THE VISIBLE THING BEING PUT INTO FORESHORTENING IS SIMILAR TO THE SAME THING SEEN THROUGH A GLASS WHERE THAT GLASS WOULD MARK THE SAID FIGURE.

The window of Albert Durer Albrecht Dürer gives the demonstration of this present Theorem; I will give here the means to practice it, and after, the demonstration will be given. Let there be a cube placed on a table, marked A, B, C, D, E, F, G. Let there also be a window placed firmly on the said table in such a way that it is in a frame and that it can be opened and closed with ease. Let there also be a thread attached to the side of the said window where there is a small bead that can be slid along the said thread. After, let a small ring be attached to the wall marked H, and let a thread pass through it where there will be a plumb line at the end, and at the other end will be attached a small rod. After, let the practice be done in this fashion: let there be one man at the end of the table and the other near the figure that one desires to foreshorten, who, with the rod to which the thread will be attached, will push it to one of the angles of the figure. Then the other man will put the other thread across the window in such a way that it can touch the other. Then, one must bring the bead near the point of contact; then, one will close the said window, and at the place where the bead touches the window, one must mark a point. And thus do for all the other angles of the figure. And one must note that one must draw the lines from one angle to the other, for fear of confusion at the same time that the points are formed. Thus you will have the foreshortening of a cube or of what you wish to foreshorten. And in the practice of this, one must consider that H is the point of view, the cube the visible thing, the table the ground line, the window the cut line, the threads A H the visual rays, the plan of the cube described on the table the Ichnography ground-plan/floor-plan, the elevation of the said cube the Orthography elevation/side-view, and the foreshortening which is described against the window the Scenography perspective representation. Now, to demonstrate that the visible thing is put into foreshortening as if it were described against a glass, let a glass be placed in the frame in place of the said window, and let the eye be at point H. It is certain that one will see in the said glass the said cube in the same form as the foreshortening made against the window, for the thread going straight from the point of view to the angles of the cube is in the same place where the visual ray would be against the glass.