MOUNT WILSON OBSERVATORY. 223 



critical study of spiral nebulae, and also of binary systems. It is plain 

 that the attack on the spirals should involve a study of the spectra and 

 motions of the matter comprising the arms, as well as a determination 

 of their parallax by any feasible means. The preliminary results of 

 van Maanen indicate motion outward along the arms, in harmony 

 with Jeans's views. But in order to secure a definitive answer to this 

 critical question, a photographic campaign, including spiral nebulae of 

 various types, must be carefully organized with this object clearly in 

 view. The large scale of the spiral nebulae at the principal focus of the 

 Hooker telescope, and the experience gained in the measurement of 

 nebular nuclei for parallax determinations, will be important factors in 

 this undertaking. The simultaneous spectrographic attack will be 

 aimed, not merely toward spirals in whose plane we lie, but also toward 

 those which stand at a sufficient angle to permit both components of 

 motion to be measured by the two methods. In this connection the 

 further development of the multiple-slit spectrograph, already applied 

 at IMount Wilson in a limited way, vdW be essential to rapid progress. 



Thus far, in our consideration of the Observatory's research program, 

 we have dealt almost exclusively with physical problems bearing on 

 the evolution of stars and nebulae. In the case of the spirals, however, 

 as in that of the red stars, it is evident that the part these objects play 

 in the system of the universe must be answered if possible. Fortun- 

 ately, the photographs obtained for the physical researches will also 

 contribute toward this end, but they must be supplemented by others. 

 This is especially necessary in attempting to answer the great out- 

 standing question whether the spiral nebulae are in reality ''island 

 universes" independent of one another and of our own sidereal system 

 or, on the contrary, lesser systems tributary to the Galaxy, which 

 dominates a single universe. Here a great need is for radial veloci- 

 ties, obtained with the most efficient spectrographs procurable, and 

 for parallaxes derived, or at least attempted, by one or more of several 

 methods. The 60-inch is quite as efficient as the Hooker telescope for 

 measuring the radial velocities of the larger spirals, but for the smaller 

 objects the latter instrument will be required. 



In dealing with problems of structure and motion in the galactic 

 system, the Hooker telescope offers some capital advantages. Hither- 

 to all studies of stellar radial velocities have necessarily been confined 

 to the brighter stars, in the vast majority of cases to those visible to 

 the naked eye. These, it is true, include some very distant giant 

 stars, but most of the stars whose radial velocities are known belong to 

 a very limited group, perhaps constituting a distinct cluster of which 

 the sun is a member, but in any event of insignificant proportions 

 when contrasted with the Galaxy. Current spectrographic work with 

 the 60-inch telescope, which will be continued as at present, includes 

 stars of the eighth magnitude and some that are even fainter. But 



