Mr. dollond’s Account, &c. 333 
move it in that direction, in the different oblique peti- 
tions of the inftrurnent, the wires are thereby put out 
of their proper fituation in every other pofition of the 
inftrurnent, except when it is in the plane of the meri- 
dian; for the equatorial wire fhould always be parallel to 
the equator, that the ftar in palling over the field of the 
telefcope may move along with it, otlierwife one cannot 
judge whether the telefcope be fet to the proper declina- 
tion, except at the inftant the ftar is brought to the inter- 
feCtion of the wires, which is only a momentary obfer- 
vation. 
The method I have now put in practice for correcting 
the refraction of the atmofphere is by applying two 
lenfes before the objeCt- glafs of the telefcope; one of 
them convex, and the other concave; both ground on 
fpheres of the fame radius, which in thofe I have made 
is thirty feet. The convex lens is round, of the fame 
diameter as the objeCt-glafs of the telefcope, and fixed 
into a brafs frame or apparatus, which fits on to the end 
of the telefcope. The concave lens is of the fame width, 
but nearly two inches longer than it is wide, and is fixed 
in an oblong frame, which is made to Hide on the frame 
the other lens is fixed into, and clofe to it. Thefe two 
lenfes being wrought on fpheres of the fame radius, the 
relraCtion of the one will be exactly deftroyed by that of 
Vol. LXIX. Y y the 
