MOUNT WILSON OBSERVATORY. 193 



and 8 M to 14 n, as indicated by the transmission screens, proved to be very- 

 similar to that of the moon. These results afford new evidence that the 

 atmosphere of Mercury, like that of the moon, is negligible in amount. The 

 agreement both in spectral distribution of energy and in the amount of 

 planetary radiation emitted by these two bodies per unit of illuminated area 

 leads to the rather surprising conclusion that the temperature of the illu- 

 minated surface of Mercury is about the same as that of the moon. This 

 result is best explained by assuming a short period of rotation for Mercury, 

 and some measurements made of the radiation from the dark side of the 

 planet favor this view. 



In the case of both Jupiter and Saturn, the measures show that a larger 

 proportion of the radiation is transmitted by the water-cell than in the case 

 of the sun. This is probably due to selective reflection. The radiation 

 from Saturn's rings is essentially reflected sunlight, although the percentage 

 transmitted by the water-cell is somewhat larger. 



RESEARCHES ON NEBUL/E. 

 DIRECT PHOTOGRAPHY. 



The direct photography of nebulae has been continued by Hubble and 

 Humason. Especial attention has been given to dark nebulae and luminous 

 nebulae showing dark markings, so many of which were discovered by Barnard 

 with the Bruce telescope. The large scale of the negatives with the 60-inch 

 and 100-inch reflectors has served to bring out a remarkable amount of 

 detail in many of these objects. Such photographs are essential to investiga- 

 tions like those of Hubble on the source of luminosity in nebulae and of 

 Russell on dust clouds in space and the nature of dark and bright nebulae. 



A study of the extraordinary variety of forms observed among the nebulae 

 has been made by Hubble with the object of dividing them into groups upon 

 which may be based a definite system of nebular classification. For this 

 purpose he has secured over 100 photographs, most of them of non-galactic 

 objects taken from the list of Holetschek, which had not been photographed 

 previously on an adequate scale. In addition, several of the larger and 

 brighter non-galactic nebulae have been photographed with the 100-inch 

 telescope. The results of this study have been placed before the members of 

 the Committee on Nebulae of the International Astronomical Union to serve 

 as a basis for discussion in the formulation of a system of classification. 



Hubble has found that the nebula N. G. C. 6822, in which a general re- 

 semblance to the Magellanic Clouds had been noted previously, is similar to 

 them in all respects and constitutes a new object of this character. It 

 consists of an irregular cloud of very faint stars, about 16 by 8 minutes in size, 

 in which are involved four patches of diffuse nebulosity, all showing emission 

 spectra, at least 3 planetary nebulae, and 12 variable stars. Attempts 

 will be made to obtain the radial velocities of some of the nebulae, the magni- 

 tudes and color indices of the brighter stars, and the light curves of the 

 variables. 



Most of the photographs of spiral nebulae required for comparison with 

 earlier photographs of the same objects have been obtained by Humason. 

 Several of these have been used by van Maanen in his measurements of 

 internal motions in spiral nebulae. For such purposes it is essential that the 

 images be very small and round, a combination frequently difficult to secure 



