" But a close precision is not reached, especially when the image
given by the crystal face is not very gooil, by superposing two images of
the same slit. It is better to place a little below the first slit, which I
shall call the signal slity and which has one of the forms/,/', another
very narrow slit, / (Fig. 2), parallel to the direction of the first, and
which I will call the cross-hair slii. It is, like the signal, crossed per-
pendicularly by a small and very fine slit, which ought to be the exact
prolongation of the one which crosses the signal slit."

** The very fine cross-hair slit does not give, usually, a clear image by
reflection on the crystal faces, but it gives by reflection in the mirror
a very clear image, colored by the glass P. It is upon this image that
the axis of the image of the signal slit, reflected by the crystal, is super-
posed, thus making use of the image of the cross-hair slit seen in the mirror
as a true cross hair. There is reached in this way an exactness which,
with good cr>stal faces, attains easily the minute and even the half-
minute."

*' It is true that with this method of working the crystal face and the
mirror are no longer strictly parallel when the crystal image of the
signal slit is superposed ui>on the mirror image of the cross-hair slit.
