Spectrographic Observations of Peculiar Stars.IV.

I. New observational results-AX Persei in January and February, 1942, showed the lines of [Fe v} and [Fe vi] with considerable intensity. Other interesting changes in the spectrum have been recorded. RW Hydrae is interesting because of the absence of fluorescence excitation in 0 m and because the He i singlets (2p1P0 - nd'D) are relatively strong. Z Andromedae has returned to a stage similar to that ob- served in August, 1940. T Coronae Borealis has bright Fe `I, but the absence of [Ne v] shows that the excitation is lower than in Z Andromedae. Z Canis Majoris has been observed near maximum light. Several Of and W stars have also been observed. II. ExcitaUon mechanisms in shells.-The importance of fluorescence excitation is discussed and is applied to the problem of selectivities among emission lines of peculiar stars. Of particular interest are the ratios of singlet to triplet intensities of He i. The great relative strength of the singlets in objects of low excitation, like RW Hydrae, is attributed (a) to the predominance of fluorescence over recombination and (b) to the presence of high radiation density in the nebulous shells of the singlet series (is'S - np1P°) of He r. Similar considerations are applied to unusual intensities of emission lines in Of shells and in P Cygni or Be shells. The influence of departures of the exciting radiation from that of a black body is discussed for novae and for certain long-period variables. When fluorescence is produced by strong ultraviolet emission lines, as in the case of Bowen's mechanism for 0 m, the gradient in the velocity of expansion of a shell produces important modifications. This may account for absence of the Bowen mechanism in Woif-Rayet stars and in other rapidly expanding shells