MOUNT WILSON OBSERVATORY. 



235 



carrying a Tessar Ic lens of 6.5 inches focus, focal ratio 4.5, loaned by 

 Mr. Edison Hoge. This lens gives extremely good definition over a 

 field more than 30° in diameter. The plates show the large features of 

 the Milky Way on a field and scale convenient for studying the relation 

 of star-clouds, absorbing regions, and dark markings. Several large 

 dark areas, not previously known, have been found at upwards of 20° 

 from the galactic plane. The photographs suggest that these dark 

 regions lie on a plane inclined to the Galaxy some 12°, which seems to 

 coincide with the plane of the local cluster of B stars. This furnishes 

 further evidence of the close relationship of B stars and diffuse nebu- 

 losity. The plates further suggest that the abrupt ending of the 

 northern branch of the Milky Way in Ophiuchus may be due to a 

 general absorption in that region. 



SPECTRA OF NEBULiE AND NEBULOUS STARS. 



Two objective prisms, one of 6° angle, the other of 15°, have been 

 used by Mr. Hubble with the 10-inch telescope for the following 

 purposes : (1) special tyj)es of nebulous stars; (2) differentiation of nebu- 

 lar spectra (whether bright Une or continuous) ; (3) study of stellar 

 spectra in dark regions. 



Nebulae which have not previously been classified have spectra as 

 follows : 



The variable nebula N. G. C. 6729, for which SUpher announced a 

 spectrum similar to Nova Aurigse, was found to possess a strong, bright 

 Ha fine. 



Attempts were also made by ]\Ir. Hubble to determine the general 

 characteristics of objective-prism spectra of extra- galactic nebulae 

 (spirals, etc., in high galactic latitudes). An exposure of 5 hours with 

 the 6° prism gives traces of practically all N. G. C. objects; 215 such 

 nebular spectra have been obtained. Lines are rarely seen, but the 

 distribution of light suggests the predominance of solar tj^es, in agree- 

 ment with previous results. Five of the 215 spectra show traces of 

 bright lines; of these, 3 are previously known cases of bright-line 

 spirals. 



Work with the 10-inch telescope has been supplemented with another 

 type of instrument — large-angle prisms mounted in front of a fast 



