horizontal circle midway between the zenith and the horizon at the hour
at which the map is supposed to be used. At other hours, of course, this
line would be differently situated.

Plates III. and V. represent fifty-two of the objects mentioned in the
above-named chapters. As reference is made to these figures in the text,
little comment is here required. It is to be remarked, however, that the
circles, and especially the small circles, do not represent the whole
of the telescope's field of view, only a small portion of it. The object
of these figures is to enable the observer to know what to expect when
he turns his telescope towards a difficult double star. Many of the
objects depicted are very easy doubles: these are given as objects of
reference. The observer having seen the correspondence between an easy
double and its picture, as respects the relation between the line
joining the components and the apparent path of the double across the
telescope's field of view, will know how to interpret the picture of a
difficult double in this respect. And as all the small figures are drawn
to one scale, he will also know how far apart he may expect to find the
components of a difficult double. Thus he will have an exact conception
of the sort of duplicity he is to look for, and this is--_crede
experto_--a great step towards the detection of the star's duplicity.


PLATES VI. and VII., illustrating Chapters VI. and VII.

The views of Mercury, Venus, and Mars in these plates (except the
smaller view of Jupiter in Plate VII.) are supposed to be seen with the
same "power."

The observer must not expect to see the details presented in the views