This image comes from the book Insignium Romae Templorum Prostectus, which the editor Giovanni Giacomo de Rossi published in Rome in 1683 and 1684. It is an engraving that Francesco Venturini did from Lorenzo Nuvolone's drawing, measures 42x18 cm and represents the elevation of the San Carlo's alle Quattro Fontana church façade, of Rome, of the architect Francesco Borromini.

The drawing shows an undulating façade. Even without knowing the building, we can easily see the first entablature undulation, the upper level and the plinth, owing to the shadows that an alleged side lighting produces on the façade. They are not shadows with precise contours, rather, those of a diffuse light that allows perception of the curve continuity and smoothness. Otherwise, the curvature would not be perceived, because the curves are projected on horizontal plans, projected in profile as straight lines.

Probably, a better way to see this curved façade would be a front or side perspective, discarding of course that such curvature can only be seen precisely in the plan drawing of the top view.

More precisely, in this elevation one can only sense the curvature. We think we are seeing it, but we do not see it in actual fact: light awakens in us memories of other past experiences and we recognize those curves that are not physically drawn. Owing to these shadows we can gaze over each part of the drawing and recognize the relief suggested.We can "see" the convexities and concavities, which parts are in front or in the rear, although we can not set its depth with accuracy. But we also see, and this is something that initially goes unnoticed, that all these movements are made along a rigid grid defined by the columns, cornices and entablatures.The stiffness that corresponds to the geometry of the facade and the plan is defined by three equilateral triangles, alternating and aligned. The geometry of the interior of the church is generated from two equilateral triangles.

In fact, what best shows the drawing is this grid, located on a plan parallel to the projection, as well as the opposition between the continuity of the curved entablature and the vertical line that ,starting from the door, reaches the oval supported by some angels, up to the lantern of the dome, a little further back. We can see a contrast between the rigidity of this grid and the smoothness of the curve, to understand that the apparent movement of the facade, the feeling transmitted by the curves, is based on a rational, immovable order. In other words, those of Giulio Carlo Argan (referring to the architecture of Borromini), the interior impulse is born out of the rigor and only by this rigor the interior impulse can be exalted into beauty.

It is also the case of the architect John Soane, in 1814, in one of his lectures at the Royal Academy in London.

There are drawings such as this one that have an enigmatic capacity that induces to be copied over and over. The value of the drawing called Insignium stands probably as its own contradiction, since the grid is the part best shown, although what is actually meaning to highlight is the curvature. It is not an objective description. It shows the real dimensions and angles, but tries to condition a way of understanding the façade. We fail to see the curves, but the illusion aroused by the shadows is stronger than the vision of the curves itself.