Emission theory (vision)
Emission theory or extramission theory (variants: extromission) or extromissionism is the proposal that visual perception is accomplished by eye beams emitted by the eyes. This theory has been replaced by intromission theory (or intromissionism), which states that visual perception comes from something representative of the object (later established to be rays of light reflected from it) entering the eyes. Modern physics has confirmed that light is physically transmitted by photons from a light source, such as the sun, to visible objects, and finishing with the detector, such as a human eye or camera.
In the fifth century BC, Empedocles postulated that everything was composed of four elements; fire, air, earth, and water. He believed that Aphrodite made the human eye out of the four elements and that she lit the fire in the eye which shone out from the eye, making sight possible. If this were true, then one could see during the night just as well as during the day, so Empedocles postulated an interaction between rays from the eyes and rays from a source such as the sun.
Around 300 BC, Euclid wrote Optics, in which he studied the properties of light. Euclid postulated that light travelled in straight lines and described the laws of reflection and studied them mathematically. He questioned that sight is the result of a beam from the eye, for he asked how one sees the stars immediately, if one closes one's eyes, then opens them at night.
The light and heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove. – Lucretius, De rerum natura
Despite being similar to later particle theories, Lucretius's views were not generally accepted; light was still theorized as emanating from the eye.
Galen, also in the 2nd century, likewise endorsed the extramission theory (De Usu Partium Corporis Humani). Due to his medical authority, his view held considerable influence in Europe for much of the following thousand years.
Evidence for the theory
Adherents of emission theory cited at least two lines of evidence for it.
The light from the eyes of some animals (such as cats, which modern science has determined have highly reflective eyes) could also be seen in "darkness". Adherents of intromission theory countered by saying that if emission theory were true, then someone with weak eyes should have his or her vision improved when someone with good eyes looks at the same objects.
Most argue that Euclid's version of emission theory was purely metaphorical, highlighting only the geometrical relations between eyes and objects. The geometry of classical optics is equivalent no matter which direction light is considered to be moving in, since light is modeled by its path, not as a moving object.
Measuring the speed of light was one line of evidence that spelled the end of emission theory as anything other than a metaphor.
Isaac Newton, John Locke, and others, in the 18th century, firmly held, by contrast, that vision was not only intromissionist or intromittist, but rays that proceeded from seen objects were composed of actual matter, or corpuscles, that entered the seer's mind by way of the eye.
Persistence of the theory
Winer et al. (2002) have found evidence that as many as 50% of adults believe in emission theory.
Relationship with echolocation
"Ptolemy’s ‘extramission’ theory of vision proposed scaling the angular size of objects using light rays that were emitted by the eyes and reflected back by objects. In practice some animals (bats, dolphins, whales, and even some birds and rodents) have evolved what is effectively an ‘extramission’ theory of audition to address this very concern. "
Note this account of the Ptolemaic theory ('bouncing back of visual ray') differs from ones found in other sources.
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