302 PHILOSOPHICAL TRANSACTIONS. ' [aNNO 1772. 



The sun's altitude might also be observed with this instrument, either by the 

 fore or back observation ; and the altitude of the moon might be taken with it in 

 the night. But the altitudes of stars could not be observed with it, nor the 

 moon's altitude in the day time, which would however be no great inconvenience, 

 as these observations might be well enough supplied by common quadrants. 



The following rules for the size of the glasses and the silvering them, and the 

 height of the telescope, may be of use. The index glass arid 1 horizon glasses, 

 should be all of equal height, and even with one another in height both at top 

 and bottom. The telescope should be moveable parallel to itself, nearer to or 

 farther from the plane of the quadrant, and the range of its motion should be 

 such, that its axis, when at the lowest station, should point about -^^ of an inch 

 lower than the top of the silvering of the horizon glasses, and when at the 

 highest station should point to the height of the middle of the unsilvered part of 

 the index-glass. The height of the glasses, and the quantity of parts silvered 

 and parts unsilvered, should vary according to the aperture of the object glass, 

 as in the following table; where the first column of figures shows the dimensions, 

 in parts of an inch, answering to an aperture of the object glass of -^ of an inch 

 in diameter ; the 2d column, what answer to an aperture of the object glass of 

 i^ of an inch in diameter ; and the 3d, what are suitable to an aperture of the 

 object glass of-,*,- of an inch in diameter. 



TTT 



lect glass of -At of an inch in diameter. 



Parts of an inch 

 Diameter of aperture of object glass 30 0.40 0.50 



Height of glasses 90 1.13 1.37 



Height of silvered part of index glass 50 0.63 0.77 



Height of unsilvered part of ditto 40 0.50 0.60 



Height of silvered part of horizon glasses ...... .25 0.33 0.42 



••' Height of unsilvered part of ditto 65 0.80 0.<)5 



If the telescope has a common object glass, the first aperture of tV of an inch 

 will be most convenient ; but if it has an achromatic object glass, one of the 

 other apertures, of -rV or -j^ of an inch, will be most proper. The field of view 

 of the telescope should be 5 or 6 degrees, and the objects should be rendered as 

 distinct as possible throughout the whole field, by applying 2 eye-glasses to the 

 telescope. The breadth of the glasses should be determined as usual, according 

 to the obliquity with which the rays fall on them, and the aperture of the ob- 

 ject glass. 



I shall conclude this paper with some easy rules, for finding the apparent an- 

 gular distance between any 2 near land objects, by the Hadley's quadrant. To 

 find the angular distance between 2 near objects by the fore observation: adjust 

 the fore horizon glass by the object intended to be taken as the direct object ; 

 and the angle measured by the fore observation on the arch of the quadrant be- 



