Fig. 50.

[ XII]
Geometrical and Perspective Figures Contrasted

As at first there may be a little difficulty in realizing the resemblance between geometrical and perspective figures, and also about certain expressions we make use of, such as horizontals, perpendiculars, parallels, &c., which look quite different in perspective, I will here make a note of them and also place side by side the two views of the same figures.

Fig. 51 A. The geometrical view. Fig. 51 B. The perspective view.
Fig. 51 C. A geometrical square. Fig. 51 D. A perspective square.
Fig. 51 E. Geometrical parallels. Fig. 51 F. Perspective parallels.
Fig. 51 G. Geometrical perpendicular. Fig. 51 H. Perspective perpendicular.
Fig. 51 I. Geometrical equal lines. Fig. 51 J. Perspective equal lines.
Fig. 51 K. A geometrical circle. Fig. 51 L. A perspective circle.
[ XIII]
Of Certain Terms made use of in Perspective
Fig. 52. Horizontals.

Of course when we speak of Perpendiculars we do not mean verticals only, but straight lines at right angles to other lines in any position. Also in speaking of lines a right or straight line is to be understood; or when we speak of horizontals we mean all straight lines that are parallel to the perspective plane, such as those on Fig. 52, no matter what direction they take so long as they are level. They are not to be confused with the horizon or horizontal-line.

There are one or two other terms used in perspective which are not satisfactory because they are confusing, such as vanishing lines and vanishing points. The French term, fuyante or lignes fuyantes, or going-away lines, is more expressive; and point de fuite, instead of vanishing point, is much better. I have occasionally called the former retreating lines, but the simple meaning is, lines that are not parallel to the picture plane; but a vanishing line implies a line that disappears, and a vanishing point implies