In two papers, Russell[396] has given a discussion of the solar and sunspot spectra, showing that ionization theory offers a very satisfactory interpretation of most of the observed phenomena. Attention was called to the anomalous behavior of barium and lithium,[397] and it was suggested that the theory of thermal ionization, while taking account of the temperature of the reversing layer, omitted to consider the effect of the absorption of photospheric radiation. This omission might cause a deviation such as is observed for barium, but appears inadequate to account for the behavior of lithium. In the case of lithium, low atomic weight, and a consequent high velocity of thermal agitation, has been suggested as the cause of the anomaly. The question of the absorption of photospheric radiation has more recently been discussed by Saha,[398] in the form of a correction to his own ionization equations. It has been pointed out by Woltjer[399] that the correction introduced by Saha and Swe may also be derived from considerations advanced by Einstein[400] and Milne.[401] The correction can be evaluated, but appears in every case to be rather small. The effect of the photospheric radiation is certainly one that must be included in a satisfactory theory, but at present, observation is probably not of sufficient accuracy to demand such a refinement. The work just quoted was qualitative. A more quantitative test of ionization theory in the solar spectrum can also be made[402] by comparing the intensities of solar lines corresponding to different excitation potentials, but belonging to the same atom. The [Pg 114] atoms which give a large number of lines in the solar spectrum are those of the first long period of the periodic table, and these, as is well known, consist of multiplets, with components of very different intensities. It appears to be legitimate to select the strongest line associated with any energy level for the comparison; the strength of this line probably represents fairly well the tendency of the atom to be in the corresponding state. Atom Excitation Potential Intensity Atom Excitation Potential Intensity Calcium 0.00 20 Chromium 0.00 10 1.88 15 0.94 5 2.53 8 1.02 5 2.70 5 2.89 2 2.92 4 Titanium 0.00 5 Iron 0.00 40 0.82 4 0.94 30 0.90 3 1.54 30 1.05 3 2.19 8 1.44 3 2.46 10 1.50 2 2.84 8 1.87 1 2.96 7 1.98 1 3.25 7 2.08 0 3.38 6 2.16 1 3.64 8 2.24 2 4.13 - 2.26 0 4.23 - 2.28 0 4.35 - 2.33 0 4.40 - 2.39 00 2.47 - 2.56 000 2.67 000