Full text: Gravesande, Willem Jacob: An essay on perspective

on PERSPECTIVE. P S, and making the Point p be the ſame as P. then join each Diviſion of this Line, to the
correſpondent Diviſion of the Line h i; and the
Dial ſought, will be drawn: p being the Foot
of the Style, and p s the Horizontal Line.

Fig. 67.

193. Demonstration .

The Baſe Line is h i; p s is the Horizontal
Line; p the Point of Sight; and E O, or C P,
of Figure 66. is the Length of the principal
Ray.

193.1.

Fig. 66,
67.

Now, ſuppoſe the Plane p s h i, to be ſet per-
pendicularly upon the Horizontal Dial, in ſuch
manner, that the Line h i coincides with H I,
and the Point o with O. Suppoſe, moreover,
that thro’ the Extremity of the Style, which we
conſider as the Eye, Lines are drawn in the Ho-
rizontal Plane, parallel to the Hour-Lines of
the Dial; it is evident, that theſe Lines will
meet the Horizontal Line p s, in the Point al-
ready prick’d down; and conſequently , the Appearances of the Hour-Lines, are the Lines
joining the Diviſions of the Lines h i and p s.

13.

194. Remark .

If the Line H I happens to meet the Meri-
dian; the common Method, by the Horizontal
Dial, is eaſier than this.

195. Prob . II. 123. To draw inclining Dials.

Theſe Dials are drawn in the ſame manner as
Vertical ones; the following Preparations being