226 CARNEGIE INSTITUTION OF WASHINGTON. 



Mr. Hubble's investigations have dealt especially with the galactic 

 nebulae and the involved stars, and with non-galactic "globular 

 nebulae, " which are very numerous. These latter objects resemble spiral 

 nebulae in spectrum and radial velocity, but even the largest of them 

 offer no evidence of spiral structure. The smaller representatives are 

 often indistinguishable from small spirals, but the globular type illus- 

 trated by M 87 apparently constitutes a distinct class, which is 

 receiving special study. Many galactic nebulae and nebulous stars 

 have also been photographed with the 60-inch and 100-inch telescopes. 

 These nebulae apparently obscure in part the stars beyond or within 

 them, and suggest by their forms the action of repulsive forces emanat- 

 ing from associated stars. 



Extended nebulae and "dark" or obscuring nebulae are found to 

 cluster about two planes — the galactic circle and the belt of bright 

 helium stars— thus supporting the h^iDothesis of a local cluster re- 

 sembling the clouds of the Milky Way (p. 251). 



Mr. Hubble's studies of variable nebulae have been continued with 

 interesting results. Small changes in Hind's variable nebula can 

 apparently be attributed to variations in absorption affecting luminous 

 regions beyond, and the same may be said of N. G. C. 2261, though in 

 this case the involved star appears to exercise some influence in its 

 immediate neighborhood. N. G. C. 7629 showed marked changes, 

 while the involved star R Coronae Australis was brightening rapidly. 

 Between the nights of August 14 and 16, 1920, a bright envelope, which 

 expanded in three days from 4" to 12" in diameter, developed about the 

 star. The highest observed rate of 4" per day, assumed to represent 

 a wave of illumination traveling outward with the velocity of light, 

 corresponds to a distance of about 290 light years, which is in good 

 agreement with the distance as deduced by two other methods (p. 252). 



Mr. Hubble's spectroscopic survey has doubled the number of known 

 gaseous extended nebulae and more than tripled that of extended 

 nebulae (p. 253) showing continuous spectra. Reference is made below 

 to his studies of the spectra of nebulous stars and their relationship 

 to those of other stars and nebulae. 



In harmony with previous results for M 33, Mr. van Maanen has 

 obtained definite evidence of outward motion along the arms of the 

 spiral nebulae M 51 and 81. As possible sources of error now seem to 

 have been excluded, this interesting result may be considered as 

 substantial support for Jean's theory, which indicates tlmt matter 

 should be thrown off from two points of a nebula after it has been 

 reduced by rotation to a lenticular form (p. 255). 



Mr. Duncan has photographed a considerable number of nebulae 

 and star-clusters, and has detected large internal motions in M 1 

 (Crab nebula), confirming the results of Lampland. These motions 

 are generally outward from the center, and show a maximuim displace- 



