It is odd but true. Although we are surrounded by cast shadows, we take very little notice of them. They are the insubstantial consequence of an object interrupting light, and because we are generally more focused on objects themselves, we tend to ignore or discount the shadows the objects cast.
The art historian Michael Baxandall, in his newly published book "Shadows and Enlightenment," sums it up by observing: "Cast shadows are what everyone sometimes attends to."
Painters perhaps attend to shadows - even cast shadows - more than most people. But even they are highly selective, as was shown by a recent exhibition at the National Gallery in London and discussed in the small book that accompanied that show called "Shadows: The Depiction of Cast Shadows in Western Art." There are cultures and periods in which painters have largely overlooked cast shadows as a factor of the visible world they depict.
But, as Prof. E.H. Gombrich, author of the book and selector of the exhibition, asserts, the absence of cast shadows in a painting is more likely to have been a conscious choice than an unconscious or forgetful omission.
"We must never assume that artists did not see what they did not paint," Professor Gombrich writes in the book.
Most cast shadows we see in moderate light are subtle and far less noticeable than the objects that cast them or the objects or surfaces on which they are cast. But in strong light, cast shadows can appear more intense and vivid than the object that interrupts the light.
When painters choose to depict cast shadows, it can be because they find them fascinating phenomena in themselves; or it can be as a tool to enhance the realism of their picture. Gombrich's exhibition and book draw attention to some of the different ways artists have used cast shadows.
Of "A Man seated reading at a Table in a Lofty Room," once considered a Rembrandt but now given to his "school" or "a follower," Gombrich observes: "The light streaming through the glass panes and the open window almost dazzles the beholder, making it hard to discover the figure and the objects."
The cast shadows in this painting also extend the viewer's knowledge of the window through which the light streams: Its divisions and panes are projected. And these shadows help us to sense the kind of surface of which the walls of the "lofty room" are made, thus adding to the specific atmosphere of this setting.
"Peasants Under the Trees at Dawn," by Jean-Baptiste-Camille Corot, is a superb example of the French painter's capacity to evoke effects of "contre-jour" (against the light). And, once more, cast shadows are used to increase the conviction that light is actually shining within, and out of, the painting. So "true" are the tones of this picture, and so convincing are the cast shadows in the foreground, that we accept the clear sky not only as vast and open space but as the source from which all the light in this picture radiates.
Gombrich comments: "Corot ... softens the shadow of the fallen tree and of the goose and thus convincingly suggests the mellow light of morning or evening. His painting therefore also illustrates the creation of a particular mood by means of shadows notably, of course, the lengthening of shadows caused by the rising or setting sun...."
Hendrick ter Brugghen's "The Concert" is a wondrous evocation of all sorts of lights and shadows (not just cast shadows) in an interior at night. Gombrich draws attention to the "conspicuous shadow thrown by the flute on the cheek of the player illuminated by the candle in the centre."
In this painting, the play of shadows seems almost to outweigh the artist's interest in the light that is the primary cause of those shadows. The shadows make for a feeling of enclosed, concentrated mystery.
Since the subject of this picture is music - something that is a succession of quickly passing moments - the emphasis on the strength of shadows, in particular that extraordinary touch of the cast shadow on the cheek, is a visual metaphor for the transitory; for the intense insubstantiality of music.