More than 175 early-type stars have been observed with a photoelectric polarimeter attached to the ~o-inch reflector at the U. S. Naval Observatory. This polarimeter was built at Amherst College, financed by the Research Corporation and described briefly in a paper presented at the Pasadena Meeting of the A.A.S.1 The observational procedure is that outlined in a preliminary paper. A quarter-wave (4350A) plate of mica was used to determine the nature of the light from HD183143. By changing the position angle of the mica we found that the polarized light could be reduced from 7 per cent to 0.7 per cent. Since no color filter was used, the residual effect may be produced by a difference in effective wave lengths. These preliminary measures therefore indicate that the light from HD183143 consists of plane-polarized and unpolarized components. - + 30 I 0 0 A -~ LONGITUDE polarization of individual stars. that the vast majority of them are located in obscured regions that can be characterized by the presence of faintly luminous nebulosities, whereas on a plate that covers the region of the North America nebula, and that reaches the fifteenth photographic magnitude, not one star with Ha in emission was found. Star No. 24 observed by Struve1 without emission lines, appears on two of our plates with fairly strong emission in Ha. As additional evidence of possible variation in emission features the writer has detected a variation in the intensity of the bright line in star No. 6 of the present list (Table I). Star No. 22 of Struve's list is variable in light with a range of at least 0.5 mag. TABLE I. STARS WITH Ha IN EMISSION Cordoba Position No. R.A. Dec. Remarks I 1fih14eo3 -26+10' 2 16 15.5 27 26 3 16 17.8 24 ~6 I 4 16 i8.o 23 0 5 16 18.5 25 3 1 6 16 i8.6 24 4 2 7 16 19.4 24 29 3 8 16 19.5 22 54 9 16 19.6 24 37 1 10 16 20.0 29 0 II 16 20.2 23 39 12 16 20.2 25 48 4 13 16 21.9 25 26 1 14 i6 23.5 23 48 15 16 23.8 26 II 5 16 16 24.0 24 9 17 i6 24.8 24 24 1 Remarks:I, very faint star; 2, apparent variation in the intensity of Ha; 3, No. 24 in Struve's list, spectral change; 4, variable as shown by plates taken in 1945; 5, star in a minute cometary nebula. On an hour Schmidt plate of the larger Sagittarius cloud, centered at a = 18h4m, ~ = 280, limiting magnitude 16, fifty-one objects with emission in Ha were found. Of these, 5 are doubtful as they could not be checked on another plate having the same limiting magnitude; 12 have a strong continuum; and 34 resemble the spectra of emission nebulae because of the absence of continuum. In this particular region Minkowski4 found 17 planetary nebulae, and before him, as far as the writer knows, only three planetaries were known. Probably most of the remaining 14 objects are new planetary nebulae. It seems that some of the planetaries found by Minkowski and some of the probable new emission nebulae are variable. I.Ap. J. 109, 92, 1949. 2.Ap. J. 90, 387, 1939. 3.Pub. A. S. P. ~8, 244, 1946. 4.Pub. A. S. P. ~8, 305, 1946; 59, 257, 1947; 6o, 386, 1948. Astronomical Observatory, Tonanzintla, Mexico.
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Hall et al. (1949) studied this question.