18 



SCIENCE 



[N. S. Vol. XLII. No. 1070 



were unsuccessful, even in the hands of 

 astronomers like Bessel and Struve. The 

 best were some observations of circumpolar 

 stars made by Struve in Dorpat between 

 1814 and 1821. The following table shows 

 some of the results at which he arrived: 



star to be near the san, and therefore to 

 have appreciable parallax. 61 Cygni is 

 not nearly so bright as a Lyrae, but has 

 a very great angular movement or proper- 

 motion among the stars. Bessel used an 

 instrument called a heliometer. Like 



Polaris and e Urs. Maj ir + O.OSStt' = + 0.075 ± 0.034: 



e Uis. Maj. and a Cass tt + 0.9627/ = — 0.136 ± 0.110 



f Urs. Maj. and 5 Cass ir + 1.0997/ = + 0.175 ± 0.127 



P Urs. Min. and a Persei ir + 0.402?/= + 0.305 ± 0.071 



Capella and P Drae 7r + 1.1477/ = + 0.134 ± 0.139 



/3 Aurig. and 7 Drac 7r -j- I.IBSt/ = + 0.020 ± 0.117 



This table has the merit of not looking Struve 's telescope, it was mounted so that 

 wildly impossible in the present state of it could be driven by clockwork to point 

 our knowledge. It has the disadvantage always at the same star. The object-glass 

 of not giving a definite parallax to each of Bessel's telescope was made by the great 

 star. For example, it is impossible to say optician Fraunhofer, with the intention of 

 how much of the 0.134" is to be given to cutting it in halves. Fraunhofer died be- 

 Capella and how much to j8 Draconis. fore the time came to carry out this deli- 

 Further, the probable errors, though really cate operation, but it was successfully ac- 

 small, are nearly as large as the quantities complished after his death, 

 determined. Delicate mechanism was provided for 

 Struve and Bessel therefore attempted turning the glass, and also for moving the 

 the problem by the differential method two halves, the amount of movement being 

 recommended by Herschel. By this time very accurately measured by screws. Bach 

 it had become easier to carry out. The half gives a perfect image of any object 

 method of mounting telescopes equator- "which is examined, but the two images are 

 ially had been devised, so that the telescope shifted by an amount equal to the distance 

 was always kept pointing to the same part one-half of the lens is moved along the 

 of the slvy by clock-work-driven mechanism, other. Thus when a bright star and faint 

 Struve chose the bright star a Lyrse, and star are looked at, one half of the object- 

 measured its distance from a faint star ^l^^^ ^^^ ^^ made to give images S and s, 

 about 40" away on ninety-six nights be- ^""^ ^he other half S and s. By moving 

 , ^T , iQor -I A <■ 1QQQ the screw exactly the right amount s can 

 tween November, 1835, and August, l»3o. ..,.,„ ., , 

 _,, „ ,, n T ■ J. ^ X. x. J be made to coincide with S, and the read- 

 In the focal plane oi his telescope he had . „ ,, . „ ,, 



, , . „ , .^. . . rni. mg of the screw gives a measure oi the 



what is called a position micrometer, ihe '' , _ , ° ^ ,, , 



,1 , ., angular distance between the two stars, 



micrometer contains two parallel spider- ^ ° , , . .,••,, 



^i ■, ^ j_ T -, J. J j-x, Bessel made observations on nmetv-eight 



threads stretched on frames, and the , -,. j. . , loor, ^ 



„ , , , ,-1 MX. nights extending trom August, 1837, to 



frames are movable by screws until the ^°^ , ^^oo mi ^ t.i j.\ i? 



. ,, ,. . , J September, 1838. The table, taken trom 



position shown m the diagram is reached: . ■, -^t • ■> ■, , ^ i ^i 



, ,. , , J, .1 _., 1-1 a report by Mam,^ shows how closely the 



the distance apart of the threads is known j, J , j_- n ■, ,^ 



„ ^, 1 J TT mean of the observations tor each month 



by the readings oi the screw-heads. Me . , ,, ■^. ,, , ,, 



„,,,,_ , , ,, nnrnri accords With the supposition that the star 



found that a Lyrse had a parallax OJbs ^Z,,, 



, ,, , riAoc" I'a'S the parallax 0.369 : 



with a probable error =b O.U2o . 



Bessel chose the star 61 Cygni as a likely ^ ^em. B. A. S., Vol. XII., p. 29. 



