222 CARNEGIE INSTITUTION OF WASHINGTON. 



The 10-inch photographic telescope has been used by Mr. Duncan and 

 Mr. Hubble for the study of areas in the Milky Way, both luminous and dark. 

 A dark marking has been found at galactic latitude +37°, the highest latitude 

 so far recorded for such objects. A small Tessar lens attached to the larger 

 telescope is giving excellent results in the photography of the larger galactic 

 features. Both of these instruments have been used by Mr. Lundmark for 

 studies in star counts and the distribution of the novse. 



Variable Nebulae. 



Mr. Hubble has continued his study of the variable nebulae with the follow- 

 ing results: 



N. G. C. 1555. Two photographs made during the year show no con- 

 spicuous change either in the nebulosity or in the associated star T Tauri. 



N. G. C. 2261, 6729. Nine photographs of 2261 and 12 of 6729 have been 

 secured during the year. The associated stars, R Monocerotis and R Coronae 

 Australis, have remained fairly constant, but the nebulae have presented con- 

 stantly varying appearances. These changes in the nebulosity can be ac- 

 counted for in the case of both objects by progressive movements of areas of 

 obscuration, possibly dark clouds of nebulous material, over a permanent 

 background of luminous nebular details. These permanent details appear 

 to vary in brightness only with variation in brightness of the associated stars. 

 The luminous details show no indications of actual motion. 



Luminosity of Nebula. 

 Mr. Hubble's investigations of galactic nebulae have led to the following 

 conclusions : 



(1) All diffuse nebulae have stars actually involved in or conspicuously 

 associated with them. 



(2) There is a direct relationship between the spectral types of stars and 

 the spectrum of the associated nebulosity. Nebulae with emission spectra 

 are found to be associated with stars of type BO or earlier, nebulae having con- 

 tinuous spectra with stars of B2 or later, while nebulae giving spectra of mixed 

 characteristics are usually associated with stars of the critical type Bl. An 

 apparent exception to these conclusions is that stars whose spectra contain 

 strongly enhanced lines may sometimes be associated with nebulae having 

 emission spectra. Such cases, however, are very rare. 



(3) The luminosity of diffuse nebulae obeys the inverse-square law with 

 respect to the associated stars as sources. 



(4) Within the range of spectrum covered by ordinary Seed 30 plates, and 

 within the probable errors of the observations, the quantity of light emitted 

 by a given area of diffuse nebulosity is equal to the amount which the area 

 intercepts from the associated star or stars. This holds for all diffuse nebulae, 

 whether their spectra are of the emission, continuous, or mixed type. 



(5) The spectra of emission nebulae never agree with the spectra of their 

 associated stars, although for the nebulae with continuous spectra the available 

 evidence indicates a fairly close agreement. 



The obvious conclusion is that luminous diffuse nebulae derive their light 

 from involved or neighboring stars. Where there are no stars properly 

 situated or of sufficient brightness to illuminate a nebula, the nebula is dark 

 and makes its presence known by obscuration. The mechanism of illumina- 



