334 THE WORLD MACHINE 



the well-known Fraunhofer lines in the spectral image ; it was 

 his careful maps which first gave them prominence. But his 

 greatest deeds were his instruments. Two of his most beautiful 

 constructions were the famous Dorpat refractor and the Konigs- 

 berg heliometer, set up in 1829. The latter was essentially 

 an equatorial telescope, designed on the Dollond plan of a 

 divided object-glass, and provided with micrometer screws, 

 permitting of angular measurements of an unheard-of accuracy. 



At this time the head of the Konigsberg Observatory was 

 the celebrated Bessel, with a brilliant record as an observing 

 astronomer, author of a vast work of tabulation, the Fundamenta 

 Astronomies, and keen to prove the powers of his new instrument. 

 As far back as 1812 he had made an especial study of a double 

 star in the constellation of the Swan, known in the catalogues 

 as 61 Cygni. It was not a brilliant star it is invisible to the 

 human eye but it had been found to have a relatively rapid 

 motion across the line of sight. It had been studied by other 

 astronomers after Bessel. Arago had spent a great deal of time 

 upon it, with the merely negative result of determining that 

 its parallax must be less than half a second. This meant that 

 it must be more than four hundred thousand times the distance 

 of the sun ; how much farther no one could say. At the Cape 

 of Good Hope, Henderson, with a fine instrument, was taking 

 observations upon a great double star of the southern hemi- 

 sphere, the alpha star of the Centaur ; Struve was busy at 

 Dorpat. It seemed certain that some sort of a result would 

 be reached soon ; as yet they were little further advanced 

 than Hooke in the seventeenth century or Herschel or Bradley 

 in the eighteenth. 



When Bessel turned his great new heliometer on the swift- 

 flying star of the Swan again, he must have felt that the goal 

 was very near. He says himself that he did not know whether 

 the measurements of its angle would turn out to be tenths 

 or thousandths of a second of arc ! Consider for a moment 

 what this means. The dial of your watch is divided into sixty 

 spaces, each representing a minute. Cut each of these into 

 six and you have the three hundred and sixty degrees which 

 make up the astronomical circle. Division of each degree by 

 sixty gives astronomical minutes ; by a sixty again gives astro- 

 nomical seconds. A tenth of such a " second," then, is not a two- 

 hundred-thousandth part of the space crossed by the watch 



