416 
ASTRONOMY: E. B. FROST 
RADIAL VELOCITIES WITHIN THE GREAT NEBULA OF 
ORION 
By Edwin B. Frost 
YERKES OBSERVATORY. UNIVERSITY OF CHICAGO 
Presented to the Academy, June 9, 191 5 
The Astrophysical Journal for October, 1914, contained an important 
paper by MM. Bourget, Fabry, and Buisson of Marseille, summarizing 
the results of their application to the Orion nebula of the photographic 
interferometric methods of M. Fabry They found var ations n the 
radial velocity from one point to another within the nebula, differences 
as large as 10 km. per second at points quite close to each other being 
indicated by local deformations of the interference rings. They also 
detected graat collective movements, the northeastern region receding, 
and the southwestern region approaching, relatively to the mean veloc- 
ity at the trapezium, with velocities of about 5 km. per second. 
It seemed of interest to repeat these determinations independently 
by the standard spectrographic method, and toward the close of the 
winter observations were begun at Yerkes Observatory with the Bruce 
spectrograph, arranged with a dispersion of one prism. The radial 
velocity was inferred from the displacements of the hydrogen lines ^ 
and 7 and the Hnes of nebulium at X 5007 and 4959. At the position of 
the western star of the trapezium. No. 619 in Bond's catalogue of stars 
in the nebula, the radial velocity of the nebula was found from measures 
of eight plates (taken occasionally during the past eleven years) to be 
-hl5.6 =t: 0.5 km. per second (recession). This is the mean of independ- 
ent measures of each plate by the writer and Mr. C. A. Maney, and it 
agrees almost exactly with the value of +15.8 km. found by the observ- 
ers at Marseille for the region of the trapezium. 
For several other positions around the trapezium, and not over 2' 
from it, we obtained values ranging from +6 to +17 km. The probable 
error for the velocity at one point, from the mean of measures by the 
two observers of a single plate, should be about =*=1.5 km., so that the 
reality of the difference may be regarded as fully confirmed. Exposures 
of from two to three hours were required at some of the positions, so 
that the accumulation of plates was slow, and additional observations 
will be required when the 'open season' for Orion returns. 
We must accordingly alter our conceptions of the nebula as an enor- 
mous mass of quiescent gas, and regard it as seething with local whirl- 
pools besides perhaps having a considerable motion of rotation as a whole. 
