234 CARNEGIE INSTITUTION OF WASHINGTON. 



Mr. Hunmson has taken the following photographs of star-fields 

 with the 10-inch telescope: 46 exposures of 60 minutes each for photo- 

 metric purposes on four areas in the Milky Way, each covering an area 

 of 340 square degrees, from 5= +24° to —40°; three photographs of 

 each of three special fields (a faint variable star, of which 18 small 

 plates were taken, was found on one of these; it appears to be an 

 eclipsing binary); five photographs of Prsesepe and 5 of M 22; 6 

 exposures in and near Orion and 3 of the double cluster in Perseus. 



VARIABLE NEBUL.E. 



Among these objects are the three known variable nebulae N. G. C. 

 1555, 2261, and 6729, all of which are under observation. A series of 

 plates of N. G. C. 2261 was made by Mr. Hubble on 10 nights with the 

 large reflectors, and N. G. C. 6729 has been photographed on 12 nights 

 to date. The variation is rapid and extensive in both nebulae. The 

 observations are not yet complete enough for thorough discussion, 

 but thus far they suggest a permanent framework of nebulous detail 

 on which occur both local brightening and local obscuration. The 

 obscured region on N. G. C. 2261 brightened up during the season in a 

 progressive manner, obhquely across the nebula and toward the 

 nucleus. No evidence has been found of actual motion of luminous 

 detail. N. G. C. 6729 shows decided changes in an interval of 24 

 hours. N. G. C. 1555, of which only one plate has been taken this 

 year, shows considerable exterior nebulosity as far as 3' from T Tauri, 

 which has not previously been photographed. 



DIFFUSE NEBULA AND NEBULOUS STARS. 



Some 40 nebulous stars and diffuse nebulae have been photographed 

 by Mr. Hubble with the large reflectors for study of structural detail 

 and for comparison with future plates for changes. Two features are 

 emphasized in these objects — the frequency in the smaller dense 

 nebulae of double stellar nuclei and the streaming of nebulosity 

 around and past involved stars, indicated by dark lanes leading away 

 from the stars. Direct photographs have been made by Mr. Hubble 

 with the 10-inch telescope for the following purposes: (1) identification 

 and classification of little-known nebulae (a weeding-out process of the 

 reflector program) ; (2) the distribution of nebulae on the general back- 

 ground of the sky; (3) structure of very large nebulae and nebulous 

 regions; (4) structure of dark markings. 



Experiments with prolonged exposures suggest that under the aver- 

 age conditions on Mount Wilson the maximum efficient exposure-time 

 for a moonless sky, with a Seed 30 plate and a focal ratio of 4.5, is of 

 the order of 12 hours. Longer exposures seem to lose more by sky-fog 

 than they gain by building up the images of nebulae and stars. 



Small-scale photographs of the Milky Way regions were obtained by 

 clamping to the tube of the 10-inch telescope a 4 by 5 box-camera 



