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 This theory has been replaced by intromission theory (or intromissionism), which states that visual perception comes from something representative of the object (later established to be rays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.

For Newton's proposal that light is emitted from luminous objects in the form of particles or corpuscles, see Emission theory.

History

In the fifth century BCE, Empedocles postulated that everything was composed of four elements; fire, air, earth, and water.[1] 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.[2] 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 400 BCE, emission theory was held by Plato.[2][3][4]

Around 300 BCE, 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.

In 55 BCE, Lucretius, a Roman who carried on the ideas of earlier Greek atomists, wrote:

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.

Ptolemy (c. 2nd century) wrote Optics, a work about the refraction of light, in which he developed a theory of vision that objects are seen by rays of light emanating from the eyes.

Galen, also in the 2nd century, likewise endorsed the extramission theory (De Usu Partium Corporis Humani).[2] Due to his medical authority, his view held considerable influence in Europe for much of the following thousand years.[5]

Evidence for the theory

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.[6]

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.

Refutation

Front page of the Opticae Thesaurus, which included the first printed Latin translation of Alhazen's Book of Optics. The illustration incorporates many examples of optical phenomena including perspective effects, the rainbow, mirrors, and refraction.

Alhazen was the first person to explain that vision occurs when light bounces on an object and then is directed to one's eyes.[7]

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.[8]

Persistence of the theory

Winer et al. (2002) have found evidence that as many as 50% of adults believe in emission theory.[9]

Relationship with echolocation

While emission theory does not correctly explain vision, it does correctly describe the mechanism underlying echolocation and sonar. Namely, rays are emitted from the sensing organism or device, and information about the environment is inferred from the rays reflected back by objects.[10]

References

1. DK frag. B17 (Simplicius of Cilicia, Physics, 157–159).
2. Finger, Stanley (1994). Origins of neuroscience: a history of explorations into brain function. Oxford [Oxfordshire]: Oxford University Press. pp. 67–69. ISBN 978-0-19-506503-9. OCLC 27151391.
3. Arnold Reymond, History of the Sciences in Greco-Roman Antiquity, Methuen and Co., Ltd., 1927, p. 182.
4. Plato, Timaeus, sections 45b and 46b.
5. "A History of the Eye"
6. Doesschate, G. T. (1962). Oxford and the revival of optics in the thirteenth century. Vision Research, 1, 313-342.
7. Adamson, Peter (2016). Philosophy in the Islamic World: A History of Philosophy Without Any Gaps. Oxford University Press. p. 77. ISBN 978-0-19-957749-1.CS1 maint: ref=harv (link)
8. Swenson, Rivka. (Spring/Summer 2010). Optics, Gender, and the Eighteenth-Century Gaze: Looking at Eliza Haywood’s Anti-Pamela. The Eighteenth Century: Theory and Interpretation, 51.1-2, 27-43.
9. Winer, G. A., Cottrell, J. E., Gregg, V., Fournier, J. S., & Bica, L. A. (2002). Fundamentally misunderstanding visual perception: Adults' beliefs in visual emissions. American Psychologist, 57, 417-424. .
10. Linton, P. (4 August 2018). "Do We See Scale?". bioRxiv 10.1101/371948.