accommodation

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Optical instruments (such as microscopes, cameras, and some telescopes) work by passing light through the curved surfaces of lenses, where the light is bent (refracted) to bring it to focus at a particular point (e.g. on the film of a camera). Their power depends on the tightness of curvature of these surfaces. To adjust focus for targets at different distances, most optical devices move one or more of their lenses closer to or further from the focusing point. The human eye contains several refractive surfaces, but it alters focus or ‘accommodates’ largely by changing the curvature of one of them — the front surface of the lens inside the eye. The more curved this is, the more powerful the lens becomes, focusing the eye on closer objects. And when the lens flattens, the eye focuses on more distant objects.

This is not the only way of focusing an eye. In chickens, accommodation is produced partly by changing the curvature of the cornea itself. Animals that move between air and water have a special problem, because immersion in water greatly reduces the optical power of the cornea (as anyone who has opened their eyes under water knows). Diving birds gain the extra focusing power that they need under water by squeezing the lens forward into the aperture of the very tough iris, which greatly increases the curvature of the front surface.

The lens of the human eye is a stiff gel of transparent protein, inside an elastic capsule. It is held in place by fine, elastic zonular fibres, which run from the equator of the lens to the muscular ciliary body, which runs around the inner wall of the eyeball, just behind the iris. Accommodation is controlled by the ciliary muscle inside the ciliary body, which encircles the lens. This muscle, which is of the ‘smooth’ type, is controlled by fibres of the parasympathetic division of the autonomic nervous system. When the ciliary muscle contracts, it does not squeeze the lens directly but reduces the tension of the zonular fibres, which relaxes the tension on the capsule of the lens, causing it to assume a more spherical shape, hence increasing the curvature of its front surface.

Accommodation can be prevented with eye drops containing the drug atropine (belladonna — originally extracted from the plant deadly nightshade), which blocks the contraction of the ciliary muscle. The nerve cells that command accommodation (and pupil constriction) are in a small structure in the mid brain called the Edinger-Westphal nucleus, close to many of the cells that direct eye movements.

With the exception of the first few months of life, when accommodation is not fully functional, accommodation becomes progressively less effective with age, with the ability to change lens power falling to zero at an age of about 50 years. This loss of accommodation, or presbyopia, is believed to be caused by changes in the stiffness of the lens proteins, rather than changes in ciliary muscle strength, or in lens size, which increases throughout life. With its accommodation frozen, the eye becomes like a cheap fixed-focus camera, which relies on the depth of field produced by a small aperture to give reasonable focus at different distances. Older people have most difficulty reading fine print (because they have to hold it far away to focus it), especially in dim illumination, when the pupil dilates and the depth of field decreases. Interestingly, people becoming presbyotic do not usually complain that things actually look blurred when held too close, but that what should be black and white looks grey.

To focus on a nearby object, as when reading, the front surface of the lens becomes more curved than when looking at a distant object


The optical quality of the human eye is far from perfect. The great German physicist, Hermann von Helmholtz, once remarked that if he were offered an instrument with such defects by a manufacturer he would send it back — but that he was gladly hanging on to his own eyes, with all their shortcomings! One striking defect, or ‘aberration’, of the eye is that it does not focus light of different wavelengths in the same plane. If deep red is in sharp focus, violet will be somewhat de-focused, and vice versa. Interestingly enough, the retinal cones that are most sensitive to blue- violet light are very few and far between compared with those that are most sensitive in the yellow-green part of the spectrum (the so-called ‘green’ and ‘red’ cones), and are absent in the central fovea (the part of the retina that we point towards things when we look directly at them). Perhaps this is an adaptation to the impossibility of focusing all wavelengths at the same time.

— Stuart Judge

See also atropine; autonomic nervous system; colour blindness; eyes; refractive errors; smooth muscle.

noun

1. The act of making suitable to an end or the condition of being made suitable to an end: adaptation, adaption, adjustment, conformation. See change/persist.
2. A settlement of differences through mutual concession: arrangement, compromise, give-and-take, medium, settlement. Law composition. See agree/disagree.

1. Process by which the shape of the lens in the eye changes so that distant or near objects can be brought into focus. Accommodation, with pupillary constriction and convergence, enables an individual to retain an object in focus as it approaches. This ability is particularly important for ball players.

2. An effect produced on sense organs by continuous and unvarying stimulation so that eventually no sensation is experienced (see habituation).

3. A social process sometimes encouraged by involvement in sporting, by which different racial or political groups adjust to each other and coexist without necessarily resolving underlying differences (compare assimilation).

4. A social process, analogous to biological adaptation, by which a society or individual adjusts to its environment.

5. In Piaget's theory, a mechanism by which a child develops from one stage to the next.

(of the eye)
The lens in the eye of land-living vertebrates focuses by changing the curvature of its surface, a process known as 'accommodation'. In fish the rigid spherical lens moves backwards and forwards as in a camera. Accommodation brings objects of interest, when fixated on the centre of the retinas in animals that have foveae, into sharp focus. This is achieved by 'hunting' for the sharpest image (or highest spatial frequency response of the eye), as there is no available signal indicating whether accommodation is set too far (insufficient curvature of the lens) or too near (too much curvature). Spectacles can correct for too great or too small a curvature of the lens, to give focused images when the eye's accommodation is incorrect or, as almost invariably happens in middle age, when the lens of the eye becomes too inflexible to change its shape to keep the image in focus for a wide range of distances of objects. Spectacles were invented in the 13th century in the West, and earlier in China. Several inventors have been suggested, especially Salvino d'Armato (d. 1312), and Roger Bacon (?1214–94), who also described the magnifying glass.

(Published 1987)

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IN BRIEF: The act of providing something (lodging or seat or food) to meet a need. Also: An adaptation or adjustment.

Often it appears that in this life of experience and accommodation we pay just as dearly for our triumphs as we do for our defeats. — Jack London, Source: The Sea Wolf

LearnThatWord.com is a free vocabulary and spelling program where you only pay for results!

i. The ability of the human eye to adjust itself to give sharp images for different object distances. It is the process by which the lens of the eye can be flattened to focus distant points on the retina or made more convex to focus nearby points on the retina.



Accommodation: adjustment of eye focus on distant object (left) and close object (right).
ii. The limits or range within which a stereo-plotting instrument is capable of operating. For example, a multiplex stereograph can adjust or accommodate for small tilts in the projector, ranging from approximately 10° about the X-axis to 20° about the Y-axis.

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Adjustment, especially adjustment of the eye for seeing objects at various distances. This is accomplished by the ciliary muscle, which controls the lens of the eye, allowing it to flatten or thicken as is needed for distant or near vision.

• amplitude of a. — the total amount of accommodative power of the eye.
• histological a. — changes in morphology and function of cells following changed conditions.

For a list of words related to accommodation, see:

• Physiology - accommodation: adjustment of eye for viewing at various distances by changing shape of lens

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Common misspelling(s) of accommodations

• accomodations

Dansk (Danish)
n. - logi, indkvartering

idioms:

• accommodation address    bekvemmelighedsadresse
• accommodation bill    akkomodationsveksel, proformaveksel, kreditveksel
• accommodation road    adgangsvej, tilkørselsvej, forbindelsesvej

Nederlands (Dutch)
woonruimte, logies, schikking, aanpassing, inschikkelijkheid, accommodatie (optisch), trein/bus etc. die overal stopt, lening noodwoning slaapgelegenheid noodwoning wasgelegenheid

Français (French)
n. - logement, arrangement, adaptation, appartements (npl), chambres à louer (npl)

idioms:

• accommodation address    boîte à lettres, poste restante
• accommodation bill    (Comm) billet ou effet de complaisance
• accommodation road    route à usage restreint

Deutsch (German)
n. - Behausung, Unterkunft, Vergleich, Anpassung, Entgegenkommen

idioms:

• accommodation address    Gefälligkeitsadresse
• accommodation bill    Gefälligkeitsakzept
• accommodation road    private Zufahrtsstraße

Ελληνική (Greek)
n. - κατάλυμα, στέγη, εκδούλευση, εξυπηρέτηση, (οικονομική) διευκόλυνση, δάνειο, συμβιβασμός
attrib. - διευκολυντικός

idioms:

• accommodation address    διεύθυνση αλληλογραφίας
• accommodation bill    (οικον.) γραμμάτιο ευκολίας
• accommodation road    οδός προσπέλασης, βοηθητικός δρόμος

Italiano (Italian)
alloggio, accomodamento, adattamento

idioms:

• accommodation address    domicilio
• accommodation bill    cambiale di comodo
• accommodation road    via di accesso
• emergency accommodation    abitazione provvisoria
• sleeping accommodation    alloggio
• temporary accommodation    alloggio provvisorio
• tied accommodation    residenza ufficiale
• washing accommodation    lavabo

Português (Portuguese)
n. - acomodação (f), alojamento (m), ajuste (m), recuperação (f), conciliação (f)

idioms:

• accommodation address    domicílio (m)
• accommodation bill    letra (f) de câmbio (Fin.)
• accommodation road    estrada (f) de servidão
• emergency accommodation    alojamento de emergência
• sleeping accommodation    dormitório (m)
• temporary accommodation    alojamento temporário
• tied accommodation    acomodação vinculada
• washing accommodation    lavanderia (f)

Русский (Russian)
приспособление, временное жилье, разрешение спора, адаптация, привыкание к чему-либо

idioms:

• accommodation address    адрес данный вместо постоянного адреса
• accommodation bill    вексель
• accommodation road    короткая, ближайшая дорога
• emergency accommodation    приспособления для чрезвычайной ситуации
• sleeping accommodation    место для сна, ночлега
• temporary accommodation    временное проживание
• tied accommodation    жилье связанное со службой
• washing accommodation    приспособления, помещение для стирки белья

Español (Spanish)
n. - hospedaje, habitación, alojamiento, arreglo, acuerdo, convenio, adaptación, reconciliación

idioms:

• accommodation address    dirección del hospedaje
• accommodation bill    letra o efecto de favor o de complacencia
• accommodation road    camino vecinal, camino de acceso

Svenska (Swedish)
n. - logi, anpassning, uppgörelse

中文（简体）(Chinese (Simplified))
适应, 调和, 调节, 和解, 预定膳宿

idioms:

• accommodation address    权宜通信地址
• accommodation bill    空头汇票, 融通汇票, 欠单
• accommodation road    专用道路

中文（繁體）(Chinese (Traditional))
n. - 適應, 調和, 調節, 和解, 預定膳宿

idioms:

• accommodation address    權宜通信地址
• accommodation bill    空頭匯票, 融通匯票, 欠單
• accommodation road    專用道路

한국어 (Korean)
n. - 적응, 화해, 융통, 대부[금], 편의, 숙박시설

日本語 (Japanese)
n. - 便宜, 宿泊設備, 好都合な事, 親切心, 適応, 順応, 調節, 和解, 収容設備

idioms:

• accommodation address    便宜的な宛先
• accommodation bill    融通手形
• accommodation road    特設道路, 私道
• sleeping accommodation    宿泊設備
• tied accommodation    付帯設備

العربيه (Arabic)
‏(الاسم) مسكن, منزل, غرفه, بنايه (الجمع) غرف, بنايات, مساكن‏

עברית (Hebrew)
n. - ‮לינה ואוכל (ברכבת וכו'), אכסון, דיור, מגורים, סיגול, התאמה, חסד, טובה, פשרה, הסדר, הלוואה, נוחות, נוחיות, חדרים מרוהטים‬

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