March 9, 1900.] 



SCIENCE. 



373 



The instruments used at the principal 

 stations are specially designed zenith tel- 

 escopes, by Wanschafif, of Berlin. The tele- 

 scope has a focal length of about 61 inches, 

 an aperture of 4J inches, and a magnifying 

 power of 104. Aside from other minor 

 peculiarities, two are especially noticeable. 

 The barrel of the telescope proper is pro- 

 tected by an outer thin metallic tube, which 

 is connected at but few points with the tele- 

 scope proper, and serves merely to protect 

 it against sudden changes of temperature. 

 This false tube is pierced at various points 

 to permit circulation of air in the space 

 between it and the tube proper. The eye- 

 piece is furnished with a reversing prism of 

 peculiar construction, such that all observa- 

 tions may be made vf^ith the observing line 

 apparently vertical and with the star ap- 

 parently moving either upward or down- 

 ward at the will of the observer. The ob- 

 servation upon one star consists of four 

 pointings, two taken while the star is mov- 

 ing apparently upward in the field of view 

 and two with the star moving apparently 

 downward, the reversing prism being turned 

 1 80 degrees between these observations. If, 

 then, the observer has a personal tendency 

 to place the observing line too far to the 

 right, this will have contrary effects in the 

 two pairs of bisections, and the personal 

 equation will be eliminated from the mean 

 result. In so far as accidental errors are 

 concerned, the few observations already 

 made indicate a high degree of accuracy, 

 the probable error of a single observation of 

 a pair being about ± 0".10. Few series of 

 observations yet made can show probable 

 errors as small as ± 0".16. 



The computed motion of the pole will be 

 nearly independent of the errors in the 

 assigned declinations of the stars, the effects 

 of the errors of declinations being eliminated 

 by the well-known group method. The 

 stars to be observed are divided into twelve 

 groups, and each group is observed for 



about two months (50 to 80 days, according 

 to the time of year). During the first half 

 of the period when group No. 2 is being 

 observed group No. 1 is also being observed, 

 and during the latter half of the period 

 group No. 3 is being observed at the same 

 time as group No. 2. Similarly, group No. 

 3 is observed first in connection with No. 2, 

 and then in connection with group No. 4, 

 and so on. The difference between the 

 mean latitude from group No. 1 and the 

 mean from group No. 2 during the period 

 when both groups are being observed is 

 obviously due to the difference of the mean 

 error in declination of the stars of the two 

 groups and to accidental errors of observa- 

 tion. This method of observation, after 

 being extended throughout the whole cycle 

 until group No. 12 overlaps group No. 1, 

 furnishes a means of determining the decli- 

 nation correction to each group to reduce it 

 to the mean of all the groups, and thus to 

 eliminate the declination errors from the 

 computed change in latitude. 



As an additional precaution, the same 

 list of stars is to be observed at all the sta- 

 tions. 



The effect of an error in the assigned 

 proper motions of the stars observed will be 

 to make the latitude of any one station ap- 

 pear to increase or decrease with lapse of 

 time, but will have no appreciable effect on 

 the value finally derived for the miction of 

 the pole ; for, the same list being used at 

 all stations, all the latitudes will appear to 

 increase or decrease together. Both the 

 Japan and the Maryland station, would 

 appear, say, to have increasing latitudes, 

 although they are nearly on opposite sides 

 of the pole, and therefore this result could 

 not be mistaken for an actual motion of the 

 pole. 



Aside from the precautions already indi- 

 cated against abnormal refractions due to 

 local conditions, the observations themselves 

 have been planned so as to guard against 



