It may be that these phenomena are a result of an abnormal abundance or distribution of the element. It is not, therefore, entirely necessary to assume that the theory is here at fault, although until the behavior of strontium has been satisfactorily interpreted, that possibility cannot be rejected. It is significant that calcium and barium show similar absolute magnitude behavior. In any case, the ionized strontium lines cannot be cited, as has sometimes been done, in demonstrating [Pg 152] that the absolute magnitude effect is due to pressure. What is actually shown is that the concentration of singly ionized atoms is more greatly increased at the expense of the neutral atoms than it is reduced by the formation of doubly ionized atoms. Since a pressure effect operates by the discouragement of recombination, it would be inferred that the recombination of singly ionized atoms with electrons to form neutral atoms is less readily encouraged than the recombination of doubly ionized atoms with electrons to form singly ionized atoms. Evidently the problem is a complex one. If the maximum of the strontium lines were at (where theory first predicted it, and where the earlier measures actually placed it) there would be no anomaly to explain; but two independent observers[428] place it definitely at or , and there can be little doubt that this is actually the correct position of the maximum. The result of the study of absolute magnitude effects is disappointing. It appears that the observed phenomena are qualitatively explained in a satisfactory manner, as due to lowered pressure, or, more accurately, to low surface gravity. There is, however, a serious discrepancy in the case of the lines whose variation with absolute magnitude is perhaps best established, and upon which the most important results have been based. The results, being empirical, are of course unimpaired, and it would seem that the theory requires to be amended. Furthermore, it does not yet appear to be possible to use the observed changes of intensity for the direct estimation of pressure differences, because of the large number of variables involved and particularly because of the superposition of the pure pressure effect upon the effect of photospheric depth. FOOTNOTES: [412] Adams and Kohlschütter, Mt. W. Contr. 89, 1914. [413] B. A. N. 19, 1922. [414] Milne, Phil. Mag., 47, 209, 1924. [415] Pannekoek, B. A. N. 19, 1922. [416] Pop. Ast., 31, 88, 1923. [417] Pop. Ast., in press. [418] B. A. N. 19, 1922. [419] In Pannekoek’s notation, a is surface brightness, is radius, and , surface gravity. [420] Stewart, Pop. Ast., in press. [421] Pub. Dom. Ap. Obs., 3, 1, 1924. [422] P. 121. [423] H. C. 258, 1924. [424] Adams, Pub. A. S. P., 28, 278, 1916; Adams and Joy, Pub. A. S. P., 36, 142, 1924. [425] Pub. Solar Phys. Com., 1910. [426] Chapter V, p. 81. [427] Chapter, V, p. 70. [428] Menzel, H. C. 258, 1924; Chapter VIII, p. 126. [Pg 153] PART III