202                            «>X   A   XKW   ELLIPTIC  ANALYSER.
provided we _(u;ive to the Xicol, not to the plate, the independent inotion which we have at our disposal.
With a view to its possible employment in such case, the instrument as originally constructed was furnished with adapting pieces enabling the Nieul and the plate to exchange places, the whole instrument being of course in that case turned round, so that thr graduation faces the incident light instead of facing the observer.
If on p. 1!»S / was called the azimuth of the major axis of the ellipse described, it was merely to facilitate the conception of the four unknown quantities that we. hud to determine. It is hardly necessary to observe that all we arc concerned with in the formula* is that, it is the azimuth of </, principal axis.]