VOL. LXIX.] PHILOSOPHICAL TRANSACTIONS. 525 



higher above the horizon than they really are ; therefore a correction for this re- 

 fraction should be made in a vertical direction to the horizon. The equatorial 

 instrument is so constructed, that the correction cannot be made by the arches 

 or circles which compose it when the star, &c. is in any other vertical arch ex- 

 cept that of the meridian, because the declination arch is never in a vertical 

 position, except when the telescope is in the plane of the meridian. To correct 

 this error, a method of moving the eye-tube, which contains the wires of the 

 telescope in a vertical direction to the horizon, has been practised ; but as the 

 eye-tube is obliged to be turned round, in order to move it in that direction, in 

 the different oblique positions of the instrument, the wires are thus put out of 

 their proper situation, in every other position of the instrument, except when it 

 is in the plane of the meridian ; for the equatorial wire should always be parallel 

 to the equator, that the star in passing over the field of the telescope may move 

 along with it, otherwise we cannot judge whether the telescope be set to the 

 proper declination, except at the instant the star is brought to the intersection of 

 the wires, which is only a momentary observation. 



The method I have now put in practice for correcting the refraction of the 

 atmosphere, is by applying 2 lenses before the object-glass of the telescope ; 

 one of them convex, and the other concave ; both ground on spheres of the 

 same radius, which in those I have made is 30 feet. The convex lens is round, 

 of the same diameter as the object-glass of the telescope, and fixed into a brass 

 frame or apparatus, which fits on to the end of the telescope. The concave lens 

 is of the same width, but nearly 2 inches longer than it is wide, and is fixed in 

 an oblong frame, which is made to slide on the frame the other lens is fixed into, 

 and close to it. These 2 lenses being wrought on spheres of the same radius, 

 the refraction of the one will be exactly destroyed by that of the other, and the 

 focal length of the object-glass will not be altered by their being applied before 

 it ; and if the centres of these 2 lenses coincide with each other, and also with 

 that of the object-glass, the image of any object formed in the telescope will not 

 be moved or suffer any change in its position. But if one of the lenses be 

 moved on the other, in the direction of a vertical arch, so as to separate its 

 centre from that of the other lens, it will occasion a refraction, and the image 

 will change its altitude in the telescope. The quantity of the refraction will be 

 always in proportion to the motion of the lens, so that by a scale of equal parts 

 applied to the brass frame, the lens may be set to occasion a refraction equal to 

 the refraction of the atmosphere in any altitude. If the concave lens be moved 

 downwards, that is, towards the horizon, its refraction will then be in a contrary 

 direction to that of the atmosphere, and the star will appear in the telescope as 

 if no refraction had taken place. 



There is a small circular spirit level fixed on one side of the apparatus, which 



