218 STELLAR PHOTOGRAPHY. 



wire by the usual method may be taken as ±0*.06. The above measures show 

 that the probable error of a single setting on the images of different stars is not 

 far from one half of this, ± O'.OS. The probable error of successive settings on the 

 same image is only ±0'.015. The observations on different stars seem fairly to rep- 

 resent the accuracy to be expected in determining the position of stars by photog- 

 raphy from transit observations. Besides errors of setting, there are included in 

 the quantity above mentioned, O'.OS, those due to lack of symmetry of the images, 

 variation in the brightness of the stars, unequal expansion or irregularities in the 

 film, so far as these could affect measures over small distances, and the various errors 

 in the measuring instrument. Many of these quantities can doubtless l)e greatly 

 diminished, but the results already obtained seem to prove the possibility of meas- 

 uring the position of stars photographically from their transits, with an accuracy at 

 least equal to that obtained in the usual manner. 



Close Polar Stars. 



A large number of photographs were taken of the vicinity of the north pole. 

 Some of these were made when the telescope was at rest, the stars leaving trails ; 

 others, principally intended for testing the position of the polar axis, were made 

 with the telescope driven by the clock. The number of stars on each plate may be 

 inferred from the fact that over one hundred stars within one degree of the pole 

 leave trails when the telescope is at rest. As each photograph extends five degrees 

 from the pole, the complete reduction would be very laborious. The photographic 

 brightness of the stars within one degree of the pole has been measured on three 

 of the plates, Nos. 7, 21, and 231. The method of measurement was the same 

 as that employed for the Pleiades and described on page 211. The attention of the 

 observer, Miss Farrar, was called to the preponderance, in the estimates of the light 

 of the Pleiades, of the zero tenths and five tenths of a magnitude, and this difficulty 

 is corrected in the present measures. Plate 7 was taken on August 8, 1885, and 

 had an exposure of 70 minutes; Plate 21 was taken on September 3, 1885, with 

 an exposure of 165 minutes, and Plate 231 was taken on December 24, 1885, with 

 an exposure of 50 minutes. The accordance of the measures is very satisfactory. 

 The total number of stars is 117, and of the separate measures 330. The average 

 value of the residuals found by subtracting the mean value of the brightness of 

 each star from the separate values, expressed in magnitude, is 0.088. Accordingly, 

 single measures of photographs taken on different nights will differ on the average 



