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.
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.
If the one set of lines exactly bisect the intervals between the others, the grating interval is practically halved, and the previously existing spectra of odd order vanish.
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.