We must bisect as far as may be, but the division is after all to be into limbs, not parts.
Driver prefers to bisect the dialogue by supposing i.
If we bisect AB and CD in P and Q respectively, and describe the arc PQ of a circle with centre 0, the length of this arc is 2(b+a)0; and b - a =AB.
Upon an axis concentric with the declination axis is carried a plane mirror, which is geared so as always to bisect the angle between the polar axis and the optical axis of the telescope.
In measuring differences of declination, where the stars are brought up by the diurnal motion, this precaution cannot be adopted, because it is necessary always to bisect the preceding star with the fixed web.