When a line is rotated about a fixed axis its changing orientations are such that they define a cone (Fig. 35a). The projected path on a stereogram is a small circle (Fig. 35b). A line with an initial orientation (labelled 1, Figs. 35a, 35b) achieves, after a given rotation about an inclined axis, a new orientation, labelled 4. The small-circle route followed by the line as it rotates is possible to draw (p. 62, Fig. 35b, 35c) but the practical procedure for constructing the rotated orientation of a line in this way is rather clumsy. An alternative method explained here makes use of the simpler procedure of rotating about a horizontal axis (pp. 64–5). In fact the procedure involves three such rotations.

The first stage (Fig. 35c) is one purely designed to simplify the problem. It consists simply of a tilting (about a horizontal axis) of both the rotation axis and the line, sufficient to make the former horizontal. In other words, stage 1 converts the whole problem to one of rotation about a horizontal axis (described on pp. 64–5). At the second stage the actual rotation can be carried out. The third stage involves the reversal of the tilting done for convenience at the first stage.

The details of this procedure are:

1. 1 Plot the line to be rotated and the axis of rotation (labelled 1 and RA, respectively in Fig. 35c, 35d).

2. […]