390 
ASTRONOMY: A. VAN MAANEN 
mean values for elliptical orbits, the central mass could be calculated. 
Even though this assumption may be far from the truth it seems worth 
while to accept certain more or less hypothetical values of the parallax 
in order to get any idea of the order of the masses with which we are 
concerned. 
Two estimates as to the parallax can be made, (1) by comparing the 
average motion of translation of 66 spiral nebulae, as given by Curtis,^ 
with those of the stars; (2) by comparing these cross-motions with the 
observed radial velocities of the few spirals for which such results are 
known. In the first case we derive a parallax of Of 005; in the second, 
of 0''0003. The corresponding central masses are in both cases very 
large, viz., 30,000 and 140,000,000 times that of the sun. Various 
objections to the acceptance of these results, even as rough guesses can 
be made, but they are the best we have at the moment. The corre- 
sponding orbital motions would be 21 and 345 km. /sec. These quanti- 
ties do not seem absurd if we remember that Wolf by spectroscopic 
methods found a rotational component of =±= 100 km. /sec. in Messier 
81.^ 
As the detailed results will soon be published I will only mention two 
more points which seem to confirm the reahty of the motions. Mr. 
Nicholson very kindly spent much time in making check-measures on 
the plates taken by Mr. Ritchey, both with the stereocomparator and 
with another measuring machine in which two microscopes were mounted 
in such a way that they were directed toward corresponding points on 
the two plates, mounted on the same plate-carrier and moved by the 
same micrometer screw. His measures give satisfactory confirmation 
of my own results. Further, I have measured two plates of Messier 
81, taken by Mr. Ritchey in 1910 and 1916, which show motion similar 
to that found above for Messier 101. It seems hardly necessary to sug- 
gest the importance of internal motions, such as are indicated here, in 
connection with the ChamberHn-Moulton hypothesis as to the origin 
of spiral nebulae, and it is to be hoped that material will soon be avail- 
able for a fuller discussion. 
1 Astronomical Journal, 27, 140 (1912). 
^ML Wilson Contr., No. Ill, 4 seq., 1916. 
* Publications Astronomical Society Pacific, 27, 216 (1915). 
* Vierteljahrsschrift Astronnmischen Gesellschaft, 49, 162 (1914). 
