Emission theory or extramission theory (variants: extromission, extromittism) is the proposal that visual perception is accomplished by eye beams emitted by the eyes. This theory has been replaced by intromission theory, which states that visual perception comes from something representative of the object (later established to be rays of light reflected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 BCE, 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 BCE, Euclid wrote Optica, 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. – On the nature of the Universe
Despite being similar to later particle theories, Lucretius's views were not generally accepted; light was still theorized as emanating from the eye.
Adherents of emission theory cited at least two lines of evidence for it.
The custom of saluting is said by some to stem from the habit of Greek soldiers putting their hands up in front of their eyes to "shade" their eyes from the powerful "light" shining from the eyes of their commanders. 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. 
Winer et al. (2002) have found evidence that as many as 50% of adults believe in emission theory.
Eyes are organs of the visual system. They provide organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and convert it into electro-chemical impulses in neurons. In higher organisms, the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system. Image-resolving eyes are present in molluscs, chordates and arthropods.The simplest "eyes", such as those in microorganisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms. From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment and to the pretectal area to control the pupillary light reflex.Eye beam
In the physics inherited from Plato (although rejected by Aristotle), an eye beam generated in the eye was thought to be responsible for the sense of sight. The eye beam darted by the imagined basilisk, for instance, was the agent of its lethal power, given the technical term extramission.
The exaggerated eyes of fourth-century Roman emperors like Constantine the Great (illustration) reflect this character. The concept found expression in poetry into the 17th century, most famously in John Donne's poem "The Extasie":
In the same period John Milton wrote, of having gone blind, "When I consider how my light is spent", meaning that he had lost the capacity to generate eye beams.
Later in the century, Newtonian optics and increased understanding of the structure of the eye rendered the old concept invalid, but it was revived as an aspect of monstrous superhuman capabilities in popular culture of the 20th century.
The emission theory of sight seemed to be corroborated by geometry and was reinforced by Robert Grosseteste.In Algernon Swinburne's "Atalanta in Calydon" the conception is revived for poetic purposes, enriching the poem's pagan context in the Huntsman's invocation of Artemis:
In T.S. Eliot's rose garden episode that introduces "Burnt Norton" eyebeams persist in the fusion of possible pasts and presents like unheard music:
The New Zealand poet Edward Tregear instanced "the lurid eye-beam of the angry Bull"— Taurus of the zodiac— among the familiar stars above the alien wilderness of New Zealand.In computer graphics, the concept of eye beams is fruitfully resurrected in ray tracing (in which the bouncing of eye beams around a scene is simulated computationally).Tapetum lucidum
The tapetum lucidum (Latin: "bright tapestry; coverlet", plural tapeta lucida) is a layer of tissue in the eye of many vertebrates. Lying immediately behind the retina, it is a retroreflector. It reflects visible light back through the retina, increasing the light available to the photoreceptors (although slightly blurring the image). The tapetum lucidum contributes to the superior night vision of some animals. Many of these animals are nocturnal, especially carnivores, while others are deep sea animals.
Similar adaptations occur in some species of spiders. Most primates, including humans, lack a tapetum lucidum, and compensate for this using perceptive recognition methods.