1907.] PERTAINING TO WORK WITH A PORTRAIT LENS. " 



graphic qualities. A long exposure, however, is required to show 

 the fainter outlying masses of nebulosity which are clearly shown in 

 the present picture. This photograph was made with the lo-inch 

 Bruce portrait lens of the Yerkes Observatory in the splendid atmos- 

 phere of Mount Wilson, California, where the writer had taken it 

 in 1905, through the courtesy of Professor Hale, for the photo- 

 graphing of the Milky Way. 



This picture exemplifies in a striking manner a peculiarity which 

 is often found in connection with these large nebulosities and to 

 which I have frequently called attention. That is, the apparently 

 free mixture of stars and nebulosity without any evidence of con- 

 densation of the nebulosity about the stars. I don't think this is 

 necessarily a case of accidental projection of the stars and nebu- 

 losity, for there are numerous similar cases in the sky. In the 

 present case one can trace out a similarity of configuration of the 

 outlines of the nebula and the massing of the stars, which would 

 strengthen the idea that they are at the same distance from us. 

 This fine object is in the Milky Way a short distance east of Alpha 

 Cygni, which star is shown at the western edge of the plate. 



A good example of the fainter and more difficult nebulosities is 

 shown in Plate IV, the nebulous region of Gamma Cygni (exposure 

 6 hours 30 minutes). These nebulosities are not visible with the 

 telescope because of their exceeding faintness. Their full extent 

 is not shown in the photograph, for they extend considerably beyond 

 the limits of the plate. It will be seen that Gamma Cygni, the star 

 in the middle of the picture, is in a region of diffused nebulous 

 matter which extends over a large area and is gathered in masses 

 of greater brightness at different points, but is in general formless 

 and diffused. 



The lower picture of Plate IV is a still finer example of the 

 photographic nebulosities — i. e., nebulosities that are too faint to be 

 seen with the telescope and for the knowledge of which we are 

 dependent on the photographic plate. This is the magnificent region 

 of the great nebula of Rho Ophiuchi (exposure 4 hours 30 min- 

 utes). Unfortunately the reproduction is a failure, for much of the 

 nebulosity and the great vacancies connected with it, that are so 

 wonderfully shown in the original, are all but lost in this half-tone. 



