MOUNT WILSON OBSERVATORY. 255 



predominates. Slipher has shown that the absorption spectra of 

 several extended nebulae agree with the spectra of the stars involved, 

 and this suggests that the luminosity of extended nebulse having ab- 

 sorption spectra is due Largely to reflection of starlight. The fact that 

 bright lines fade so smoothly into a brightening continuous spectrum 

 as the stellar type advances, and that nebulosity around the few known 

 cases of late-type nebulous stars averages considerably fainter than that 

 around stars of earlier types, further suggests the possibility of an 

 excitation which produces continuous spectrum, decreasing with the 

 temperature of the involved stars, superposed on the reflected illumi- 

 nation of the nebulae. This question can be studied through color- 

 indices of the nebulae, and attempts are being made to gain the neces- 

 sary information. 



Nebulous Stars. 



The investigation of the spectra and colors of nebulous stars by 

 Mr. Seares and Mr. Hubble has been continued, and results for most 

 of the known objects brighter than the 13th magnitude are complete. 

 An excess of color over that corresponding to the spectrum of the star 

 is almost always present, though to a varying degree. The most 

 remarkable case is that of the fainter stars near o Persei, many of 

 which are involved in the nebulosity I. C. 1985 or its outlying masses. 

 The average color excess for 17 stars is over half a magnitude, and for 

 several of them it amounts to 0.8 or 0.9 mag. Broadly speaking, 

 those closest to the center of the nebula show the largest excess of 

 color. As usual, the stars are of early type, mostly B's with a few 

 A's. Assigning absolute magnitudes in accordance with the luminosity 

 curves of Professor KaptejTi, we have for the mean parallax of the 

 group 0^0045=1=0 ''0003. The spectroscopic parallax of the nearby 

 nebulous star B. D. +31°597 is 0':005. I. C. 1985 and the luminous 

 nebulosity surrounding B. D. +31°597are presumably associated with 

 the obscuring material that fills this part of the sky, and it is there- 

 fore probably not accidental that the two parallaxes are the same. 



Intern.\l Motions in Spiral Nebul.e. 



In the last report mention was made of the measures of the spiral 

 nebula M 33 by Mr. van Maanen. The results were derived from a 

 pair of plates taken with a 10-year interv^al at the 25-foot focus of the 

 60-inch reflector. The earlier plate was secured by IMr. Ritchey, the 

 new one by Mr. Duncan. Measures by Mr. van Maanen of two photo- 

 graphs of the same object taken by him at the 80-foot focus with an 

 interval of 5 years have given strong corroboration of the earlier results. 



A new stereocomparator was built during the year, in which consid- 

 erable improvements have been effected as compared with the Zeiss 

 instrument. After testing, it was put into regular use in April 1921; 

 the results are very satisfactory. With this instrument Mr. van 

 Maanen has measured two other spiral nebulae, M 51 and M 81. For 



