excited), for the numbers in 

 other states can then be calculated by thermodynamic theory, know- 



FiGURE 1.- — Diagram showing the energy levels of the sodium atom. The dots repre- 

 sent the Individual atomic states. The.y are set at heights corresponding to the 

 energy content of the atom. The lines represent those transitions which can 

 actually occur and produce spectral lines. An atom in the lowest energy state 

 absorbs only a certain series of spectral linos, one in the next highest energy state, 

 two other series of lines. (See text.) 



Figure 2. — Similar diagram for the spectrum of titanium. The number of energy 

 states is very large, and the transitions between them give a very complicated 

 spectrum 



ing the sun's temperature. In this way the numbers of neutral atoms 

 have been found for many elements. Most ionized atoms have all 

 their best lines in the far ultra-violet and are not amenable to our 

 treatment, but there are several whose numbers can be found, Now 



102992—32 15 



