[Rule 10]
Horizontals in the same plane which are drawn to the same point on the horizon are parallel to each other.
Fig. 40.
This is a very important rule, for all our perspective drawing depends upon it. When we say that parallels are drawn to the same point on the horizon it does not imply that they meet at that point, which would be a contradiction; perspective parallels never reach that point, although they appear to do so. Fig. 40 will explain this.
Suppose S to be the spectator, AB a transparent vertical plane which represents the picture seen edgeways, and HS and DC two parallel lines, mark off spaces between these parallels equal to SC, the height of the eye of the spectator, and raise verticals 2, 3, 4, 5, &c., forming so many squares. Vertical line 2 viewed from S will appear on AB but half its length, vertical 3 will be only a third, vertical 4 a fourth, and so on, and if we multiplied these spaces ad infinitum we must keep on dividing the line AB by the same number. So if we suppose AB to be a yard high and the distance from one vertical to another to be also a yard, then if one of these were a thousand yards away its representation at AB would be the thousandth part of a yard, or ten thousand yards away, its representation at AB would be the ten-thousandth part, and whatever the distance it must always be something; and therefore HS and DC, however far they may be produced
and however close they may appear to get, can never meet.
