MOUNT WILSON OBSERVATORY. 225 



(2) A discussion of the total motions of 67 spirals from the measurements 

 of Curtis and 82 spirals from those of Lundmark. 



(3) A comparison of the internal motions in 7 spirals measured by van 

 Maanen with the spectroscopic radial velocities. 



These three methods give values ranging from 0''0001 to OTOOIO, and appear 

 to indicate that the spiral nebulae are at distances considerably less than 

 those required by the island-universe theory. 



Spectra of Nebula and Nebulous Stars. 



Mr. Hubble is continuing his spectroscopic investigations of stars associated 

 with diffuse nebulae. The radial velocities indicate that a very large propor- 

 tion of those with spectral types earlier than B8 are spectroscopic binaries, 

 and the orbits of several will be investigated to study the possible effects of 

 the nebulosity on orbital motion. 



Spectrograms of the nuclei or central stars of 10 planetary nebulae have 

 been obtained by Mr. Hubble with the Cassegrain spectrographs. These 

 all show much the same general characteristics and belong to types Od and Oe. 



Mr. Humason, using the 10-inch photographic telescope and an objective 

 prism, has discovered a new planetary nebula, and Mr. Hubble and Mr. 

 Lundmark, with the same instrument, have secured spectra of several diffuse 

 nebulae and nebulous objects. 



Mr. Sanford has used the small slit spectrograph at the primary focus of 

 the reflectors for work on a few nebulae. With this instrument, adapted 

 for use with two prisms, and a very short camera, an exposure of 21 hours 

 under very unfavorable conditions upon the nucleus of N. G. C. 2681 showed 

 a continuous spectrmn which extended to the violet of the K line. The 

 spectral type is estimated as F8 and measurements of the radial velocity give 

 a value of 4-700 km. 



Color Indices of Spiral Nebula. 

 Photographs have been taken by Dr. Lundmark for the purpose of studying 

 the color of the spiral nebulae and differences of color in different portions of 

 the same nebula. Two methods have been employed; first, direct photog- 

 raphy with and without color screens; second, exposures through a large 

 objective grating attached to the 60-inch reflector. This grating, designed 

 and first used by Professor Hertzsprung at Mount Wilson in 1912, has recently 

 been refitted in the Pasadena shops. Dr. Lundmark's first photographs were 

 made on objects of known color for the purpose of deriving a scale and zero- 

 point, and his results, so far as preliminary reductions have been made, con- 

 firm those of Seares and those of Lindblad and Lundmark made at Upsala. 



TRIGONOMETRIC PARALLAXES AND PROPER MOTIONS. 

 During the year Mr. van Maanen, assisted by Mr. Gingrich, has obtained 

 353 photographs with 641 exposures at the 80-foot focus of the 00-inch 

 reflector. The presence of an exceptional amount of astigmatism in the 60- 

 inch mirror during a period in March and April made some of these photo- 

 graphs unavailable for measurement. The plates necessary for the deriva- 

 tion of the parallaxes of 16 fields have been secured, the total number of fields 

 completed to date being 160. The measurement of the fields for the deter- 

 mination of the proper motions of the comparison stars has been continued 



