15 January, 2009



This figure is similar to the previous one, except that the extremity A of the given line is raised from the ground, but the same demonstration applies to it.





And now let us suppose the vertical plane K to be a sheet of glass, and the given line AE to be the visual ray passing from 24 the eye to the object A on the other side of the glass. Then if E is the eye of the spectator, its projection on the picture is S, the point of sight. If I draw a dotted line from E to little a, this represents another visual ray, and o, the point where it passes through the picture, is the perspective of little a. I now draw another line from g to S, and thus form the shaded figure ga·Po, which is the perspective of aAa·g.


Let it be remarked that in the shaded perspective figure the lines a·P and go are both drawn towards S, the point of sight, and that they represent parallel lines Aa· and ag, which are at right angles to the picture plane. This is the most important fact in perspective, and will be more fully explained farther on, when we speak of retreating or so-called vanishing lines.

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