[ 33 * ] 
XXIII. An Account of an Apparatus applied to the equa- 
torial Jnjlrument for correcting the Errors arijing from 
ifoe RefraClion in Altitude. By Mr. Peter Dollond, 
; Optician ; communicated by the Afronomer Royal. 
HE refraction of the atmofphere occafions the liars 
or planets to appear higher above the horizon 
than they really are ; therefore, a correction for this re- 
fraction fhould he made in a vertical direction to the 
horizon. 
The equatorial inftrument is fo conftruCted, that the 
correction cannot be made by the arches or circles which 
compofe it when the ftar, See. is in any other vertical 
arch except that of the meridian, becaufe the declination 
arch is never in a vertical polition but when the tele- 
fcope is in the plane of the meridian. 
To correCt this error, a method of moving the eye- 
tube which contains the wires of the telefcope in a ver- 
tical direction to the horizon has been praCtifed ; but as 
the eye-tube is obliged to be turned round in order to 
Head March 4, 1779* 
move 
