250 CARNEGIE INSTITUTION OF WASHINGTON. 



snowfall 48 inches. Mean temperature 54°; maximum 94°, July 7. 

 Minimum 14°, February 15. Average wind velocity, 10.4 miles per 

 hour; maximum velocity, 60 miles per hour, February 22. 



RESEARCHES ON NEBULA. 



Mr. Bubble's chief subject of study during the year has been the 

 galactic nebulae and the stars involved therein. Since the Milky 

 Way is not always in position for observation, a particular class of 

 non-galactic nebulse has also been chosen for investigation. Four- 

 hundred and ninety-two plates were taken during the year, represent- 

 ing 500 hours of actual exposure on 122 separate nights. The longest 

 exposure was 19 hours on the p Ophiuchi region, made on four nights 

 with the 15° objective prism on the 10-inch Cooke lens. 



The non-galactic objects, which have been designated as "globular 

 nebulae," are more numerous than those of any other class. The 

 brightest and largest of them, such as M 49, 60, and 87, show no trace 

 of spiral structure, although their spectra and radial velocities are of 

 the same character as those of the spirals. M 87, at least, has a large 

 number of faint stars, none brighter than about the 19th magnitude, 

 clustering around its borders. The appearance is not that of a typical 

 star-cluster, although it approximates this more closely than it does 

 the appearance of a spiral. 



The smaller members of the class become more numerous with de- 

 creasing size and brightness until they fade into the general mass of 

 faint blotches on the photographs, which can hardly be distinguished 

 from star images. Many of these will undoubtedly prove to be spirals 

 when greater magnification can be employed, and some will become 

 spindles when longer exposures bring out the ansae. An uncertain 

 percentage, however, should remain in the class of M 87, for the 

 sequence in size is complete to the limits of the telescope. 



To gather information on this subject, certain rich fields of small 

 non-galactic nebulae have been selected, and these are being photo- 

 graphed with the 10-inch, 60-inch, and 100-inch telescopes with com- 

 parable focal ratios and exposure times. Classification of nebulae 

 appearing on these plates should give some indication as to the rela- 

 tion between class and scale. Photographs are also being made with 

 a single telescope and different exposure times, in an effort to correlate 

 class with density of the image. On nights of the finest definition, 

 plates of long and short exposures are made with the 100-inch reflector 

 to study the minute structure of the largest and brightest globular 

 nebulae. Those photographed to date are: 



M 49, 00, 84, 86, and 87. 



N. G. C. 524, 4278, 4742, and 5826. 



The investigation is proving of considerable interest, but conclusions 

 are not yet definitive. 



