The Dark Room

During a partial solar eclipse, if you stand under a leafy tree and look down, the ground is covered in crescents. Hundreds of them, scattered across the dirt and the pavement, each a curved sliver of light, all leaning the same way. They are images of the sun — the sun as it is at that moment, bitten into a crescent by the moon — and they are being cast by the gaps between the leaves. Every small opening in the canopy is doing it. When the eclipse passes, the crescents fill back into discs; the leaves have been projecting round images of the whole sun all along, every ordinary afternoon, and you never noticed because a blur of light on the ground looks like nothing at all. Nobody built this. It is just what light does when it passes through a small hole.

That is the whole principle of the camera obscura. It was never really invented. It was noticed, again and again, by people who had no contact with one another. In a Mohist text traditionally dated to the fourth or fifth century BCE, someone in China wrote it down — light entering a darkened room through a pinhole, forming an inverted image on the far wall, the ray from a person's foot arriving at the top and the ray from the head at the bottom, crossing at the aperture. A Greek text handed down under Aristotle's name, the Problemata, asked why sunlight through a square gap or a wicker sieve still lands as a round spot, and why the eclipse makes it a crescent. Around the year 1000, in Cairo, Ibn al-Haytham built dark chambers to study it properly and gave the crucial proof: he set several lamps outside a room with one small hole, and inside saw several separate spots of light, each lined up straight with its own lamp, none of them mingling. The image was not something the room manufactured. It was already there in the light, fully formed and sorted, and the hole merely let a thin slice of it through onto the wall where the darkness let it show.

This is the strange thing about the device, and it is easy to walk past. Every other optical instrument does something to the light to give you more of it: a lens gathers, a telescope magnifies, a microscope resolves. The pinhole does almost nothing. It takes light away — you darken the room — and admits a single narrow cone through one small opening. What that reveals is that the world has been painting itself, ceaselessly, on every surface, through every aperture, and that we fail to see these paintings only because each is drowned in all the others: every point of every wall lit from every direction at once, the images layered into a wash. The darkness clears the wash; the hole selects one image out of it. The device does not make the picture. It isolates one that was there the whole time — in colour, moving, upside down.

Upside down, and reversed left to right — not flipped like a mirror but rotated a full half-turn, because every ray travels straight and they all cross at the pinhole: the top of the scene sent to the bottom of the image, the left sent to the right, in the same crossing. For a long time this inversion was not a curiosity but a scandal, and the scandal was about the eye. The old optics, Ibn al-Haytham's included, held that the sensitive part of the eye was the lens at the front, and it resisted the obvious idea that an image forms at the back of the eye precisely because that image would be upside down — an inverted picture of the world seemed an impossible thing for the soul to be handed. Elaborate refractions were proposed at the back of the lens to keep the internal picture the right way up.

It was Kepler, in 1604, who stopped defending against the inversion and simply accepted it. In the same book he gave the device its name — camera obscura, "dark chamber" — and located the same dark chamber inside the skull. The eye is one: the pupil is the hole, the lens focuses, and a real, inverted image falls on the retina at the back, which is the screen. He called it a pictura, a painting. Fifteen years later a Jesuit named Christoph Scheiner cut away the back coats of an ox's eye until they were translucent — some accounts say a human one — held it up, and saw the little inverted world glowing on the retina directly, exactly as Kepler had said. The dark room and the eye turned out to be the same object at two scales.

Here Kepler did something stranger than answering. Having gotten the optics exactly right — retina, inversion, focus — he declined to explain why we see the world upright anyway. That, he wrote, was not a question of optics or of anatomy; it happened somewhere in the brain, and he left it there. He had followed the light to the back of the eye and stopped. The geometry ends at the retina; the seeing is on the far side. A century later philosophers were still arguing about why the world looks upright.

There is one more turn, because the dark room did not stay in the laboratory. Once della Porta recommended it to painters, and Barbaro fitted a lens and a stop to brighten and sharpen the picture, you could trace the projection on the wall. The lens made the dark room, at last, an instrument like the others — one that adds light rather than only taking it away. And that raises a question about a great deal of Western painting. The artist David Hockney and the physicist Charles Falco argued that the sudden, uncanny realism of European art from the 1420s onward came in part from artists secretly tracing optical projections — that the "look" of truth we credit to genius was, in places, transcription. Their evidence is entirely in the paintings: a carpet's perspective that shifts as if refocused, highlights on Vermeer's bread and metalwork rendered as soft glowing discs, what Falco reads as the signature of a lens throwing an out-of-focus point of light, which the naked eye does not produce. The strongest objection is not that they are wrong but that it may be unprovable: there is no document, no notebook, no purchase order for most of it, only the paintings testifying about themselves, and a lens leaves aberrations the paintings mostly do not show. It is a live and unfinished fight. The dark room hands us that too — a picture so faithful you cannot be sure whether a hand or a lens laid it down.

We shortened the name in the end. When the box got small enough to carry and someone put a light-sensitive plate where the wall had been, "camera obscura" lost its second word, and the dark chamber became simply the camera — the thing in every pocket now. The oldest version of it needs no plate and no lens and no pocket. It is running under the tree during the eclipse, throwing its crescents on the ground.