Vol. 7, 1921 
ASTRONOMY: J. C. DUNCAN 
179 
All settings of the mirrors have thus far been made by hand, and con- 
siderable time is consumed between measures. Two screws are now 
being mounted on the beam, driven by a single motor, which for any 
separation will keep the outer mirrors equidistant from the fixed inner 
mirrors, and thus greatly facilitate the operation of the interferometer, 
^hese Proceedings, 7, 1921 (143-146). 
CHANGES OBSERVED IN THE CRAB NEBULA IN TAURUS 
By John C. Duncan 
Mount Wilson Observatory, Carnegie Institution of Washington 
Communicated by George K. Hale. Read before the Academy, April 26, 1921 
Changes in the structure of the Crab nebula (N. G. C. 1952, M. 1) have 
recently been detected by Lampland 1 from a comparison of seventeen 
photographs made with the 40-inch Lowell reflector during a period of 
eight years. An excellent negative of this nebula had been made by 
Ritchey with the 60-inch Mount Wilson reflector, 1909, October 13, using 
a Seed 27 plate and giving an exposure of three hours with the aperture 
stopped down to 54 inches. For comparison with this plate, the writer 
made a negative with the same instrument, 1921, April 7, using full aper- 
ture and a Seed 30 plate and giving an exposure of 1 hour 20 minutes. 
Though the exposure of this plate was necessarily shorter than that of 
Ritchey's because of the proximity of the object to the sun, the larger 
aperture and the faster plate made the photograph comparable in density 
with the earlier one. 
The two plates were compared in the stereocomparator, and it was 
seen at once that changes in the relative luminosity of different parts of 
the nebula had occurred, particularly in the bright region north-west of 
the center; and also that certain filaments and condensations had appar- 
ently moved, the motion in general being away from the center, though 
not always exactly along a radius. 
Measures in two coordinates were made with the micrometer of the 
stereocomparator upon twelve nebulous condensations and thirteen com- 
parison stars. The nebulous points chosen are sufficiently symmetrical 
to permit measures of about equal accuracy in the two directions. They 
were chosen in the outer, faint parts of the nebula because of the diffi- 
culty of seeing isolated points in the bright portions. One comparison star is 
the north component of the double star that lies near the center of the 
nebula; each of the others lies near one of the measured nebulous points. 
The plate constants were derived from the measures of the comparison 
stars by a least-squares solution ; with these constants the motions of the 
nebulous points were then referred to the mean of the comparison stars. 
