106 



EECENT MEASURES OF " a. CENTAUEI." 



By a. B. Biggs. 



As au illustration of tlie efficiency of the Micrometer 

 described in my former paper, I give the following series of 

 measures, in their order, extending from 26th May to 21st 

 November, 1888 :— 



Distance Readings 15"-60 16"-01 17"-13 17"-40 16"-93 17"-16 

 Position Angle 201°-6 203°-6 204° 204°-2 205°-4 203°7 



Summary Table. Mean Date 1888-71 ] Total No. of 

 „ Distance 16"" 71 > Observations 



„ Posn. Angle 203°-7 ) 25. 



I also give for comparison, measures taken with the Filar 

 Micrometer, from 19th March to 26th May, 1888 ; and from 

 19th September to 21st November, 1888 :— 



Mean of both 

 Columns. 



Mean Date 



„ Distance ... 



„ Position Angle 

 Total No. of Observa" 



1888-35 1888-82 1888-58 



16"-45 17"-10 16"-77 



203°-34 204-6 203°-97 



14 29 43 



I reckon the variation at the present time at + l"-00 per 

 annum for distance, and + 0°-7 for position angle. To 

 the foregoing means of measures up to epoch 1889-00, we 

 shall have to multiply these rates of variation by (1889 - 

 1888-71=) 0-29:— and (1889-1888-58 = ) 0-42 respectively. 

 Applying the corrections thus obtained, we may make the 

 following comparisons. In the third column I give the 

 corresponding figures from my Ephemeris— (Society's Vol.,. 

 1887, page 82) :— 



EPOCH, 1889-00. 



Micrometer ABB Filar Ephemeris 



Distance l7"-00 17"-19 17"-00 



Position Angle 203°-90 204°-26 202°-90 



I think it probable that the Ephemeris is in error about. 

 1 degree in Position Angle. 



The measures of distance by the Filar were all taken as 

 differences of Declination, and were reduced to direct distance 

 by the secant of the Position Angle. 



The specially favourable conditions which this star affords 

 for double star observations, as well as the particular interest 

 which attaches to it on many accounts, especially to us in the 

 South, will, I trust, be sufficient excuse for my having dealt 

 with so much detail. 



