emission theory (vision)

In the fifth century BC, Empedocles postulated that everything was composed of four elements; fire, air, earth, and water.DK frag. B17 (Simplicius of Cilicia, Physics, 157–159). 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.{{cite book |last=Finger|first=Stanley |title=Origins of neuroscience: a history of explorations into brain function |publisher=Oxford University Press |location=Oxford [Oxfordshire] |year=1994 |pages=67–69 |isbn=978-0-19-506503-9 |oclc=27151391 }} If this were true, then one could see during the night just as well as during the day, so Empedocles postulated that there were two different types of emanations that interacted in some way: one that emanated from an object to the eye, and another that emanated from the eye to an object. He compared these outward-flowing emanations to the emission of light from a lantern.

Around 400 BC, emission theory was held by Plato.Arnold Reymond, History of the Sciences in Greco-Roman Antiquity, Methuen and Co., Ltd., 1927, p. 182.Plato, Timaeus, sections 45b and 46b.

Around 300 BC, Euclid wrote Optics and Catoptrics, in which he studied the properties of sight. Euclid postulated that the visual ray emitted from the eye travelled in straight lines, described the laws of reflection, and mathematically studied the appearance of objects by direct vision and by reflection.

Ptolemy (c. 2nd century) wrote Optics, a work marking the culmination of the ancient Greek optics, in which he developed theories of direct vision (optics proper), vision by reflection (catoptics), and, notably, vision by refraction (dioptrics).

Galen, also in the 2nd century, likewise endorsed the extramission theory (De Usu Partium Corporis Humani). His theory contained anatomical and physiological details which could not be found in the works of mathematicians and philosophers. Due to this feature and his medical authority, his view held considerable influence in the pre-modern Middle East and Europe, especially among medical doctors in these regions.[https://www.stanford.edu/class/history13/earlysciencelab/body/eyespages/eye.html "A History of the Eye"]

Adherents of emission theory cited at least two lines of evidence for it.

File:Raccoon red eye.JPG's eyes brightly reflect a camera flash]]

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 their vision improved when someone with good eyes looks at the same objects.

Doesschate, G. T. (1962). Oxford and the revival of optics in the thirteenth century. Vision Research, 1, 313–342.

Some argued that Euclid's version of emission theory was purely metaphorical, highlighting mainly the geometrical relations between eyes and objects. The geometry of classical optics is equivalent no matter which direction light is considered to move because light is modeled by its path, not as a moving object. However, his theory of clarity of vision (the circular appearance of far rectangular objects) makes sense only if the ray emits from eyes. Alternatively, Euclid's can be interpreted as a mathematical model whose only constraint was to save the phenomena, without the need of a strict correspondence between each theoretical entity and a physical counterpart.

Measuring the speed of light was one line of evidence that spelled the end of emission theory as anything other than a metaphor.{{cn|date=August 2023}}

File:Thesaurus opticus Titelblatt.jpg

Alhazen was the first person to explain that vision occurs when light reflects from an object into one's eyes.{{cite book|last=Adamson|first=Peter|title=Philosophy in the Islamic World: A History of Philosophy Without Any Gaps|url=https://books.google.com/books?id=KEpRDAAAQBAJ|date=2016|publisher=Oxford University Press|isbn=978-0-19-957749-1|page=77}}

The rise of rationalist physics in the 17th century led to a novel version of the intromissionist theory that proved extremely influential and displaced any legacies of the old emissive theories. In Cartesian physics, light was the sensation of pressure emitted by surrounding objects that sought to move, as transmitted through the rotatory motion of material corpuscles.{{cite book|title=Revolutionizing the Sciences: European knowledge in transition, 1500-1700|edition=3rd|first=Peter|last=Dear|publisher=Princeton University Press|location=Princeton|orig-date=2001|year=2019|lccn=2018963374|isbn=978-0-691-19434-9|pages=91–94}} These views extended to Isaac Newton's corpuscular theory of light,Geoffrey Cantor, Optics after Newton: Theories of Light in Britain and Ireland, 1704-1840 (Manchester: Manchester University Press, 1983), 11-12, 24-26. and would be adopted by John Locke and other the 18th-century luminaries.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. {{doi|10.1353/ecy.2010.0006}}.

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

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. [https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12094435&dopt=Abstract].

{{Clarification|reason=Which adults? Describe the experiment, the hypothesis, sample size, etc. This is a bold claim|date=August 2023}}

Rupert Sheldrake claims to have found evidence for emission theory through his experiments in the sense of being stared at.{{cite book|last=Sheldrake|first=Rupert|title=The Sense of Being Stared at: And Other Unexplained Powers of Human Minds|url=https://books.google.com/books?id=ZU2buVGmevQC|date=2003|publisher=Park Street Press|isbn=9780099441533}}

Sometimes, the emission theory is explained by analogy with echolocation and sonar. For example, in explaining Ptolemy's theory, a psychologist stated:{{cite bioRxiv|biorxiv=10.1101/371948|first1=P.|last1=Linton|title=Do We See Scale?|date=4 August 2018}}

"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.Lindberg, D., Theories of Vision from Al-kindi to Kepler, University of Chicago Press (1976), pp. 15–17, Smith, A. (2018). Greek Optics. In A. Jones & L. Taub (Eds.), The Cambridge History of Science (The Cambridge History of Science, pp. 413–427). Cambridge: Cambridge University Press, p.418

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Category:Obsolete theories in physics

Category:Visual perception

Category:History of optics