The interior of San Carlino picks up and intensifies the alternating convex and concave wall treatment, with such a profusion of columns and niches that it is hard to discern the complicated geometric floor plan. Nor do you get any help in grasping the true shape of the church from looking up at the dome, which is oval-shaped and does not seem to be an extension of the highly confined area for the human being below - but rather a soaring intrusion into Heaven above, bathed in light from semi-hidden windows. Symbols are central to Borromini's approach to building. Many of the shapes were taken from ancient Roman buildings and pther architectural models, but their use by Borromini was revolutionary for he was trying to design buildings which were not so much walls and floors and ceilings as undefinable magical dimensions which force the beholder into spiritual contemplation. In the cloister, for instance, there are no corner columns holding up the heavy colonnade above, and the alternating shapes of the bays prevent the eye from coming to rest.
Borromini had to build his church on a highly limited and asymmetrical corner site. Consequently the
plan of the church is indeed complicated, and to that epoch-making in the history of architecture. One could say that the basic design
of the plan is elliptical, but the already existing fontain on the cut-off corner was to the effect that the architect had
to omit a chapel from an earlier design. Borromini decided to create a spatial unity by incorporating the chapels into the
ellipse. The shape of the site didn't admit any radiating chapels. The design of the plan shows, that the architect had basic
geometry in mind. Although the adornment of the interior conceals most of the architectural structure, the entablature reveals
the design of the plan. The plan isn't really an authentic ellips; it's a rhomb with circularly respectively elliptically shaped
corners. In reality the plan of the interior is composed of two equally sized equilateral triangles put together. The common
base of the triangles constitutes the transverse axis of the interior plan. The billowing circumference of the executed plan
follows this rhomboid shape with great precision. But, then again, the ground plan can be interpreted in a variety of ways...
The interior of the church is bright, and the dominating colour is white. Sixteen partly engaged columns of the Composite
order support the high entablature. Apart from the main altar there are two lateral altars. You could say that the ellipse of
the plan is indented at four places creating four irregular spaces. Two of these are small chapels. The French painter Pierre
Mignard (1612-1695) has executed the high altar painting St. Carlo Borromeo contemplates the Holy Trinity with Sts. Jean de
Matha e Félix de Valois. Above is the symbol of the Trinitarian order, a cross formed by a red vertical and a blue horizontal bar.
This symbol is also found on the tabernacle, on the front of the altar and on the Trinitarian habit. The statues in the niches
above the doors on the sides of the main altar are of Sts. Jean de Matha (left) and Félix de Valois. The tiny chapel to the right
of the entrance has three devotional paintings (1653) attributed to the 17th century painter Giuseppe Milanese. The Crucifixion
is over the altar, with Scourging of Christ on the left and Crowning with Thorns on the right. The right lateral
altar is dedicated to the Trinitarian St. Miguel de los Santos (1591-1625), and the painting (1847) by Amalia de Angelis shows the
saint in ecstacy, during which the Saviour exchanges hearts with him.
However, the principal and most important architectural feature in the church is the elliptical dome.
Within it are geometric designs (crosses, octagons, hexagons) and a lantern with the symbol of the Holy Spirit. Most of the light
comes from the dome, which has no drum; further light, though not as much, comes through the window above the entrance. A ring
of large, stiffly carved leaves stands out in silhouette against the complicated coffered pattern of the dome, like some strange
crowning balustrade. Borromini set about in an ingenious way to be able to apply the dome onto the irregularly shaped interior.
He designed a complex intermediate surface consisting of coffered semi-domes and pendentives. Oval stucco medallions in the
pendentives illustrate episodes in the lives of the order's founders: Sts. Jean de Matha and Félix de Valois meet,
Pope Innocent III approves the order, The Founders receive the habit, The Ransoming of Captives
(see image at the top of this page). The stucco work in the dome, including the medallions, is by Giuseppe and Giulio Bernasconi.
The cloister of San Carlino is a veritable architectural gem. It is both intimate and monumental, and it has some very striking features typical of Borromini, the most interesting being the subtle neutralisation of the corners. Borromini is supposed to have said that the corner is the enemy of all good architecture, and certainly he almost always manages in some way to deny it. There are several examples of "denial of corners" in his architecture. You find it in the interiors of Oratorio dei Filippini, Santa Maria dei Sette Dolori and Cappella dei Re Magi. Further, in Borromini's architecture a room - even an out-door room such as a cloister - is never a mere aggregate of flat surfaces; it is a stereometrically conceived "hollow form" enclosing the spectator on all sides, and this impression is accentuated when the corners are cut off. In the cloister of San Carlino each corner thus consists of one bay of the colonnade curving slightly inwards together with the balustrade of the upper storey; only the architrave of the upper storey remains straight in these bays. Typical of Borromini is also the rhythmical interplay of flat and round-headed openings: the middle of the short side is an arch flanked by two curved flat-headed bays, while the centre of the long side is a flat-headed bay flanked by two arches.
Another rather unconventional feature in the cloister is the balustrade. Here the balusters are not circular, as is generally the case in High Renaissance architecture, but triangular with slightly concave sides, and every second baluster is upside down, giving a strange impression of flickering movement. Such balustrades were to be a common feature in Borromini's architecture.
The crypt or subterranean church of San Carlino repeats the ground plan of the main church, but in a more simple way. It remains unfinished. The altar niche has an 18th century fresco depicting Jesus on the Cross with the Virgin Mary and St. John. When I had struggled my way up to the church of San Carlino, I felt both warm and a bit tired. The crypt gave me a pleasant coolness. And the small chapel to the left intended to be Borromini's last resting place is still empty...
Some further architectural reflections...