In our Treatise on Simplified Perspective, we have described this tool, which we reiterate here.

This instrument serves simultaneously to choose the space to be used for the painting and to determine the Point of View. It is represented here by figure 3 in plate 8. Here is how to use it. Hold the instrument with the thumb and index finger on branch a, ensuring it is vertical, and look through the small opening C. The ends a and b of the small branches placed at point B of the instrument will indicate the limits of the chosen picture. Move the instrument across the landscape in view, keeping one eye on opening C and the other closed. Stop at the most pleasing scene and choose the space between the ends a and b of the small branches as the canvas.

Recall that we mentioned on page 19 (Drawing a Box from Life), and on page 35 (Drawing a Landscape from Life), that one should move back two or three times the largest dimension of the desired representation. In landscapes, width is almost always the largest dimension. Note that in this instrument, the diameter ab, which determines the width of the painting, is half of EF, the instrument's length. Thus, there's no need to step back by more than twice the width of the tableau. In effect, the diameter ab being equal to half the distance between the eye and this diameter means that both the width and the foreground are determined by this diameter, and the distance from the eye to this foreground is precisely twice the width as seen between a and b.

Additionally, above the branches described and exactly facing opening C, is another small opening through which one can again see the tableau, determining the Point of View. For example, if one is looking at a building, the point of this building opposite the center of this opening will be the Point of View, and all receding lines of objects at right angles with the painting must converge to this point.

Suppose the observer views the landscape depicted (page 8) by figure 3.

Mark on the paper a horizontal line forming the painting's base or ground line, represented here by line AB. Then use the guide-pencil to compare the total height from the line where the chosen tableau begins up to the tree's peak with the total width. Here, height HG equals either one-third or two-fifths of width AB. Therefore, on the paper, a height equal to two-fifths of the width is taken. Also compare the distance between the ground line and the base of the mound with the entire tree height. Here HG equals seven-ninths of the entire height. Likewise, compare the mound’s height to the tree’s height. Finally, compare all previously found widths and heights, and the height of the small hill represented here by DIJEG with the mound height CG.

Once the first plan is established, the others follow easily. The first plan consists of objects on the painting's first line and closest to the observer, like in a theatre; the first row of decorations. The second plan has objects immediately following the first; as here, the river may come next. The third plan involves the building, followed by a large mountain with a building on top. Other mountains offer a fifth, sixth, and seventh plan, etc. The second plan, being occupied here only by the river, leads us to search for the third. Holding the guide-pencil horizontally (plate 3, figure 1), such that it conceals a starting point of the water's edge represented here by L, observe the height at which the guide-pencil intersects the mound represented here by CG. Here, L intersects the mound at point R, half the height CG, marking the paper with a point at the mound's mid-height. Here, the shore forming the foreground's beginning presents a receding line from the represented commencement.

Translation Notes:

• Point de Vue: Directly translated as 'Point of View', often used in art to describe the perspective from which the scene is observed.
• Line AB: Represents a baseline or ground line in artistic compositions, crucial for establishing perspective and relative scale in landscapes.