XVI PRELIMINARY GENERAL CATALOGUE OF STARS FOR 1900 
more rapidly with the time than for the standard stars. For instance, there is a 
large number of stars whose right ascensions or declinations have the greatest 
weight somewhere between 1880 and 1890, or later, with probable error of cen¬ 
tennial motion, o'.'6, or more. The manner in which the probable error increases for 
this class of stars can best be illustrated by an example. For this purpose take 
No. 26 of the Catalogue, for which the probable error at the epoch 1888.3 * s ± ” r 3 
and of the centennial proper-motion is ± T06. The following table gives computed 
probable-errors and weights (unit of weight corresponds to p.e., ± "3) for the 
respective epochs. 
Star No. 26 in Declination. 
Year. 
p.e. 
Weight. 
1888.3 
± '.'i 3 
5-3 
1900 
± . 18 
2.8 
1910 
± . 26 
i -3 
1915 
± . 31 
0.9 
1920 
± • 3 6 
0.7 
Thus a weight of determination that was 5.3 in 1888 has fallen to one-fourth 
of this amount in 1910. That is below the weight of the mean of two observations 
in the more precise of modern catalogues of observation. This rapid diminution 
of weight seems worthy of mention here, because its existence often appears not to 
be specifically and properly appreciated even in works aiming at precision. 
RECAPITULATION. 
Before entering into detail in reference to the proper understanding and use of 
the present Catalogue we may venture to recapitulate the statement of its principal 
characteristics as follows: 
1. The primary object of the Catalogue is to give the proper-motions of the stars as they 
result from a precise discussion of all readily available observations in each case. 
2. It aims, also, to furnish a Standard Catalogue that shall be practically exhaustive of the 
material of observation both in extent and in thoroughness of discussion, and 
which shall include all stars likely to be needed at present for standards in zone- 
observations and the like. 
3. It gives the right-ascensions of the stars substantially free from the error arising from 
magnitude-equation. 
4. It furnishes with each star the means for a complete estimate of the probable accuracy 
attained in respect to the catalogue position and motion of that star. 
5. As has been intimated, and as will later be shown more specifically, the catalogue-data 
offer convenient means for incorporating with due weight, for purposes of revision, 
observations additional to those on which the catalogue results are based. 
