Rosa, Salvator: Desolate Landscape with Two Figures (1660)

In Flann O'Brien's novel, The Third Policeman, we're introduced " mainly in the footnotes " to the ideas of a fictional mad scientist called De Selby. De Selby propounded several untenable theories, for example, that the world was not round but shaped liked a sausage. Most memorably, he 'held that darkness was simply an accretion of 'black air', ie, a staining of the atmosphere due to volcanic eruptions too fine to be seen with the naked eye, and also to certain 'regrettable' industrial activities involving coal-tar by-products and vegetable dyes'.

The comedy is partly in the ideas themselves, but more in the earnest attempts of various learned commentators to take De Selby seriously and give a patient hearing to his unfeasible speculations. This notion that darkness is a kind of subtle gas or vapour that increases at certain times of day, and collects in certain dark places " surely, they argue, there are some grounds for scepticism.

'One difficulty is precisely this question of terms. A 'dark place' is dark merely because it is a place where darkness 'germinates', and 'evening' is a time of twilight merely because the 'day' deteriorates owing to the stimulating effect of smuts on the volcanic processes. De Selby makes no attempt to explain why a 'dark place' such as a cellar need be dark.' No, and he doesn't have much of an answer to candles and electric light, either, save 'the statement that 'black air' is highly combustible, enormous masses of it being instantly consumed by the smallest flame, even an electrical luminance isolated in a vacuum'. In his experiments designed to 'bottle night', De Selby unfortunately always used opaque vessels.

De Selby may be cracked. But his sense of darkness as a positive, active force, as not just an absence of light, has a long pedigree. His 'black air' theory seems to owe something to John Ruskin's genuine belief that, as the 19th century progressed, the world was getting darker. It was infected by a plague wind, a wind of darkness, that blighted even the pure atmosphere of his beloved Alps. 'The light which once flushed these pale summits is now umbered and faint; the air which once inlaid the clefts of all their golden crags with azure is now defiled with languid coils of smoke, belched from worse than volcanic fires.'

Ruskin's vision (or hallucination) seems to be a premonition of industrial pollution, expressed with the imagery of biblical pestilence and infernal fumes " and that imagery is ancestral. Night is an old enemy. The powers of darkness are powers indeed. Milton's hell is filled with 'darkness visible'. The plague of darkness in the Book of Exodus is 'a darkness which may be felt'.

These powers are at work in the art of painting, too. Painters have always had bottled night at their fingertips " it's called black paint. And paintings often seem to be infused with De Selby's 'black air', with an excessive and gratuitous darkness that gathers more thickly and spreads more pervasively than the apparent light sources in the pictures would strictly permit.

Even in the most realistic pictures, shadow can easily get out of control. The pitch-dark backgrounds that provide such a dramatic setting in the work of Caravaggio and his followers " they seem, on reflection, a little far fetched. But the pictures usually get away with them. The eye can't really judge. Who knows? There could be a great pocket of darkness there. But some pictures deliberately strain our eyes' credulity.

Salvator Rosa's Desolate Landscape with Two Figures depicts a small scene, and pre-industrial. But it's devoted to darkness and menace and spatial disorientation. It puts its view in an upright format. The shape of the picture doesn't offer a scene that can be scanned horizontally. It creates a vertical experience of landscape, something to be looked up and down. The terrain itself corresponds.

It consists of a steep crag that tumbles through the picture space. There's an opening to daylight in the top left corner, but there is no horizon, no receding vista. The crag is upfront and flat on. And there isn't much sense of a ground level, either. You can't really say what is supporting the two little figures who sit on the logs down below, or how they really relate to what is beyond them. They are there to look small and powerless.

There's no 'foothold' for the eye. There's just the wall of crumbly rock and foliage, which extends off-picture in all directions, its slopes ascending above the top of the frame, and its cavernous dells sinking beneath the bottom, a sheer face, a sheer drop. Two great broken branches poke into the scene diagonally, coming out of nowhere, crossing in an 'X' in the centre of the picture, in mid- air, making a barrier that blocks our way and further confusing our sense of space.

And behind them, all is darkness " more or less impenetrable, more or less unmotivated. The bottom half of the picture is lost in a void of obscurity. True, you could explain this realistically by imagining that the light falls on to this scene through a dense forest. But realism is hardly the point. Darkness is given its head here, treated as a heavy rolling downpour.

The central effect is a kind of mistaken identity. For surely this upright landscape should really be a waterfall. It has the structure of a mountain torrent, with the water cascading in, top right, rushing down, breaking and scattering on rocks, bringing branches with it, spreading out, filling a wide pool at the bottom. But instead of a torrent of water, an engulfing flood of darkness falls through it. The whole scene is like an enactment of what John Ruskin (who, of course, had quite an eye for these things) said of Salvator Rosa himself: 'Gloom gained upon him and grasped him.'

THE ARTIST

Salvator Rosa (1615-73) invented a sensibility. The Neapolitan artist was a master of gloom and doom. He became a byword for a new type of scary fun, 'horrid beauty', encompassing all that was wild, savage, rude, desolate " landscapes full of ragged rocks, blasted tree-trunks and hair-raising foliage, peopled by hermits, brigands, witches and ghouls. He was an early exponent of 'the artist as outsider', famously cross and melancholy, insisting on his independence and his inspiration, the force of his unbiddable genius. His self-portrait shows him as a scowling, jagged, rugged form swathed in darkness against a cloudy sky.