170 REPORTS ON INVESTIGATIONS AND PROJECTS. 



The exposures on these objects have ranged from 3.5 to 16 hours. 



From an examination of the negatives Mr. Fath concludes that essentially 

 all of the globular star clusters photographed are approximately of the 

 F type of spectrum on the Harvard system of classification, while the spiral 

 nebulae are of the G type. A few of the photographs are under-exposed, 

 and it is possible that these results may be modified somewhat when more 

 spectra are available. The spectrum of N. G. C. 650-1 shows 7 bright lines 

 coincident with those found in the spectra of gaseous nebulae, and it is pos- 

 sible that this object is a transition type between gaseous and spiral nebulae. 



Professor Kapteyn's Investigations. 



A considerable portion of the time of Professor Kapteyn this year was 

 devoted to the working out of a part of the program of the 60-inch reflector. 

 Last year a series of photographs of the spectra of spiral nebulae and star- 

 clusters was undertaken. Since these objects are, as a rule, so faint that 

 spectra can be obtained only of the brighter ones, and even then with low 

 dispersion and with exposures which extend over a whole night or even two 

 nights, especial attention has been given to the question in what way these 

 laborious observations can be made useful in the solution of the fundamental 

 problem whether there is an appreciable selective absorption of the light of 

 the stars in its passage through space. For this purpose a number of obser- 

 vations have been added to the program of the reflector. 



With the aid of Mr. Seares a program has also been worked out for the 

 photographic determination of standards of magnitude for stars of every 

 degree of brightness down to the faintest stars that can be obtained with a 

 moderate exposure with the 60-inch reflector. Several observatories are 

 already devoting much labor to this problem, and the great light-gathering 

 power of the Mount Wilson reflector makes it particularly desirable that 

 this Observatory should take part in the work, not only in order to aid in 

 establishing a scale of absolute magnitudes, but also in order to extend the 

 scale to include fainter stars than could be reached at most other observa- 

 tories. 



The main part of Professor Kapteyn's work, however, has been devoted 

 to an investigation of two very extensive systems of stars of the Orion type 

 of spectrum. These systems have already been shown to include probably 

 more than half of the stars known to belong to this spectral type. It seems 

 probable that further investigations will lead to the conclusion that these 

 two systems are identical with two main star-streams, in which case the 

 movement of these streams, as shown by the Orion stars, would be seen in 

 its "purest" or least perturbed state, owing to the vast distance of stars hav- 

 ing this type of spectrum. However this may be, there seems to be little 

 doubt that as soon as a sufficient number of radial velocities are available 

 the nature of these systems will allow us to derive the individual distances 

 of all of the members of the groups of stars, and therefore of at least half 



