GOMMENTATfO ASTRONOirrCA. 231 



« fRese qiianlilies being applied: with proper siijns. 



« Now i partakes of Ihe error er uiicerlainty of each of these quantities, " 



Qua quidem incertitudine amplo explicita excussu et pouderata comparalionis vero- 

 aimilitudine , ita pergit : 



« It may also perhaps be suggesüed , that the mural circle'may be used wilhoul ap- 

 « plying index error, as was done with respeot to the observations given in the appeiidix 

 « of Mr. Pond's first paper. But the knowledge of the stability of the index error du- 

 « ring six or eight naonths , depends on the reduclions by the Standard stars; and there- 

 « fore the above sources of uncertainly remain. Mr. Pond rcmarks , that between July 

 « and March the index error may have oscillated a small fraclion of a second on each 

 « side the mean, and not more , so that I Ihink no important conclusion ean be dedu- 

 « ccd from the resuMs in that appendix. " 



« I hope I have so expressed myself, that I shall be understood to mean, Ihat I 

 « consider Ihe results of observations hilherto madc by the Greenwich circle inconclu. 

 • sive as to the existence or non-exislence of parallax , merely from the uncertainly of 

 « the elemcnts used in llje reductions , not from any errors of the observations , or from. 

 « any defects in the construclion of the Instrument." 



« I more particularly offer to the consideration of aslronomers the prececding re- 

 V marks , as in ihe present State of astronomy , the relative fitness of instruments for as- 

 « certaining with precision the smaller motions , whelher real or apparent,. cf Ihe fixed^ 

 « Stars, is an object of imporlance.^ 



« In instruments similar to that beJonging to Ihe observatöry of Trinity College, Du- 

 tt blin , the index error is found by reversing the Instrument , the position of the verti— 

 (I cal axis being ascertained by a plumb line. Thus the deterraination of the index error 

 « is not malerially affected by any of the uneertainties above referred to. Therefore , by 

 « its principle , this inslrumenl should appear particularly adapted foi enquiries relative- 

 « to the annual parallax, annual Variation, etc. etc." 



K From the fixed telescopes we are probably to look för the fiiial cfecision of the- 

 « question of parallax. hx first sight these seem to offer a very simple and certain cri*- 

 « lerion. However, a little consideration will point out probable sources of difficully. 

 « Suppose the star under examination be compared with a star opposite in AR , or wilh* 

 « one as nearly so as can be conveniently had. Besides the uncertainly respecting^ Ihe- 

 « annual Variation, even the uncerlainty in the quantity oF aberration may lend Im 

 « some degree to conceal the parallax , unless the minimum of aberration in decliualjon 

 <t of each star be at the same time-, and tlie observations are made pretiy eqjially ou? 

 « bolh sides of tfiis time. The star /3 Aurigae has been judiciöusly ehosen by ITr; Pond* 

 « to eumpare with » Cjgni^ A moxe proper stac could not ha-ve beeit ehosen.; yet tReve- 



a die- 



