The text describes a method for drawing a rectangular box using linear perspective techniques. It explains how to find specific points and planes on a drawing to represent different sides of the box. The document concludes by emphasizing the importance of these techniques for drawing not only rectangular but also spherical objects from nature.
The width of the ABCD plane, represented on our pencil holder by CL, is thus half of the height AB. Therefore, you take on your drawing half of the line representing AB; you project it horizontally from the point you previously marked to represent point L; you thus find on your drawing the point that will represent point C, and you connect it with a line to the point you already had representing point B. You then compare the side CD with side AB: you find that CD seems to be three-quarters of AB. You then take on your drawing a size equal to three-quarters of the line representing AB, and you project this size on your drawing from the point representing C, raising a perpendicular on this point which will represent CD, three-quarters of AB; you then draw a line on your drawing connecting the two points representing A and D, thus finding the plane ABCD.
With the side ABCD found, side BAEF is found by the same operation.
It remains to find the plane DAEG, forming the top of the box. Since you already have the three points D, A, E, you only need to find point G. Assuming from point G, which represents angle G to your eye, you let a plumb line descend, as figure 8 indicates, or your pencil holder held in such a way that it forms a plumb line, as seen in figure 9, it will be found that here, by the manner the box is placed, you will have points G and A on the same vertical line. You must therefore continue the height vertically representing AB on your drawing, knowing that the point representing G will be on this extension. You then compare the height of aG with AB, always by eye and pencil holder. aG here is a small quarter of AB: you take on your drawing a small quarter of the line that represents AB, and this size will give you the point corresponding to G on your extended AB line. You draw the lines DG and GE and this gives you the plane DAEG forming the top of the box.
IMPORTANT RESULTS FROM THE PREVIOUS CHAPTER.
THE ELEMENTARY NOTIONS OF LINEAR PERSPECTIVE, DEDUCED FROM DRAWING BOXES FROM NATURE.
Drawing from nature of the rectangular box, in the three positions we have taken as examples, although extremely easy, is of great importance. The operations just demonstrated would similarly apply to all the rectangular bodies found in nature. This will be further developed as we indicate the means to draw spherical-shaped bodies from nature.
Translation Notes:
1. "porte-crayon" translated as "pencil holder" refers to a specific drawing tool in historic art education.
2. "fil d’aplomb" or "plumb line" is a simple tool used for establishing a vertical line.