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.