C ™9 ] 



Diameter of aperture of object-glafs 



Height of piaffes 



Height of filvered part of index-glafs 

 Height of unfilvered part of ditto 

 Height of filvered part of horizon-glaiTes 

 Height of unfilvered part of ditto 



Parts of an Inck 



,30 



>9Q 



>5° 



,40 



■2$ 



,65 



0,40:0,50 



1,1 : 



o ; 6; 



0,50 



o»3'3 



i.37 



0,77 

 0,60 

 o>42 



0,8010,95 



If the telefcope has „a common objedt-glafs, the 

 firfi; aperture of T %.ths °f an mcn w ^ ^e l ~n°& con- 

 venient 5 but if it has an achromatic object-gla'fs, one 

 of the other apertures of -/-o-ths or ^.thsof an inch, 

 will be moft proper. The field of view of the 

 telefcope mould be 5 or 6 degrees, and the objects 

 mould be rendered as diftinct as poflible throughout 

 the whole field, by applying two eye-glaffes to the 

 telefcope. The breadth of the glafTes mould be de- 

 termined as ufual, according to the obliquity with 

 which the rays fall on them and the aperture of the 

 object-glafs. 



I mall conclude this paper with fame eafy rules 

 for finding the apparent angular diltance between 

 any two near land obje&s by the Hadley's qua- 

 drant. 



To find the angular difhnce between two near 

 objects by the fore-obfervation. Adjuft the fore- 

 horizon-glafs by the object intended to be taken as 

 the direct object 5 and the angle meafured by the 

 fore-obfervation on the arch of the quadrant between 

 this object and any other object feen by reflection will 

 be the true angle between them as feen from the 

 centre of the index-glafs. But, if the quadrant be 



already 



