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RECENT ADVANCES IN SCIENCE 



ASTRONOMY. By H, Spencer-Jones, M.A., B.Sc, Royal Observatory, 

 Greenwich. 



lonisation in Stellar Atmospheres. — If, on the occasion of a 

 total eclipse of the sun, the slit of a spectroscope be set upon 

 the bright limb of the sun and a plate exposed immediately 

 before the total phase commences or immediately after its 

 termination, the bright line or " flash " spectrum of the upper 

 layers of the solar chromosphere is obtained. It has long been 

 known that this spectrum is characterised by those lines which 

 are relatively stronger in the spark than in the arc spectra, 

 lines which were designated by Lockyer as enhanced lines. 

 Moreover, it is only such lines as are enhanced which reach to 

 the highest levels of the chromosphere. The reason why 

 such substances as calcium occur at the highest levels to the 

 apparent exclusion of other and lighter substances has been 

 the subject of much discussion. An explanation could doubt- 

 less be given if the cause of the enhancement in the spark of 

 certain lines could be stated. This is connected with the 

 physical mechanisms of the arc and spark. Lockyer supposed 

 that passage from the arc to the spark meant a great though 

 localised increase in temperature. That this could be the 

 explanation of the enhancement of lines in the flash spectrum 

 is not probable, for it would involve the supposition that the 

 ■ outer chromosphere is at a higher temperature than the lower 

 layers or photosphere. It is, perhaps, most convincingly dis- 

 proved by the experiments of Anderson at the Mount Wilson 

 Observatory, who has recently been studying the spectra 

 produced by fine wires, exploded by the discharge of a large 

 condenser. The light source so produced had an intrinsic 

 brightness of about loo times that of the solar surface, 

 corresponding to a black-body temperature of 20,000° C, yet 

 the principal enhanced lines were absent. 



The more plausible theory has been put forward that the 

 enhanced lines are due to radiations not from a normal atom 

 of the element but from an ionised one, i.e. an element which 

 has lost an electron. Thus, for instance, in the case of calcium 



