The Beginnings of Sight

May 26, 2009

In the 5’th century BC, the Greek philosopher Empedocles caused a sensation by explaining how eye sight worked.

The eyes, said Empedocles, were like a lantern. Within each eye blazed a special kind of fire – the fire of the goddess Aphrodite. Her magical flames burned without causing injury, casting forth an ‘inner light’ that emanated outwards from the eye until hitting something… and whatever the light hit, you instantly saw. The inner light projected by your ocular lantern reached around you like sensory tentacles. Sight was the consequence of your light touching whatever you looked at.

Empedocles’ theory resonated with many of his fellow Greeks, who had probably noticed that in order to see something, they had to look at it with their eyes open. It was plain that their eyes were filled with flame, for who had not seen an eye flash, or glint, or shine from within like a beacon?

Not everyone was convinced. If your eye was a lantern, then if you looked hard enough with a truly fiery gaze, surely you should be able to see in the dark! Which, try as they might, no one could do. And if the eye emitted light, how was it that you saw both what was near and what was far simultaneously? How was it that the light of Aphrodite’s fire took the same amount of time to travel distances both short and long?

Empedocles and his followers were unperturbed by these minor empirical details. Greek philosophers of the time perceived reality as but an imperfect manifestation of their carefully constructed models – an implementation detail. If reality did not quite work like the model claimed, then reality must (probably) be broken. The philosophers were typically so confident of their logic that most never bothered verifying anything experimentally – who cared if the data fit the model, as long as the model was pretty?

Plato, who was born not long after Empedocles died, refined his theory to address the concerns raised by its detractors. The reason you could not see in a dark room, he explained, was because you needed more than the inner light of your eye lantern to do so. You also had to open the window and let in some external light. External light was made by light sources such as lamps and the sun and was different, but kindred to internal light. In the Timaeus, Plato wrote that eyes emanated a “a substance akin to the light of every-day life” that flowed “in a stream smooth and dense”. By itself, this “stream of vision” was inert, but the catalytic light of an “external and kindred fire” transformed it. It could then “diffuse the motions” of whatever it touched, “causing that perception which we call sight”.

Plato used his optical model to explain how you saw your face in a mirror. You saw your face because of the interplay between the internal light from your eye, some external light, and a third kind of light being given off spontaneously by your face! Sight happened when your vision stream ran into the light being given off by the objects you were looking at. Your vision stream smacked into the light coming off your face precisely on the surface of the mirror. And on that surface, you saw your ugly mug. 

Plato’s model of vision had most of his students and fellow Greeks nodding in agreement. Except his best student, Aristotle, who thought it such bunk that he ridiculed it for posterity in his book On Sense and the Sensible. Aristotle was convinced that the tired old philosophers of the past had it figured backwards. What looked like fire in the eyes was only reflection and optical illusion. The claim that sight should happen at the point of contact between the many sorts of light – was labored and “irrational”! The simpler and more logical explanation was that sight happened within the eye!

Aristotle theorized that objects somehow managed to perturb the medium (air) that lay in between them and the observer. These perturbations spanned the medium instantaneously, entered the eye and triggered sight. The reason you had to have your eyes open to see was not to let your inner light get out, but to let the perturbations and changes in the medium get in! And color was just variations in perturbation. Different colors generated differing changes in the medium, each of which triggered a unique sensation in the eye.

Aristotle argued his case as only he could – with logical derivation from observation accompanied by the absence of concrete experimental data. But he made no long term headway against the Platonic model, perhaps because unlike Plato, he offered no conjecture on how mirrors worked. For most Greeks, eyes continued to behave like lanterns.

In the 4th Century BC, the Atomist philosopher Epicurus offered a radical alternative, derived from the work of the Atomist pioneer Democritus. Epicurus believed you saw because of what came off the objects you looked at. All sensations, said Epicurus, were the consequence of contact. Vision was the sensation caused by contact with images. And images were immeasurably thin atomic replicas or outlines of objects that the objects perpetually gave off – like a constant shedding of skin. The images or eidola floated away from the objects into the void until they drifted into your eyes, and thence into your mind, where they produced the sensation of sight.

The implications of a universe seething with eidola were immense.  For one, it meant that you did not actually have to look at something to see it – all you had to do is run into its eidola. And eidola also implied that you could potentially see in the dark. Most interestingly of all, eidola could give you a permanent feeling of déjà-vu and genuine hind-sight, since technically, any eidola that was not sensed by an eye just lived on. Critics wondered how the eidola, despite being atomic replicas of their sources and hence of the same size, squeezed through the small aperture of pupil into the insides of the eye! It is not known how deeply Epicurus thought about the miraculous implications of his model (or the logic defying ones) – but clearly, the Epicureans were none too concerned, as they adopted the theory enthusiastically.

Euclid gave Plato’s ideas a mathematical foundation by applying geometry – a subject he knew a thing or two about. In his treatises Optics and Catoptrics, Euclid used the concept of the ray to explain sight, reflection and perspective. There were two kinds of rays – sight rays and light rays. Rays travelled in straight lines and with infinite speed, arriving at their destinations, whether near or far, instantaneously. Why the rays did any of this, or if they did any of this at all, Euclid did not state – probably because he had no idea. Euclid’s sight rays (internal light) shot out of the eye lantern in an expanding cone. Sight rays mixed with light rays (external light) and produced sight by colliding with the light rays emitted by individual objects. Mirrors operated by reflecting sight rays. Euclid’s ray geometry supplied the first understanding of perspective.

And so it went, until by the 1st Century CE, Ptolemy had extended and adapted the Euclidean principles to explain refraction and other optical mysteries. The Euclidean/Ptolemaic theory effectively ended the debate over the mechanics of sight – at least in the Greek and Roman world, for there is evidence that thinkers in ancient India and elsewhere may have developed to a more modern theory of vision. But in much of the world, the lantern of the human eye shone brightly for a thousand more years.

It was not until the 11’th century CE that an Arab genius from an intellectual hotbed called Basra (yes, the same one, and it had already existed for 1300 years before Saddam was even born) finally saw the world with a fresh set of eyes – a distinctly familiar and modern set of eyes. And by the time the great Alhazen was done looking, he had invented the scientific method and penned one of the greatest scientific works of all time: The Book of Optics. Alhazen produced a methodical and experimentally sound theory of sight, of reflection, refraction, of lenses, mirrors, ocular structure, pin-hole cameras, and a whole host of other optical subjects that nobody had ever thought correctly about. Alhazen offered not conjecture, but theories backed by experimental verification! Alhazen was the first recorded individual to scientifically prove that sight was the consequence of rays of light, reflecting off objects and entering the eye to form an image!

The science of Alhazen and the giants who followed – Newton, Huygens, Maxwell – should have extinguished the fires of Aphrodite for good. But they failed. In 2002, a study published in the American Psychologist journal showed that nearly 50% of college students continue to believe that the eye gives off the light that produces vision! Empedocles’ lantern of the eye and the flames of Aphrodite still light the visible world.

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