MOUNT WILSON SOLAR OBSERVATORY. 255 



1. The radial velocity of the nucleus is +1,180 km. per second, a result 

 in good agreement with Slipher's value of +1,100 km. 



2. Within the limits of error of measurement, the linear rotational 

 velocity varies uniformly with the distance from the nucleus. A 

 least-squares solution gives an equation of the form ?/= —2.78 a:+ 1,180, 

 in which y is expressed in kilometers and x in seconds of arc. Thus at a 

 distance of 120" from the nucleus the rotational velocity amounts to 

 nearly 335 km. 



3. This velocity may be interpreted as representing the motion of a 

 body rotating as a soUd, or as movement along the arms of the spiral 

 convolutions. It is opposed to the hypothesis of orbital motion of the 

 matter producing the spectrum about a central nucleus. 



4. The prominent dark streak parallel to the major axis of the spindle 

 may be composed of cool material at the periphery of the nebula which 

 absorbs or scatters the Ught. It is perhaps associated with the rapid 

 movement of rotation or translation along the arms of the spiral. 



5. The assumption that the observed radial motions for N. G. C. 4594 

 are of the same order as the measured angular displacements of 

 M 101 leads to a parallax of about 0''0002 for the former, which is 

 similar to the average value obtained by Curtis from a discussion of 

 proper motions. 



Two photographs have been made of different portions of the spiral 

 nebula M 33 at the Cassegrain focus of the 60-inch reflector. The first 

 of these was on the bright knot 10' distant from the nucleus; the second 

 was on the nucleus itself. The condensation gives a spectrum consisting 

 of bright lines, and the measurement of the three lines H7, H/S, and 

 X5007 yields a radial velocity of —278 km. The spectrum of the 

 nucleus, on the other hand, is continuous with absorption lines of type 

 A, and this type extends outward to a distance of at least 1' on 

 either side of the nucleus. The linear scale of the spectrum in this 

 photograph is extremely small and measurements are difficult. The 

 value of the radial velocity as obtained by two observers is of the order 

 of -70 km. 



PROFESSOR KAPTEYN'S INVESTIGATIONS. 



Professor Kapteyn, who on account of European conditions has been 

 unable to come to the Observatory during the year, has devoted his 

 time mainly to two questions: 



1. The first is a continuation of the investigation of the relation 

 between the proper motions and radial velocities of stars of the spec- 

 tral classes F, G, K, and M (see Report for 1915). Professor Kapteyn 

 has found the exceedingly interesting result that the motion of the 

 First Star-Stream is accelerated. As this would be the first directly 

 observed evidence of the action of forces in the general stellar system, 

 it has been deemed necessary to collect, not only for F, G, K, and M 

 stars, but also for types B and A, all available data for both proper 



