204 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1931 



in astrophysics by an Indian physicist, Saha, and has revolutionized 

 our understanding of the subject. 



In the solar spectrum the arc lines predominate. Enhanced lines, 

 though numerous, are usually inferior in strength, though by far 

 the strongest lines in our whole spectrum arise from ionized calcium 

 (Ca+). "Band lines" due to compounds are not conspicuous, but 

 some were recognized by Rowland as due to compounds of carbon. 

 Many more have since been detected, and it is probable that a great 

 number of the faint lines which are as yet unidentified may turn out 

 to be due to compounds whose spectra have not yet been measured 

 with adequate precision for comparison. 



A decisive test of the ionization theory is found in the spectra 

 of sun spots. The spots, though darker and redder than the rest of 

 the sun's surface, and obviously cooler, give off light of their own. 

 This exhibits a very distinctive spectrum, similar in general to that 

 of the sun, but with many differences. Some lines are much weaker, 

 others are stronger, and some greatly strengthened. Comparison 

 with the data of the laboratory shows that the weakened lines are 

 practically all enhanced lines, and the strengthened lines are lines 

 of easily ionized metals, while hundreds of band lines appear in 

 the spots alone. At the lower temperature of the spots new com- 

 pounds form which are completely decomposed over the disk, and 

 the compounds otherwise present are increased in quantity. Ioniza- 

 tion diminishes, so that enhanced lines fade, and arc lines strengthen. 

 The agreement with theoretical prediction persists to the minutest 

 detail. For an element of difficult ionization, like silicon, the en- 

 hanced lines are very much weakened, and the arc lines little, if at 

 all, strengthened, while for one easy to ionize, such as strontium or 

 scandium, the enhanced lines hardly change and the arc lines are 

 enormously strengthened in the spot. This agrees perfectly with 

 calculation, which shows that for such elements most of the atoms 

 are ionized even above the spots, but the percentage of neutral atoms, 

 while still small, is greatly increased there. There are three elements, 

 lithium, rubidium, and indium, which appear only in the spot spectra. 

 All are easy to ionize, and must be so completely ionized at the ordi- 

 nary photospheric temperature that their arc lines disappear. The 

 strongest lines of the ionized atoms are out of reach in the ultra- 

 violet, and so we would lose them altogether from our list, were 

 it not for the relative coolness of the spot. This actually happens 

 for caesium, the easiest of all elements to ionize; and there are a 

 good many other elements, such as barium and the rare earths, which 

 reveal themselves only by their enhanced lines. 



Elements of difficult ionization are also at a disadvantage in the 

 sun, but for a different reason. Not all the lines, even of a neutral 



