Page:An introduction to linear drawing.djvu/76

 ﻿the other lines, extended in the same manner, form angles of 27 and 14 degrees.

The relative size of the angles is best represented on the back of the eye, where they are cut by a single circle, and where the distance between the lines gives the apparent size of the object as seen by the spectator. Thus, the distance between the dotted lines on the back of the eye is the apparent length of the farthest arrow, and so of the other lines, which may be easily traced to their proper arrows.

If the distance of the third arrow reduces the angle from 92 to 14 degrees, it must be evident that, if the arrow or object be much farther removed, it will form no angle at all with the eye, but will become a mere point, and then disappear or become invisible.

This point, at which the object ceases to form any angle with the eye, is called the Vanishing Point of the object; and that point in the eye where the lines cross each other is called the Point of Sight.

In figure 1, the arrows are represented of their real size, to show why they appear smaller as they recede. In figure 2, they are represented at their apparent size, to show the vanishing point to better advantage.

In the above figure the real size of the object is distinguished from the apparent size by dots, and as the vanishing point falls short of the last arrow, the conclusion is that this arrow is invisible to the eye of the observer.

The vanishing point is familiarly represented by the sides of a long and straight road, which seem to approach each other at a distance; or by the lamps of a long bridge, which, although in parallel lines, apparently meet when seen from either end of the bridge.