Fig. 1.
I. The Sight-Point.—
Let A B C D, [Fig. 1.], be your sheet [p5] ]of paper, the larger the better, though perhaps we may cut out of it at last only a small piece for our picture, such as the dotted circle N O P Q. This circle is not intended to limit either the size or shape of our picture: you may ultimately have it round or oval, horizontal or upright, small or large, as you choose. I only dot the line to give you an idea of whereabouts you will probably like to have it; and, as the operations of perspective are more conveniently performed upon paper underneath the picture than above it, I put this conjectural circle at the top of the paper, about the middle of it, leaving plenty of paper on both sides and at the bottom. Now, as an observer generally stands near the middle of a picture to look at it, we had better at first, and for simplicity’s sake, fix the point of observation opposite the middle of our conjectural picture. So take the point S, the center of the circle N O P Q;—or, which will be simpler for you in your own work, take the point S at random near the top of your paper, and strike the circle N O P Q round it, any size you like. Then the point S
is to represent the point opposite which you wish the observer of your picture to place his eye, in looking at it. Call this point the “Sight-Point.”
II. The Sight-Line.—
Through the Sight-point, S, draw a horizontal line, G H, right across your paper from side to side, and call this line the “Sight-Line.”
This line is of great practical use, representing the level of the eye of the observer all through the picture. You will find hereafter that if there is a horizon to be represented in your picture, as of distant sea or plain, this line defines it.
III. The Station-Line.—
From S let fall a perpendicular line, S R, to the bottom of the paper, and call this line the “Station-Line.”
This represents the line on which the observer stands, at a greater or less distance from the picture; and it ought to be imagined as drawn right out from the paper at the point S. Hold your paper upright in front of you, and hold your pencil horizontally, with its point against the point S, as if you [p6] ]wanted to run it through the paper there, and the pencil will represent the direction in which the line S R ought to be drawn. But as all the measurements which we have to set upon this line, and operations which we have to perform with it, are just the same when it is drawn on the paper itself, below S, as they would be if it were represented by a wire in the position of the leveled pencil, and as they are much more easily performed when it is drawn on the paper, it is always in practice, so drawn.