MOUNT WILSON OBSERVATORY. 253 



object culminates. Assuming the phenomenon to be a wave of 

 illumination moving with the velocity of light, the most rapid radial 

 growth, about 2" per day, would correspond to a distance of about 

 290 light-years. The probable absolute magnitudes of four nebulous 

 stars in the same dark area as the variable nebula indicate a distance 

 of 300 light-years. Distances of this order would require that the 

 moving area of obscuration referred to above travel with the velocity 

 of light. This velocity would correspond to a maximum possible 

 distance of 330 light-years. 



Plates made in June 1921 show changes in brightness of faint lumi- 

 nous details, some 90" preceding the nucleus, a region not obviously 

 connected with the variable nebula. The spectrum of R Coronse 

 Australis seems not to be of the nova type, as suggested by Slipher 

 (Lowell Bull., No. 81), but is rather like that of T Tauri, except that 

 the bright lines of hydrogen and iron are reversed as^Tnmetrically, 

 and no bright H and K lines have been found. There is, however, a 

 strong bright Ha line. 



N. G. C. 3550 and I. C. 48. — These nebulae have been suspected of 

 variability, the latter on the authority of Barnard. Both prove to be 

 spirals. N. G. C. 3550 has a faint star close to the nucleus. If the 

 observed changes in brightness are real, they might be accounted for 

 on the assumption that novse appeared in the nebulae. 



N. G. C. 7662. — Barnard announced the variability of the nucleus 

 of this planetary some years ago. Photographic and photovisual 

 observations made here give no indications of variability during the 

 year. 



Spectra of Nebula. 



Long exposures by Mr. Hubble with objective prisms on the 10-inch 

 Cooke lens and with the smaller cameras have added to the list of ob- 

 jects whose spectra were known 13 planetaries, 13 extended nebulae with 

 emission spectra, and 25 extended nebulae with continuous spectra. 

 Of these objects, 3 planetaries, 6 emission and 5 continuous-spectrum 

 nebulae were listed in last year's report. The survey has doubled the 

 number of gaseous extended nebulae previously known, and more than 

 tripled that of extended nebulae showing continuous spectra. 



The planetaries are 



