Vol. 7, 1921 ASTRONOMY: A. VAN MAANEN 3 
as they seem to be involved in the nebular knots and, with the exception 
of the centre, are decidedly blue. Fourteen points are probably nebular, 
being seemingly involved in nebulosity but not decidedly blue, or blue 
but not so clearly involved in the nebula. For 20 points it is quite un- 
certain whether or not they form part of the nebula. 
N t 
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SCALE 
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Internal motions in the spiral nebula Messier 33. The dotted- and full-line arrows show the 
motions of points in the nebula derived from each of two pairs of photographs. The 
comparison stars are enclosed in small squares and circles. 
For each set of plates the proper motions \x a and /jl s were derived for 
all the points measured with respect to the 24 and 23 comparison stars 
used. These motions for the points belonging to the nebula are pre- 
sumably due partly to a translation of the nebula as a whole, and partly 
to internal motion. By a method analogous to that followed for Messier 
101 the motion of translation of the nebula was found to be 
M a = -0".007, M5 - -0".002. 
