III. THE STATION-LINE.—From _S_ let fall a perpendicular line, _SR_, 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 wanted to run it through the paper there, and the pencil will represent the direction in which the line _SR_ 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. IV. THE STATION-POINT.—On this line, mark the distance _ST_ at your pleasure, for the distance at which you wish your picture to be seen, and call the point T the “Station-Point.” [Illustration: Fig. 2.] In practice, it is generally advisable to make the distance _ST_ about as great as the diameter of your intended picture; and it should, for the most part, be more rather than less; but, as I have just stated, this is quite arbitrary. However, in this figure, as an approximation to a generally advisable distance, I make the distance _ST_ equal to the diameter of the circle _NOPQ_. Now, having fixed this distance, _ST_, all the dimensions of the objects in our picture are fixed likewise, and for this reason:— Let the upright line _AB_, Fig. 2., represent a pane of glass placed where our picture is to be placed; but seen at the side of it, edgeways; let _S_ be the Sight-point; _ST_ the Station-line, which, in this figure, observe, is in its true position, drawn out from the paper, not down upon it; and _T_ the Station-point. Suppose the Station-line _ST_ to be continued, or in mathematical language “produced,” through _S_, far beyond the pane of glass, and let _PQ_ be a tower or other upright object situated on or above this line. Now the _apparent_ height of the tower _PQ_ is measured by the angle _QTP_, between the rays of light which come from the top and bottom of it to the eye of the observer. But the _actual_ height of the _image_ of the tower on the pane of glass _AB_, between us and it, is the distance _P′Q′_ between the points where the rays traverse the glass.