MOUNT WILSON SOLAR OBSERVATORY. 



245 



MEASUREMENT OF PARALLAXES AND PROPER MOTIONS. 



During the past year 387 direct photographs, including 518 expo- 

 sures, have been taken at the 80-foot focus of the 60-inch reflector. 

 Of these, 404 exposures were for the determination of parallaxes, 93 

 for the determination of proper motions of large proper-motion stars, 

 and 21 for miscellaneous purposes. 



For 17 parallax fields, each with 12 to 20 exposures, the measures and 

 reductions have been finished, making a total of 34 finished fields. All 

 stars with a very few exceptions were selected from Boss's "PreUmin- 

 ary Catalogue" and are of spectral types F to N, with magnitudes less 

 than 5.5, and proper motions less than 0''5. The mean probable error 

 of a final parallax is ±0^006. From the first 20 fields evidence was 

 derived that systematic errors due to a magnitude error and to the 

 neglect of quadratic terms of the coordinates of the comparison stars 

 are negligible. 



The work on the large proper-motion stars has been continued during 

 the middle of the nights. For 126 stars in the northern hemisphere 

 first-epoch plates have now been secured, while second-epoch plates 

 (one year's interval) have been taken of the following four stars of 

 exceptionally large proper motions: 



The photovisual magnitude of the companion of Pi 2'' 123, according 

 to Mr. Shapley, is 11.50. Assuming the parallax of the companion to 

 be the same as that of Pi 2^ 123, its absolute photovisual magnitude is 

 about 12.5. 



During the past year five plates of the spiral nebula Messier 101 were 

 measured and discussed for evidence of internal motion; two of these 

 were taken by Mr. Ritchey at the 25-foot focus of the 60-inch reflector, 

 while the other three were loaned by the Lick Observatory. The 

 plates, combined in three pairs with intervals of 5, 9, and 14 years, 

 were measured with the stereocomparator, which presents special 

 advantages for the investigation of small and not easily determined 

 displacements. The measures are differential and the possibility 

 of setting in quick succession on corresponding points of different 

 photographs tends to decrease the errors. On the Mount Wilson plates 

 87 nebulous points were measured, while 46 and 69 points were 

 measured on the two pairs of Lick plates. The same 32 stars for 

 comparison purposes were used throughout. After correcting the 



