336 
Mr. dollond’s Account , See 
c, The circular fpirit level, which fhews when the ob- 
long lens is in a vertical arch. 
d, The quadrant to which the fpirit level is fixed, for 
fhewing the angular elevation of the telefcope. 
E, The milled head fixed to a pinion, by which the 
whole apparatus is turned round on the end of the 
telefcope, in order to fet the oblong lens in a vertical 
arch. 
F, Another pinion for fetting the quadrant to the angu- 
lar elevation of the telefcope. By means of thefe two 
pinions the air bubble mull be brought to the middle 
of the level. 
act , Is the fcale, with divifions anfwering to minutes ancl 
half minutes of the refraction occafioned by the con- 
cave lens. 
