In earlier papers (Varshni, 1978, 1979) we have emphasized the need of obtaining the absorption-line spectra of quasars with high resolution and accuracy (about 0.1 Å). Recently a number of papers have appeared in which high-resolution studies of quasars have been reported (Chaffee et al., 1983, 12 km/s. Carswell et al., 1984, 0.25 Å; York et al., 1984, 10 km/s), though these investigations cover only a narrow region of the spectrum. Clearly , the techniques are now available. It would be most desirable to obtain the spectrum of 0805+046 over a wide wavelength interval (3200-5000 Å) at a high resolution and with a high signal-to-noise ratio.
Absorption lines have been noted in the low-resolution spectra of 0138-381 and 0316-203. It would be of considerable interest to obtain high resolution absorption-line spectra of these two quasars. An intercomparison of the absorption lines in 0805+046 can provide clues to the evolution of the shell in a quasar. It would also be of interest to look for the helium discontinuity at 3422 Å in 0138-381, 0316-203, and 1124+571.
During the last twelve years since the PLS model was first proposed, progress has been made in theoretical studies and in experimental observations of population inversion in an expanding plasma. We note here one point which is of direct relevance to the present paper. Zhinzhikov et al. (1978) have observed population inversion in helium in supersonic plasma expansion . The experimental data confirm the operation of a recombination mechanism for the population of the He I levels under the experimental conditions. Population inversion has also been observed in some helium- like ions (Apruzese et al., 1978 ; Bhagavatula and Yaakobi, 1978; Zhizhan and Guangyu, 1983).
Finally, we may also note here that He I 4713 emission is known to occur in the spectra of certain novae and nova-like stars.