1192 
Astronomy. — “The variability of the Pole-star.’ By Dr. A. PANNr- 
KOEK. (Communicated by Prof. B. F. van DE SANDE BAKHUYZEN). 
(Communicated in the meeting of January 25, 1913). 
A slight variability of « Ursae minoris has already several times 
been suspected by different observers (SrmpEL, Scumipt). When in 1889 
and 1890 I executed a great number of observations (estimates with 
the naked eye after ArGELANDER’s method) for the determination of 
the brightness of the stars of the 2°¢ and 34 maenitudes, such 
great differences showed in some of these stars, that they were being 
observed as regularly and as often as possible in the following years 
with a view to probable variability. Among these stars was also the 
Pole-star *). In 1890 I found that the period was about 4 days; 
each time 2 days after a great intensity came a faint one and the 
reverse. I did not succeed, however, in finding an accurate value 
for the period. From the observations in December 1890 I found 
two maxima on Dee. 7.0 and Dee. 29.8 (in reality they occurred 
on Dec. 6.6 and Dee. 30.4), which yielded a probable period of 3.8 
days; this however did not agree with the observations of that winter. 
After all it must indeed have been hopeless to derive the elements 
of the variation from these observations only. As the mean error of 
an estimate amounted to 0.7 of the whole amplitude, as appeared 
later on, it might even happen that a maximum and a minimum 
seemed to have changed places owing to errors of observations. 
Moreover the remembrance of the results of previous days may spoil 
an observation. If on one particular day the star has, perhaps wrongly, 
been estimated very faint, one expects to see it very bright two 
days afterwards, and this may influence the estimate. On the other 
hand the small number of observations in a given interval of time, 
say a month, owing to bad weather, did not allow to counteract 
the uncertainty of the separate estimates, by uniting a great number 
into a normal place. I have long continued the observations of this 
star, up to 1899, in order to have material for a closer investigation, 
in case the variability should be proved and the period should be 
accurately known. 
In 1898 CamrBerr, discovered that the radial velocity of this star 
is variable and hence that it is a spectrocopic double star with a 
1) The other stars in which [ consider variability to be probable, although I 
cannot prove it with certainty owing to the smallness of the amplitude, are ¢ Tauri 
(period of a few days), 40 Lyncis (26 days) and x Herculis (14 months); the 
latter two are of a red colour, 
