O B S 



NEUTRAL Point. See Elastic bodies., equilibrium of. 



NIGHT-GLASS, or Siveeper. 



These are Telescopes of t\vo, or two and a half feet in length, with 

 large apertures, the object glass either a single lens of 3 or 3| inches dia- 

 meter, or an achromatic of 2% ; their magnifying power, 6, 8, or 10 

 times ; field of view 5 or 6 degrees : they are occasionally furnished with 

 a system of cross wires, and a diagonal eye piece. Their use is for a ra- 

 pid survey of any part of the heavens, and for fixing upon such objects 

 as may be proper for examination with finer telescopes. They are also 

 useful, provided the observations are recorded, in detecting minute 

 changes in the heavens upon a subsequent review ; or in searching for 

 any object supposed to be movdable, as an asteroid. For this purpose 

 delineations of the telescopic constellations, near the place where it is 

 suspected to be, should be drawn upon paper ; and after some days in- 

 terval, the moving star will be discovered. This can only be done with 

 a night glass of very low magnifying power. Herschel's small sweeper 

 was a Newtonian reflector of 2 feet focal length, aperture 4,2 inches, 

 magnifying power 24, and field of view 2 12'. (Phil. Trans, ) 



NONIUS. See Vernier. 

 NORMAL. See Subnormal. 

 NORMAL, equation to See Tangent. 

 NUTATION of the Earth's axis. See Precession. 



O 



OBSERVATORY. 



TABLE, 



Of the Latitudes and Longitudes of the principal Observatories of Europe, 

 from the most recent and accurate determinations. (Lax.) 



195 



