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.
To test this parallelism the single web must be made to bisect the images of both components simultaneously, as in fig.
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.
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.