( 317 ) 



(6) In 1833 — 39 Bessel, at the Heliometer, measured not only 

 distances of all the satellites from both limbs of the planet, but also 

 angles of position of the centre of the planet to III and IV. 



His heliometer was the first big instrument of the sort made in 

 the establishment of Fralxhofer; the objective had an aperture of 

 70-2 Par. lines and a focal distance of 11314 Par. lines = 7 feet 

 10 inches 3-4 lines, Paris measure, (15-84 and 255-22 c.M.). The 

 mean error of a single observation of distance (which properly was 

 the mean of eight pointings) appeared to be 



for I + 0"26, for the mean distance resulting from all the measures, + 0"055 

 , II ±0-24, „ „ „ „ „ r, , . r, ±0-067 



, Hi ±0-31, „ „ , r, , » , , „ ±0042 



, IV ±0-43, „ „ „ „ , n n . n ±0-045 



Mean. ±0-31, „ , , „ , » , , , ±0-052 



ScHUR, at Göttingen, used the heliometers which were made by 

 Merz at Miinchen for the observation of the transits of Venus in 

 1874 and 1882. The aperture of the objectives of these instruments 

 w^as 34 Par. lines, something less than half that of the heliometer 

 of Koningsberg; the focal distance was 3| feet (113-7 cM.). 



At these heliometers the reading, instead of being made on the 

 drums of two micrometers, was made by a microscope at right 

 angles to two scales fitted to the two halves of the objective. As 

 however in this way more time was required than for reading the druns 

 of a micrometer of Bessel's instrument, Schur, instead of taking the 

 mean of eight pointings, was content with the mean of four pointings, 

 which also make a complete measuremeiit. 



The mean errors of each observation obtained by Schur for a 

 complete set of four measures was : 



for I ± 0"-34, 



„ II ± -44, 



„ III ± -37, 



„ IV ± -42, 



Mean: ± 0"-39, 



a result, which, taking into account the shorter focal distance, may 

 be considered fairly good. Bessel as well as Schur aimed not so 

 much at the determination of the positioJi of the orbits of the 

 satellites as at that of the mass of Jupiter. 



