Join _EC_, and produce it to cut _GF_ in _F_. Then _F_ is the vanishing-point required. For, because _AEC_ is an angle in a semicircle, it is a right angle; and therefore the line _EF_ is at right angles to the line _AB_; and similarly all lines drawn to _F_, and therefore parallel to _EF_, are at right angles with any line which cuts them, drawn to the vanishing-point of _AB_. And because the semicircle _ADC_ is in a vertical plane, and its diameter _AC_ is at right angles to the horizontal lines traversing the surface of the inclined plane, the line _EC_, being in this semicircle, is also at right angles to such traversing lines. And therefore the line _EC_, being at right angles to the steepest lines in the plane, and to the horizontal lines in it, is perpendicular to its surface. * * * * * The preceding series of constructions, with the examples in the first Article of the Appendix, put it in the power of the student to draw any form, however complicated,[30] which does not involve intersection of curved surfaces. I shall not proceed to the analysis of any of these more complex problems, as they are entirely useless in the ordinary practice of artists. For a few words only I must ask the reader’s further patience, respecting the general placing and scale of the picture. As the horizontal sight-line is drawn through the sight-point, and the sight-point is opposite the eye, the sight-line is always on a level with the eye. Above and below the sight-line, the eye comprehends, as it is raised or depressed while the head is held upright, about an equal space; and, on each side of the sight-point, about the same space is easily seen without turning the head; so that if a picture represented the true field of easy vision, it ought to be circular, and have the sight-point in its center. But because some parts of any given view are usually more interesting than others, either the uninteresting parts are left out, or somewhat more than would generally be seen of the interesting parts is included, by moving the field of the picture a little upwards or downwards, so as to throw the sight-point low or high. The operation will be understood in a moment by cutting an aperture in a piece of pasteboard, and moving it up and down in front of the eye, without moving the eye. It will be seen to embrace sometimes the low, sometimes the high objects, without altering their perspective, only the eye will be opposite the lower part of the aperture when it sees the higher objects, and _vice versâ_.