that one wants to represent. One could compare this frame to a glass partition that would completely bisect the room while the observer, placed behind this partition, at one end of the room, would draw everything he could see through this transparent partition.

One can freely determine on the paper, the height or width of the room, on a vertical or horizontal line. If the room is higher than it is wide, one determines the height first; otherwise, it is the width that is first defined. Here, we begin with the width. Having chosen a line AB as the width, it is found that the height BC of the model room is equal to four-fifths of its width AB. Thus, one takes on the paper a height BC, equal to four-fifths of AB, and thus the square corresponding to ABCD of the model room is established. Next, one must find the walls and floor of the room.

The intersection of this floor with the back wall gives a line marked here by ab. To find this line, one holds the pencil horizontally, as in plate 3, figure 11 (page 41). When the pencil is placed so as to hide from the eye this line here represented by ab, it will also cut the line BC, which marks the room's height in the foreground. The pencil will cut this line BC at a point e, at a small quarter of BC, from point B. Thus, one marks on the paper a point e, at a small quarter of height BC, already drawn from point B. From this point e, a horizontal line is drawn.

Similarly, the intersection of the right side of the room with the back wall gives a line bc. By holding the pencil horizontally, such that the opposite right side extremity aligns with the line represented here by bc, and then, without disturbing the hand, placing the index finger opposite the line represented here by BC, one has on the pencil what is the entire length, in perspective, of the right side represented here by BC to bc. Then, without moving the hand, one compares ab with width AB. One finds that this bc is three-thirds of AB. On the horizontal line previously drawn through point e, on the drawing, one takes, from point e, a size of three small thirds of the line AB of the drawing. This size gives the point b on the drawing, and on this point b, a vertical is erected in the direction marked here by bc.

Assuming the observer is placed just at an equal distance from points A and B, and facing the middle point of the opposite side of the room, it is evident that the left side ADab appears absolutely the same size as the right side BCcb. If one were to seek, with the aid of the pencil, the height of point a, one would find that this point is placed on the horizontal line ef that extends to g, and one would find this point g, by taking a size from point f equal to b, equal to the small thirds of AB. On this point a, a vertical is also erected. It remains only, to complete the square abcd of the back of the room, to determine points c and d. It is evident that the latter square, in reality, equal and similar to ABCD, although it appears smaller due to perspective. It has been seen that BC equals four-fifths of AB: therefore, one will take ab, and similarly bc equal to four-fifths of ac, and cd equal to 1/4, and keeping the line de, in ar aba, representing the face and the background of the room's side opposite the observer. Connecting by lines the points or angles B and b, C and c, D and d, and A and a, one will have the meeting lines or sides of the lateral planes, the floor, and the ceiling, lines that, sufficiently prolonged, meet at a point that would be just at the observer's eye height. This would be the viewpoint, marked here by point V. All the receding lines (forming a right angle with the observer) will head, in your drawing, to this point of view, which will be their point of concurrence.

The main lines, or, according to the accepted term, the great mass of the drawing being thus found, one then deals with the details. One will begin with the windows.

Holding the pencil horizontal and always parallel to oneself, one takes on this pencil the distance as seen from the object EF of the first window, to BC forming the start of the room on the right side, for the observer. One compares this distance with the entire width of the side of the room, from BC to bc. Here the distance fH equals one-quarter of the width EF. One takes on the paper, and from the BC line, a dimension equal to one-quarter of the width of this side of the room.

Translation Notes:

- 'porte-crayon' has been translated as 'pencil' for simplicity, but it signifies a tool used for maintaining lines parallel during drawing.