when using an alt-azimuth, to observe objects as nearly as possible due
south. Of course in many cases this is impracticable, because a
phenomenon we wish to watch may occur when an object is not situated
near the meridian. But in examining double stars there is in general no
reason for selecting objects inconveniently situated. We can wait till
they come round to the meridian, and then observe them more comfortably.
Besides, most objects are higher, and therefore better seen, when due
south.

Northern objects, and especially those within the circle of perpetual
apparition, often culminate (that is, cross the meridian, or north and
south line) at too great a height for comfortable vision. In this case
we should observe them towards the east or west, and remember that in
the first case they are rising, and in the latter they are setting, and
that in both cases they have also a motion from left to right.

If we allow an object to pass right across the field of view (the
telescope being fixed), the apparent direction of its motion is the
exact reverse of the true direction of the star's motion. This will
serve as a guide in shifting the alt-azimuth after a star has passed out
of the field of view.

The following technical terms must be explained. That part of the field
of view towards which the star appears to move is called the _preceding_
part of the field, the opposite being termed the _following_ part. The
motion for all stars, except those lying in an oval space extending from
the zenith to the pole of the heavens, is more or less from right to
left (in the inverted field). Now, if we suppose a star to move along a
diameter of the field so as to divide the field into two semicircles,
then in all cases in which this motion takes places from right to left,