The Ancient Idea That Vision Worked By Shooting Elemental Fire Out Of Your Eyeballs, And Touched The Objects You Were Seeing

That all makes sense, is something we all get our heads around in childhood, and has the obvious advantage of being correct. But it took quite a few whacky missteps before we got there. 

One of those missteps (keeping in mind that we are apes who have somehow developed intelligence, give ancient people a break) is that several ancient thinkers believed vision was caused by a substance being emanated from your eyeballs and out into the world, like a kind of feeler. 

The idea, though it seems odd to modern thinking, was popular with respected intellectuals, including Ptolemy, Galen, Euclid, and Plato. 

This attempt to explain eyesight evolved over time, but the basic hypothesis was that some substance came out of the eye, either feeling the objects directly or else interacting with light in the environment. Essentially, your eyes were thought to store this substance, which they beamed out into the world like Superman's laser vision.

"As when a man who intends to make a journey prepares a light for himself, a flame of fire burning through a wintry night," Empedocles wrote, "in the same way the elemental fire, wrapped in membranes and delicate tissues, was then concealed in the round pupil."

Again, it may seem to be a silly idea, but these were attempts to explain what we observed around us, with no handy textbooks lying around to point out the errors. 

In particular, this idea was thought to explain why objects tend to blur the further you get away from them: they were being hit less by the substance you have inside your eyeballs, of course, because they were further away.

As well as this, the concept was thought to explain why you get dizzy while looking at something that spins: your eye feelers are getting all tangled up as they try to keep track of the rotating object. 

It also perfectly described why your vision goes away whenever you close your eyes and could be used to explain (wrongly, of course) why people can see light when pressure is applied to their eyeballs.

More than meets the eye

Time went on, and people began to notice that this version of events didn't explain a lot of what we saw around us. For example, why the Sun hurts our eyes, or why we can see distant stars and planets at all (once we had some sort of sense of how unfathomably distant they are). 

When you look at the Moon, are you sending your eye juice all the way up to have a feel of its craters? Actually, scratch that, when you are looking at poop, does that mean your eyeball is touching the poop?

Of course, it wasn't just a case of going "Aha, then light must be bouncing into our eyeballs!" One new idea was that every object instead emitted a perfect, tiny little copy of itself, and it was this that made it into your head.

"The concept of some copy of objects, carried through the air to the eye, was to have widespread and long lasting appeal, and it was referred to by many names, including eidola, simulacra, species, images," a paper on the topic explains. 

"There was an obvious source of observational support for such a theory: the image of an object could be seen reflected from the eye of an observer."

Around 1000 CE, Abū 'Alī al-Hasan ibn al-Hasan ibn al-Haytham, known as Alhazen for short, found substantial evidence against this idea and published it in his seven volume work on optics. Though it will appear pretty basic now, it was the result of some innovative scientific thinking.

One experiment involved Alhazen  standing in a darkened room with a hole cut in one of the walls. Behind this wall, he placed two lanterns at different heights, and noted that depending on which lamp was illuminated, the beam would shine on a different part of the room. 

As well as this, covering the lanterns resulted in the spot disappearing, and revealing the lantern made it reappear again. 

It may sound like a simple game of peekaboo with a lamp, but Alhazen had shown that certain objects emit light, which travels in straight lines, rather than being shot out of everyone's eyeballs.

In a second, more sophisticated example, he again utilized a hole. This time it was a tiny pinhole in a dark room, through which light from outside could get in and create a camera obscura effect, projecting the outside scenery onto the back of the dark room. 

This showed, though it would still take us a long time to really pin things down, that images could be created with no need for eyeballs that shoot out feelers all the way to the freaking Moon. Which, on balance, is a shame.