Supposing this figure represented, not a pyramid, but a triangle on the ground, and that _QD_ and _QP_ are horizontal lines, expressing lateral distance from the line _TD_, still the rule would be false for _QP_ and true for _QD_. And, similarly, it is true for all lines which are parallel, like _QD_, to the plane of the picture _AB_, and false for all lines which are inclined to it at an angle. Hence generally. Let _PQ_ (Fig. 2. in Introduction, p. 6) be any magnitude _parallel to the plane of the picture_; and _P′Q′_ its image on the picture. Then always the formula is true which you learned in the Introduction: _P′Q′_ is to _PQ_ as _ST_ is to _DT_. Now the magnitude _P_ dash _Q_ dash in this formula I call the “SIGHT-MAGNITUDE” of the line _PQ_. The student must fix this term, and the meaning of it, well in his mind. The “sight-magnitude” of a line is the magnitude which bears to the real line the same proportion that the distance of the picture bears to the distance of the object. Thus, if a tower be a hundred feet high, and a hundred yards off; and the picture, or piece of glass, is one yard from the spectator, between him and the tower; the distance of picture being then to distance of tower as 1 to 100, the sight-magnitude of the tower’s height will be as 1 to 100; that is to say, one foot. If the tower is two hundred yards distant, the sight-magnitude of its height will be half a foot, and so on. But farther. It is constantly necessary, in perspective operations, to measure the other dimensions of objects by the sight-magnitude of their vertical lines. Thus, if the tower, which is a hundred feet high, is square, and twenty-five feet broad on each side; if the sight-magnitude of the height is one foot, the measurement of the side, reduced to the same scale, will be the hundredth part of twenty-five feet, or three inches: and, accordingly, I use in this treatise the term “sight-magnitude” indiscriminately for all lines reduced in the same proportion as the vertical lines of the object. If I tell you to find the “sight-magnitude” of any line, I mean, always, find the magnitude which bears to that line the proportion of _ST_ to _DT_; or, in simpler terms, reduce the line to the scale which you have fixed by the first determination of the length _ST_.