2 
ASTRONOMY: A. VAN MAANEN 
Proc. N. A. S. 
like appearance. For this same object two plates of short exposure, taken 
at the 80-foot focus of the 60-inch reflector in 1915 and 1920, were also 
available. 
Both these pairs of plates have been measured with the monocular 
arrangement of the Zeiss stereocomparator. It has been shown that 
with temperature properly controlled this instrument is admirably adapted 
to such measures, because it allows the bisection in quick succession of 
corresponding points on the two photographs; the possibility of not bi- 
secting a nebulous and asymmetrical point in the same way on both plates 
is thereby greatly reduced, if not wholly avoided. A note on the unre- 
liability of the stereocomparator for accurate measures has recently been 
published by Kreiken; 4 but, as I have pointed out several times it is only 
necessary to keep the temperature of the room constant in order to secure 
good results with the Zeiss instrument. In our case the change in tempera- 
ture during a series of measures can be kept within a few tenths of a de- 
gree. The influence of this variation is extremely small; but, as an extra 
precaution, we have adopted the plan of measuring the n points, first in 
the order 1 n, then in the order n 1. In this way practically 
all temperature effect can be eliminated, and it is easy to prove that the 
measures of the present series, which are of moderate length, cannot be 
affected by more than 10 or 12 per cent. For a series of several hundred 
points, whose measurement would take several days, the conditions are 
not so favorable, and in the present case it was thought best to await the 
completion of a new stereocomparator, now under construction in the 
instrument shop of the Observatory, in which the principal sources of 
trouble have been avoided, before undertaking the complete measurement 
of the plates. 
For a preliminary discussion, 24 comparison stars and 23 nebulous 
points were selected on the plates taken at the 25-foot focus. When 
the measures had been finished, Mr. Seares was kind enough to call my 
attention to some photographs of Messier 33 taken by him with and with- 
out color-screens. A few years ago Seares found that the outer points of 
spiral nebulae are decidedly blue in color. A rough comparison of his 
plates showed that I had been fortunate in the choice of both comparison 
stars and nebular points. Of the first only three are blue, while of the 
latter all but two are decidedly blue ; these two points have been omitted 
in the following discussion, since they may not belong to the nebula. 
On the plates taken by me at the 80-foot focus all measurable points, 
a total of 89, were used. After the measures were finished I selected, with 
the help of the photographs taken by Mr. Ritchey and by Mr. Seares, 
23 objects as comparison stars; these are all starlike in appearance and 
reddish in color. Ten points which are either very bright or very faint 
or for which the measures are uncertain were excluded. Of the remaining 
points, 22, including the centre, almost certainly belong to the nebula, 
