MOUNT WILSON OBSERVATORY. 195 



The central regions of the few spirals which have been observed seem to 

 follow the same general law, and the results suggest that amorphous non- 

 galactic nebulae are built on a nearly uniform pattern. 



INTERNAL MOTIONS IN SPIRAL NEBULAE. 



The principal investigation of this character was the measurement by- 

 van Maanen of 400 points in the great spiral nebula Messier 33 on negatives 

 taken in 1910 by Ritchey and 1922 by Humason. The results confirm 

 strongly those obtained from six spirals measured previously in showing the 

 presence of internal motions which can be interpreted either as rotation or as 

 a motion outward along the spiral arms. Further investigations will be 

 deferred until photographs taken with the 100-inch telescope and separated 

 by an adequate interval of time are available. 



In summing up the results of this difficult and very important research, 

 mention should be made of the strong character of the evidence which leads us 

 to place full confidence in the values obtained. 



(1) Of the seven nebulae measured, three are left-handed and four right- 

 handed spirals. The photographs measured were obtained for the most 

 part at the primary focus of the 60-inch reflector, but include negatives taken 

 at the Cassegrain focus of this instrument and a few made with the Crossley 

 reflector of the Lick Observatory. None of these photographs shows any 

 displacement for the comparison stars such as is found in the nebulae. 



(2) Measurements of Messier 51 by Kostinsky, Lampland, and Schouten 

 from observations with three other instruments give results similar to those 

 found by van Maanen. 



(3) The negatives used by van Maanen were also measured by other 

 observers, in the case of Messier 101 by Nicholson and of Messier 33 by 

 Lundmark, with confirmatory results. 



(4) Three different measuring-instruments were used in the course of the 

 work. The same machines, when used in the measurement of proper motions 

 on ordinary star-fields, have never shown any rotary displacements. 



From a combination of the results of various methods used in deriving 



the distances of the larger spirals, it appears that their parallaxes probably 



lie between a few ten-thousandths and a few thousandths of a second of arc. 



This would indicate diameters between a few light-years and several hundred 



light-years. 



COLOR INDICES OF SPIRAL NEBULA. 



Previous to his return to Upsala in May, Dr. Lundmark obtained a con- 

 siderable number of photographs of nebulae taken through the large objective 

 grating attached to the 60-inch reflector. These include in particular con- 

 densations in the arms of spiral nebulae, small non-galactic nebulae, and 

 former novae. The results will be made the subject of a study of color indices 

 which Lundmark is preparing. 



SPECTROSCOPIC OBSERVATIONS OF NEBULAE. 



Most of the spectroscopic observations made during the year have been 

 those of Hubble on the nuclei in planetaries, a few related objects such as 

 N. G. C. 2359 and 6888, and the novae about which nebulosity has been 

 observed. The detailed discussion of the spectrograms has not been com- 

 pleted, but a simple inspection shows that the planetary nuclei for the most 

 part are normal early-type stars with continuous and absorption spectra 



