In order to attain the maximum degree of consistency, the estimates used in this chapter were derived chiefly from the two series of plates mentioned in Chapter VIII. All the plates used were made with the same dispersion (two 150 objective prisms) and were of comparable density, and of good definition. The data furnished by the writer’s own measures were supplemented by some estimates derived by Menzel[479] from a similar series of plates, of the same dispersion and comparable quality. The estimate of the marginal appearance of potassium was very kindly suggested by Russell from solar observations. The observed marginal appearances of all the lines that are available are summarized in the table that follows. Successive columns contain the atomic number and atom, the series relations, the wave-length of the line used, and the Draper classes at which the line is observed, [Pg 182] respectively, to appear, to reach maximum, and to disappear. Asterisks in the last column denote the ultimate lines of the neutral atom, which are strongest at low temperatures, and have no maximum. TABLE XXVII Atom Series Line Classes Atom Series Line Classes 1 H 4340 - - 22 Ti 3999 * * 2 He 4471 4862 * * 5015 4867 * * 4388 4856 * * He+ 4542 - 4536 - - 3 Li 6707 * * - 4535 - - 6 C+ 4267 23 V 4333 * * 11 Na 5889 * * 4330 * * 5896 * * 24 Cr 4290 * * 12 Mg 5184 - ? 4275 * * 5173 - ? 4254 * * 5167 - ? 4497 - 3838 - ? 25 Mn 4034 * * 3832 - ? 4033 * * 3829 - ? 4030 * * Mg+ 4481 - 4084 - 13 Al 3962 * * 4041 - 3944 * * 26 Fe 4325 - 14 Si 3905 - 4811 Si+ 4128 30 Zn 4811 4131 4722 19 K 4044 * * 38 Sr 4607 * * 4047 * * Sr+ 4078 - 20 Ca 4227 * * 54 Ba+ 4555 - ? 4455 - Ca+ 3933 - - [Pg 183] Estimates by Menzel are indicated by a dagger; those marked by a double dagger were taken from dyed plates made with slightly smaller dispersion. METHOD OF ESTIMATING RELATIVE ABUNDANCES If the physical conception of marginal appearance above outlined is correct, the number of atoms of a given kind above the photosphere will practically determine the class at which the corresponding line is last seen.[480] Now at marginal appearance the number of suitable atoms is only a small fraction of the total amount of the corresponding element that is present in the reversing layer, and this fraction is precisely the “fractional concentration” evaluated by Fowler and Milne. If then it be assumed that the number of atoms required for marginal appearance is the same for all elements, the reciprocals of the computed fractional concentrations at marginal appearance should give directly the relative abundances of the atoms.