810 Dr. Megh Nad Saha on 



pretation of these lines was given by Kirchhoff in 1859, who 

 showed that most of the principal lines can be attributed 

 to the absorption of light of proper frequency by the cooler 

 layers of the vapour lying above the photosphere. Since this 

 epoch-making discovery, it has become a part of the routine 

 work of astrophysicists to catalogue and properly measure 

 the wave-length of these lines, and identify them with the 

 emission lines of elements obtained in the laboratory. 



The most extensive mapping of the Fraunhofer spectrum 

 is due to Rowland, who counted and catalogued about 

 20,000 lines, but of these only 6000 have been identified 

 with the lines of known elements. By this means, the pre- 

 sence of thirty-six elements has been definitely established 

 in the sun, with doubtful indications of eight or more. The 

 following are the elements of the existence of which in the 

 sun no evidence has yet been obtained * : — 



(A). Rubidium, Caesium; Nitrogen, Phosphorus, Boron, 

 Antimony, Bismuth, Arsenic ; Sulphur, Selenium; Thallium, 

 Praseodymium. 



Doubtful indications have been obtained of the existence 

 of the following elements : — 



(B). Radium | ; elements of the inert group with the 

 exception of Helium, which is obtained in the flash spectrum, 

 Osmium, Iridium, Platinum, Ruthenium, Tantalum, Thorium, 

 Tungsten, Uranium. 



The following elements are represented by very faint 

 lines in the Fraunhofer spectrum : — 



(C). Potassium, Copper and Silver; Cadmium and Zinc ; 

 Tin and Lead ; and Germanium. 



(D). Chlorine, Bromine, Iodine, Fluorine, Tellurium, and 

 many other elements have not been investigated at all. 



No satisfactory explanation has yet been offered of the 

 complete non-existence of the lines of elements mentioned 

 in group (A) or (B), or of the faint occurrence of the lines 

 of elements mentioned in group (C). Similarly, it has not 

 yet been made clear why certain elements like 0a, Fe, V, Ti 

 are so unusually prominent in the solar spectrum. They are 

 represented not only by the absorption lines of the neutral 

 atom, but also by the absorption lines of the ionized atom 

 (enhanced lines). 



It is sometimes assumed that these phenomena are due to 

 the chemical composition of the sun — in other words, the 



* Pringsheim, Physik der Sonne, p. 116. 



t For the controversy regarding the existence of Radium, and the 

 members of the inert group in the flash-spectrum, see ' Observatory,' 

 vol. xxxv. pp. 297, 357, and 402. 



