334 M r . dollond’s Account of an Apparatus 
the other, and the focal length of the objeCt-glafs will 
not be altered by their being applied before it; and if 
the centers of thefe two lenfes coincide with each other, 
and alfo with that of the objeCt-glafs, the image of any 
object formed in the telefcope will not be moved or fuffer 
any change in its petition. But if one of the lenfes 
be moved on the other, in the direction of a vertical 
arch, fo as to feparate its center from that of the other 
lens, it will occafion a refraction, and the image ill 
change its altitude in the telefcope. The quantity of the 
refraCtion will be always in proportion to the motion of 
the lens, fo that by a fcale of equal parts applied to the 
brafs frame, the lens may be fet to occafion a refraCtion 
equal to the refraction of the atmofphere in any altitude. 
If the concave lens be moved downwards, that is, towards 
the horizon, its refraction will then be in a contrary di- 
rection to that of the atmofphere, and the ftar will ap- 
pear in the telefcope as if no refraCtion had taken place. 
There is a l'mall circular fpirit level fixed on one fide 
the apparatus, which ferves to fet it in fuch a petition, 
that the centers of the two lenfes may be in the plane of 
a vertical arch. This level is alfo ufed for adj lifting a 
fmall quadrant, which is fixed to it, and divided into de- 
grees, to fhew the elevation of the telefcope when di- 
rected to the itar; then the quantity of refraCtion an- 
fwering 
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