TEL 



5. In refracting telescopes, if A be the linear aperture of the object 

 glass, the density of rays in the picture upon the retina varies as 



A2 F'g 



pa 



And in the Newtonian telescope, as 

 A 2 F'a 

 ~7*~" 



6. To place a telescope in the meridian by tlie pole star.-( Wollaston. ) 

 Calculate the time of the meridian passage of the star correctly, and 



apply that to your chronometer. Then having the str.r in 1 ho field of 

 your telescope (the instrument being first truly adjusted, and the adjust- 

 ing screw for azimuth between your finger and thumb) and keeping it 

 bisected, or covered by your meridian wire till the exact instant calcu- 

 lated, clamp the instrument there in azimuth, and you will find it very 

 neaily in the meridian indeed. 



Having thus placed the telescope very nearly in the meridian ; we may 

 adjust it accurately so, by either of the following formulae : 



Formula for correcting the error of a Meridian Telescope by the observa- 

 tion of any circuinpolar star above and beloiv the po!c. 



If the western interval be greater than the eastern one, Ihe telescoro 

 points to the east of that end of the true meridian which lies under the 

 the elevated pole (be that N. or S.) and v. v. 



The angle of this deviation may be investigated thus : 



To the log-, of half the difference between the intervals in seconds (or 

 the difference between either interval and lh. sid. time.) 



Add the log. tangent of the star's PD. 



And the log. secant of the lat. of the station. 



The sum (abating from the Index) will give the log. of a number of 

 seconds of sid. time; which converted into degrees, c. will express tho 

 angular deviation of tho instrument from the true meridian, to be ap- 

 plied as above. 



This method depends not at alt upon knowing truly the R A of tho 

 star; nor its PD with any very great accuracy: the ZD or alt. read 

 oS' with the instrument, as it passes the meridian, will give the latter 

 with fully sufficient precision. 



Formula, from which the above rule is deduced. (Maddy.) 



Deviation. - l^JT-^^ 1 ? > where t and t' are the two intervals, 

 cos.J. tan. 



$ the star's declination, and I the latitude of the place. 



295 \ 



