MOUNT WILSON OBSERVATORY. 221 



sary. Needless to say, this plant will be of great service in pushing to 

 the highest attainable limit some of the investigations described in this 

 report and others of similar character. The Observatory will also be 

 aided materially by having access to the powerful X-ray apparatus and 

 the large physical library to be installed in the Bridge Laboratory. 

 More important still, constant stimulus and enlightenment will be 

 derived from close association with Dr. Millikan's researches on atomic 

 structure and Dr. Noyes's chemical investigations of problems bearing 

 directly on astronomy. The new step is therefore an epoch-making 

 one in the progress of the Observatory. 



Turning to other aspects of the year's work, we may first mention 

 those which bear directly upon this closer alliance with physics and 

 chemistry. For many years certain peculiarities of solar and stellar 

 spectra have baffled all attempts at solution. As an example, it has 

 been impossible to understand why the H and K lines, which certainly 

 belong to calcium, an element of comparatively high atomic weight, 

 nevertheless extend to the highest levels in the solar atmosphere, 

 far outreaching the lines of sodium, magnesium, and other lighter 

 elements. Dr. Megh Nad Saha, Assistant Professor of Physics in the 

 University of Calcutta, has recently offered an explanation which 

 appears to be generally applicable to the interpretation of many of 

 the most puzzling phenomena of solar and stellar spectra. According 

 to this view, the H and K lines are the enhanced lines of a calcium atom 

 which has lost one electron, whereas the fundamental line of neutral 

 calcium is X4227. In the higher levels of the chromosphere, where the 

 ionization, which is only partial at the higher pressures of lower levels, 

 becomes complete, neutral calcium and hence the X4227 Ime disap- 

 pears, while H and K, representing the ionized atoms, remain as con- 

 spicuous lines. The D and b lines of sodium and magnesium are due 

 to the neutral atoms, which are not present at high levels, and the lines 

 corresponding to the ionized atoms of these and other elements fail to 

 appear because they lie in the extreme ultra-violet, with the possible 

 exception of X4481 of magnesium. Space is lacking to give further 

 details, but Dr. Saha has already pointed out many possible applica- 

 tions of his theory, and others will rapidly develop. In evidence of 

 this, attention is called to the important results obtained by Dr. 

 Henry Norris Russell, Research Associate of the Observatory, who has 

 extended the theory to the case where atoms of several kinds are pres- 

 ent, and tested it in a preliminary study of the spectra of sun-spots 

 (p. 240). Among the interesting results of this work is the discovery 

 in the sun of rubidium, shown by the presence in the spot spectrum 

 of two lines in the infra-red, as predicted by Saha. A general attack 

 on solar, stellar, and laboratory spectra from this point of view, in 

 which Dr. Russell and other members of the staff will take part, is 

 being organized. In this connection it is expected that the determina- 



