and
. These pressures are of course to be regarded as upper limits, for it is possible to miss several lines at the violet end of the series, and Wright, with larger dispersion, does indeed record twenty-four Balmer lines in
Cygni; on the other hand it is not likely that the estimated number will exceed the actual number of lines.
The pressures in the reversing layer, as derived from the observed limit of the Balmer series, are then of the same order as the pressures derived by the other methods outlined above. This is of especial interest because the method, if applicable, is a direct one, and gives results for individual stars, whereas all the other methods, excepting the one based on pressure shifts, are essentially indirect.
(g) Ionization and Chemical Equilibrium.—The evidence adduced by Russell and Stewart[88] has been greatly amplified by Fowler and Milne,[89] and by the data bearing on their theory which were subsequently published by the writer[90] and by Menzel.[91] It is not intended to present the evidence from ionization theory here in support of the low pressures inferred by the other methods for the reversing layer. The pressure derived in the present chapter, and considered as independently established, will be used in [Chapters VII] ff. to derive a stellar temperature scale, for the reversing layer, from the line-intensity data presented.
SUMMARY
The following tabulation contains a synopsis of the reversing layer pressures derived by the methods that have been outlined.