424 BARNARD— ASTRONOMICAL PHOTOGRAPHY [April 20, 



I think there is no other region in the entire sky so remarkable 

 as this of which Rho Ophiuchi appears to be the center. The great 

 nebula itself, which seems to cover almost this entire region with its 

 extensions, and its association with the extraordinary star vacancy 

 here are very puzzling, and lead one to believe that the apparent 

 paucity of small stars at this point is due in some way to the presence 

 of the nebula. The great dark lane or rift running to the east, 

 extends as far as the region of Theta Ophiuchi and seems to be a 

 part of the system of vacancies that occur to the east and south 

 of Theta. 



The great nebula is full of remarkable details. There are a 

 number of principal condensations, that of Rho Ophiuchi being 

 perhaps the most striking. The nebula extends to, and involves the 

 bright naked-eye star Sigma Scorpii in a strong condensation full 

 of details. In several wave-like masses it involves and reaches 

 beyond Antares, one of the brightest stars in the sky. It seems to 

 faintly cover a great part of the sky here, extending so far north, 

 perhaps, as to connect with the remarkable nebula about Nu Scorpii. 

 There are traces of it extending as far south as Tau Scorpii. 



Perhaps as remarkable as anything in connection with this nebula 

 is the fact that it is so faint that the eye, armed with the most 

 powerful telescope, cannot see it. Its light seems to be almost 

 entirely photographic, and though too faint to be seen in the tele- 

 scope it is doubtless very bright to the photographic plate. 



At the lower part of this plate, a half inch to the left of the 

 small cluster (M 4), is apparently an ordinary star. This is the 

 bright red star Antares which is the brightest in this region of the 

 sky, but which, from its red color, appears quite small and insig- 

 nificant on the photograph. A half inch above the cluster is the star 

 Sigma Scorpii which is much less than Antares. Sigma Scorpii is 

 the center of a bright condensation of the nebulosity which in the 

 original is seen to connect with the larger nebulosity (in the middle 

 of the plate) about the star Rho Ophiuchi. The dark lanes running 

 from the nebula east, though strong and conspicuous in the original, 

 are nearly lost in the reproduction. 



The first picture in Plate II is a photograph of the region of the 

 double cluster of Perseus (exposure 5 hours 55 minutes), which 



