ASTRONOMY: F. G. PEASE 
517 
THE ROTATION AND RADIAL VELOCITY OF THE SPIRAL 
NEBULA N. G. C. 4594 
By Francis G. Pease 
MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE IMSTITUTION OF WASHINGTON 
Received by the Academy, July 18. 1916 
The question of the rotation of nebulae has received a considerable 
amount of attention within recent years. The extreme faintness, how- 
ever, of these objects, particularly of the spiral nebulae, makes spectro- 
scopic investigation most difficult and laborious; and for only a very few 
of them are direct photographs available of such quality and separated 
by such intervals of time as to provide a reasonable hope of detecting 
changes of position by direct measurement. 
In the case of the planetary nebulae Hartmann^ in 1902 believed that 
he had found spectroscopic evidence of rotation in the large nebula in 
Draco and in Struve No. 6. His values, however, are not in accordance 
with the measures of Campbell and Moore, who recently have pubKshed 
definitive values for several of these nebulae.^ 
In his report for 1913 Wolf^ stated that the lines in the spectrum of 
Messier 81, N. G. C. 3031 Ursae Majoris appeared doubly bent near 
the nucleus, indicating a rotation of 100 km. per sec. near the nucleus, 
the east side approaching. In Lowell Observatory Bulletin, No. 62, 
Slipher announced the rotation of the nebula which is the subject of 
this communication, and a few months later gave the numerical data 
referred to below. Later Wolf^ mentions attempts to detect rotation 
in the Ring and Dumb-bell nebulae, but his apparatus was not well 
adapted to the work. 
Van Maanen,^ from measures of direct photographs, obtained evi- 
dence of rotation in Messier 101, together with a possible outward com- 
ponent for the individual knots, which seem to move along the arms 
of the spiral. 
The great lengths of exposure necessary for spectrum work on faint 
nebulae has suggested the following modification of the slit end of the 
focal plane spectrograph so that several points in a nebula may be ob- 
served simultaneously. In this arrangement the metal jaws of the slit 
are replaced by a silvered glass plate. This plate and a direct photo- 
graph of the nebula taken at the same focus as that at which the spec- 
trograph is to be used are mounted on a comparator, and for each bright 
spot in the nebula a slit (fig. 1) is cut in the silver film with a dividing 
attachment, the cut projecting beyond the nebulosity far enough to 
allow the addition of a comparison spectrum. Spots are chosen so 
