252 CARNEGIE INSTITUTION OF WASHINGTON. 



work is being carried on in the hope of using such a filter for radial 

 velocities of the approximately stellar nuclei of certain non-galactic 

 nebulse. 



Variable Nebula. 



N. G. C. 1555. — Mr. Hubble has obtained three long, two moderate, 

 and three short exposures of Hind's Variable Nebula with the 100-inch 

 telescope. The long exposures show a wealth of detail, both of illu- 

 mination and absorption, within a circular area 3 minutes of arc in 

 diameter, about T Tauri as a center. Very small changes in relative 

 brightness have occurred, mostly in the brightest portion south fol- 

 lowing the star, which can be attributed to areas of absorption cutting 

 out luminous detail. The short exposures show at least three small 

 flares jutting out from T Tauri, south preceding, south following, and 

 to the north. The first is the brightest and registers in 15 seconds on 

 Seed 30 plates. No change has been detected in these details, although 

 it must be understood that definite conclusions can be drawn only from 

 plates made with unusually good seeing. There was no great change in 

 brightness of T Tauri itself during the period covered by Mr. Hubble's 

 plates. The exterior nebulosity showed marked changes as compared 

 with a plate made by Mr. Pease with the 60-inch in 1913. 



iV. G. C. 2261. — Ten photographs were made during the year, but 

 observing conditions were not always good. Obvious changes oc- 

 curred, but not on so large a scale as those of the previous year. All 

 observed changes can be accounted for by obscuring clouds wandering 

 over a permanent background of luminous nebular detail, together 

 with an occasional brightening and fading of the nebulosity close 

 about the star at the apex — R Monocerotis. R itself did not vary 

 appreciably during the year. 



N. G. C. 6729. — Forty spectrograms on 25 nights were made with 

 the 100-inch reflector during the year. Two runs of several succes- 

 sive nights each were secured, one when the star R Coronse Australis 

 was brightening rapidly, and the other when the variable was practi- 

 cally stationary at the top of its curve. Only minor changes were 

 noted during the latter run. Rapid changes, however, occurred while 

 the star was increasing in brightness; and these, as in the case of 

 N. G. C. 2261, can be interpreted as the temporary obscuration of a 

 permanent background of luminous detail, together with an intense 

 brightening of nebulosity in the immediate neighborhood of the star. 

 Obvious changes within 24 hours were observed. 



Between the nights of August 14 and August 16, 1920, a bright 

 envelope developed around R Coronae Australis that looked reddish 

 to the eye. Its diameter increased as follows: Aug. 13, 4"; Aug. 14, 

 6"; Aug. 15, 8"; Aug. 16, 12"; Aug. 17, 12"; Aug. 18, 11"; Aug. 19, 10". 

 The measurements are rough and subject to uncertainties due to vary- 

 ing atmospheric conditions at the great zenith distance at which the 



