L 377 ] 



LXXV. On the Adjustment of the Line of CoUimation of the 

 Transit Instrument. 

 Gentlemen, Bath, November 10, 1823. 



T HAVE recently met with a description of a mode of ad- 

 ■* justing the Hne of colhmation of the transit instrument, 

 which, although published 7/pivards of thirty years ago, does 

 not seem to have been generally (if at all) practised; but 

 which appears capable of great accuracy. It is to be found in 

 a work entitled Fixarum prcEcipiiarum Catalogus Novns, by 

 F.deZach, published at Gotha in 1792; in page 18 of which 

 he directs the observer to note the exact time of the transit 

 of a star (near the pole) from the frst to the middle wire of 

 the telescope (M. Zach's telescope having only three wives) : 

 which being done, he is then to invert the telescope, end for 

 end, and note the exact time of its passing the last wire, whicli 

 is obviously the very sa?ne wire as that which was called the 

 frst in the former position. If the two intervals correspond, 

 the line of collimation may be considered accurate : but, if 

 not, the proper corrections must be made to bring them so. 

 As this method is very simple, and must be well known to 

 many pei'sons, I am sur^irised it has not been more generally 

 adopted. 



The same author, who has, I am informed, written other 

 works on Astronomy, adds that a similar method may be suc- 

 cessfully pursued with stars 7iot near the pole. In this case, 

 two stars must be selected which differ but little from each 

 other in declination, and which can be observed without 

 moving the telescope. As to their difference in right ascen- 

 sion, all that is required will be sufficient time for inverting 

 the telescope. Let this difference in right ascension be well 

 ascertained, either by actual observation, or deduced from 

 the best catalogue. Let the transit of the first star be ob- 

 served : then, after inverting the telescope, let the transit of 

 the second star be observed. If the interval between the two 

 transits made in this manner corresponds with the difference 

 between the correct right ascensiolis of the two stars, the line 

 of collimation is, in this case also, accurate : if not, it must be 

 corrected as before mentioned. I am not aware of any ob- 

 jection to either of these methodb : but, as I have not had 

 much experience in practical astronomy, I submit them to 

 such of vour readers as may be curious in these matters : in 

 order tluit, by a more general circulation, the method may 

 have a fairer chance of being tried ; and, if found successful, 

 of being universally adopted. 



I am, gentlemen, your very obedient servant, 



Zeno. 

 Vol. G2. No. 307. Nov. 1823. 3 IJ LXXVI. List 



