MOUNT WILSON OBSERVATORY. 



199 



PHOTOGRAPHS OF NEBULA AND CLUSTERS. 



A considerable number of photographs of spiral and irregular 

 nebulae were made by Mr. Ritchey at the primary focus of the 60-inch 

 reflector. Among these were N.G.C. 1023, 7479, and 7782, Messier 

 81, H.V. 44, and H.I. 199. Eight photographs of the Andromeda 

 nebula and five of Nova Persei were also obtained. The exposure- 

 times on these objects varied from 30 to 330 minutes. 



The study of the photographs of the Andromeda Nebula has resulted 

 in the discovery of six additional novae, one of which was found by 

 Dr. Duncan in August of this year. The number of novae in this 

 nebula found from the Mount Wilson photographs now amounts to 

 eight. 



The announcement by Professor Barnard of a nebula near Nova 

 Persei of 1901 led to the photography of this star with plates of dif- 

 ferent sensitiveness and with various exposure-times. The negatives 

 showed the presence of a sharply defined continuous ring of nebulosity 

 about the star, 16" in diameter. No certain evidence of motion was 

 found on the successive photographs. 



Mr. Pease obtained 12 photographs, mainly of spiral and gaseous 

 nebulae, during the autumn of 1917, with exposure-times varying from 

 20 to 320 minutes. These were as follows: 



N. G. C. 



955 Spindle type. Variable? 



1186 Supposed variable. No change since 1914. 



1491 Faint, irregular. 



1555 Very faint, irregular, with change since Nov. 1916. 



1700 Nebulous patch 0'5X0'8 with strong center. 



2024 Gaseous, faint and irregular. 



2245-7 Irregular nebulae in dark lane. 



