T E L 



Formula for the correction of a Meridian Telescope by Hie observation of 

 two stars differing considerably in polar distance. 



If the southern of two stars passes a meridian telescope too soon for the 

 calculated difference of apparent R A between them (whether its passage 

 be before or after the northern one, is immaterial) the telescope when. 

 turned down towards the south horizon will point to the east of the true 

 meridian, and v. v. This holds universally, whether the latitude of the 

 station be N. or S. 



The angle of this deviation from the meridian may be found thus: 

 The quantity of sidereal time, by which the observed difference of R A 

 varies from the calculated difference between the stars, being- reduced to 

 seconds of time ; 

 To the log. of that number of seconds ; add 



the log. cosines of the declination of each star ; 

 the log. cosecant of the difference between them in declination ; 

 and the log. secant of the lat. of the station : 



The sum (abating 40 from the Index) will give the log. of a number of 

 seconds of sidereal time ; which reduced to degrees, &c. will express the 

 angle made by the instrument and the true meridian. 



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



Let T T' be the difference of right ascensions of the two stars from 



the Tables. 



t t' the difference of right ascension as observed by the tele- 



scope, 5 and & the declinations, I the latitude, then 



Deviation 



= $T - T' -(-*<)?. 

 3 



-- 



cos. Asm. ($3 



7. To find the field of view of a telescope. 



Direct the telescope to a star in the equator, or very near it, which 

 will answer quite well enough for all usual purposes, and observe the 

 number of seconds occupied in its passage across the field of view, and 

 multiply this number by 4, to obtain in degrees a measure of the field. 



It would evidently be inconsistent with the limits of this small work 

 to enter into any explanation of the nature, use, and adjustment, of 

 mathematical instruments j nevertheless as a telescope is in the hands 

 of almost every one at all conversant in scientific pursuits, the follow- 

 ing practical observations on this instrument, selected from the works 

 of eminent practical astronomers, may not be unacceptable to the inex. 

 perienced observer. 

 296 



