Emission theory (vision)
Emission theory or extramission theory (variants: extromission) or extromissionism is the proposal that
History
In the fifth century
Around 400 BC, emission theory was held by Plato.[2][3][4]
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).[2] 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.[5]
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 their vision improved when someone with good eyes looks at the same objects.[6]
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.[citation needed]
Refutation
The rise of
Persistence of the theory
Winer et al. (2002) have found evidence that as many as 50% of adults believe in emission theory.[11][clarification needed]
Relationship with echolocation
Sometimes, the emission theory is explained by analogy with echolocation and sonar. For example, in explaining Ptolemy's theory, a psychologist stated:[12]
"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.[13]
References
- DK frag. B17 (Simplicius of Cilicia, Physics, 157–159).
- ^ OCLC 27151391.
- ^ Arnold Reymond, History of the Sciences in Greco-Roman Antiquity, Methuen and Co., Ltd., 1927, p. 182.
- ^ Plato, Timaeus, sections 45b and 46b.
- ^ "A History of the Eye"
- ^ Doesschate, G. T. (1962). Oxford and the revival of optics in the thirteenth century. Vision Research, 1, 313–342.
- ISBN 978-0-19-957749-1.
- LCCN 2018963374.
- ^ Geoffrey Cantor, Optics after Newton: Theories of Light in Britain and Ireland, 1704-1840 (Manchester: Manchester University Press, 1983), 11-12, 24-26.
- .
- ^ 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. [1].
- bioRxiv 10.1101/371948.
- ^ 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