MOUNT WILSON OBSERVATORY. 239 



is extended by the presence of other elements in the case of an element 

 of easy ionization, but diminished in the case of an element harder than 

 the average to ionize. 



(3) When multiple ionization takes place, by the successive removal 

 of two (or more) electro as from the atom, the number of successive 

 stages of ionization in which a sensible proportion of atoms can 

 simultaneously exist is only two (at least under any conditions which 

 are likely to be met with). 



The ordinary (flame or arc) lines are absorbed by non-ionized atoms, 

 the enhanced lines by ionized atoms, and certain "super-enhanced" 

 lines by double ionized atoms. Many facts connected with the spec- 

 tral sequence among the stars are explainable u'pon these principles. 

 For epcample, the fact that the arc lines disappear in class B before the 

 "super-enhanced" lines appeajr is a direct consequence of (3). 



The following results have been obtained by a test of the theory in 

 sun-spot spectrum: 



The principal lines of rubidium, XX7800.29 and 7947.64, which are 

 absent in the solar spectrum, appear in the spot spectrum as diffuse 

 lines of intensity 1 and 0, respectively, show strong Zeeman effect, 

 and are very similar in character to the lithium line at X6708. 



The lines of the principal series of potassium are much strengthened 

 in the spot spectrum. The infra-red pair of sodium at XX8183, 8194 is 

 also much strengthened. 



The lines of the subordinate series of potassium, lithium, and rubi- 

 dium appear neither in the sun nor in spots. 



All these phenomena are in excellent accordance with the theory of 

 Saha^ who predicted most of them. Rubidium appears to be so com- 

 pletely ionized in the sun that there are not enough neutral atoms left 

 to absorb the ordinary lines, except at the lower temperature of the 

 spots. Lithium, however, should be less ionized than sodium, and the 

 absence of its lines in the spectrum of the sun can be explained only on 

 the assumption that very little of the element is present. 



The alkaline earths show a similar behavior. Barium, which is 

 the most easily ionized, appears in the solar spectrum mainly by its 

 enhanced lines ; and the presence of even the most prominent line due 

 to the neutral atom (X5535) appears to be doubtful both in sun and 

 spots. The lines of zinc, which have a high ionization potential, are 

 much weakened in spots. 



IONIZATION OVER FACUL^. 



On the Bohr theory of atomic structure, radiation of the normal 

 type occurs only when an electron falls from one orbit to another of 

 lower potential energy. The ordinary enhanced lines of an element 

 are thought to be produced when the atom is ionized, that is, when one 

 electron has been removed completely. Saha has developed an ex- 

 pression for the relative proportion of ionized to un-ionized atoms as a 



