July 2, 1915] 



SCIENCE 



19 



1837 



Effect of 

 Observed Parallax 



Mean Date Disappointment 0"369 



August 23 + o''l97 + o"212 



September 14 + 0.100 + 0.100 



October 12 + 0.040 — 0.057 



November 22 — 0.214 — 0.258 



December 21 — 0.322 — 0.317 



1838 



January 14 — o"376 — 0^318 



February 5 — 0.223 — 0.266 



May 14 + 0.245 + 0.238 



June 19 + 0.360 + 0.332 



July 13 + 0.216 + 0.332 



August 19 ■+ 0.151 + 0.227 



September 19 + 0.040 + 0.073 



Simultaneously with these determina- 

 tions of the distance of a LyrjB and 61 

 Cygni, the distance of a Centauri, one of 

 the brightest of the southern stars, was 

 found by Henderson from observations of 

 zenith distance made by him at the Cape 

 between April, 1832, and May, 1833. He 

 learned just before the termination of his 

 residence at the Cape that this star had a 

 very large proper-motion. Suspecting a 

 possible parallax, he examined the observa- 

 tions when he had taken up his new office 

 of Astronomer Royal for Scotland, and 

 found a parallax amounting to 0.92". He 

 did not, however, publish his results until 

 he found that they were confirmed by the 

 right ascensions. In a communication to 

 the Royal Astronomical Society in Decem- 

 ber, 1838, he states that it is probable that 

 the star has a parallax of 1.0". 



The great and difficult problem which 

 had occupied astronomers for many gen- 

 erations was thus solved for three separate 

 stars in 1838 (see table). 



Henderson's observation is interesting 

 because a Centauri is, as far as we yet 

 know, the nearest of all the stars to us. 

 But by far the most valuable of these ob- 

 servations is Bessel's. The heliometer, 

 which he devised, proved itself to be by far 



the most serviceable instrument for deter- 

 mining stellar parallax until the applica- 

 tion of photography for this purpose. 



(The unit of distance is that from the earth to 

 the sun.) 



The somewhat dramatic manner in which 

 the distances of three stars were determined 

 in the same year, after several centuries of 

 failures, may have led to the hope that the 

 range of many more stars would soon be 

 found. This was not the case, however. 

 Each star had to be measured separately, 

 and involved many nights of observations. 

 The quantities to be measured were so 

 small that they taxed the resources of the 

 best instruments and best observers. In 

 1843 Peters published the parallaxes of 

 half a dozen stars determined with the ver- 

 tical circle at Pulkova, but the parallax of 

 only one of these, Polaris, is obtained with 

 much accuracy. With Bessel's heliometer, 

 Schliiter and Wichmann measured the dis- 

 tance of Gr. 1830, the star which had the 

 largest known proper-motion. In the 'six- 

 ties, Auwers with the same instrument de- 

 termined the parallax of several quick-mov- 

 ing stars, and also of the bright star Pro- 

 cyon. With the Bonn heliometer, Krueger 

 in the 'sixties measured the distance of 

 three stars, and Winnecke two more. Other 

 observations were made, amongst others, by 

 Maclear, Otto Struve, Briinnow and Ball; 

 but as these observers had not such suitable 

 instruments, their results were not of the 

 same high standard of value. A generous 

 estimate would place the number of stars 

 the distance of which had been satisfactor- 



