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87-558: The arch is a semi-circular aluminum structure, which opened like the iris of a human eye. It was designed to be used as a mechanical curtain for the Olympic Medal Plaza's stage. The design was inspired by Utah's natural stone arches , such as Delicate Arch . At the time of its construction, the arch was the largest unfolding structure in the world. It took Hoberman four months to design the arch (with support from Buro Happold ). Specialized knuckle assemblies, which allowed

174-455: A fovea area which gives acute vision. In the acute zone, the eyes are flattened and the facets larger. The flattening allows more ommatidia to receive light from a spot and therefore higher resolution. The black spot that can be seen on the compound eyes of such insects, which always seems to look directly at the observer, is called a pseudopupil . This occurs because the ommatidia which one observes "head-on" (along their optical axes ) absorb

261-460: A central point. The nature of these eyes means that if one were to peer into the pupil of an eye, one would see the same image that the organism would see, reflected back out. Many small organisms such as rotifers , copepods and flatworms use such organs, but these are too small to produce usable images. Some larger organisms, such as scallops , also use reflector eyes. The scallop Pecten has up to 100 millimetre-scale reflector eyes fringing

348-428: A cluster of numerous ommatidia on each side of the head, organised in a way that resembles a true compound eye. The body of Ophiocoma wendtii , a type of brittle star , is covered with ommatidia, turning its whole skin into a compound eye. The same is true of many chitons . The tube feet of sea urchins contain photoreceptor proteins, which together act as a compound eye; they lack screening pigments, but can detect

435-446: A dark ring encircling the iris on some individuals, but is a result of the optical properties of the region between the cornea and sclera , not of pigments in the iris. Iris color is a highly complex phenomenon consisting of the combined effects of texture, pigmentation, fibrous tissue, and blood vessels within the iris stroma , which together make up an individual's epigenetic constitution in this context. An organism's "eye color"

522-427: A factor of 1,000 or more. Ocelli , some of the simplest eyes, are found in animals such as some of the snails . They have photosensitive cells but no lens or other means of projecting an image onto those cells. They can distinguish between light and dark but no more, enabling them to avoid direct sunlight . In organisms dwelling near deep-sea vents , compound eyes are adapted to see the infra-red light produced by

609-641: A few facets, each with a retina capable of creating an image. With each eye producing a different image, a fused, high-resolution image is produced in the brain. The mantis shrimp has the world's most complex colour vision system. It has detailed hyperspectral colour vision. Trilobites , now extinct, had unique compound eyes. Clear calcite crystals formed the lenses of their eyes. They differ in this from most other arthropods, which have soft eyes. The number of lenses in such an eye varied widely; some trilobites had only one while others had thousands of lenses per eye. In contrast to compound eyes, simple eyes have

696-407: A focusing lens , and often an iris . Muscles around the iris change the size of the pupil , regulating the amount of light that enters the eye and reducing aberrations when there is enough light. The eyes of most cephalopods , fish , amphibians and snakes have fixed lens shapes, and focusing is achieved by telescoping the lens in a similar manner to that of a camera . The compound eyes of

783-612: A high refractive index, decreasing to the edges; this decreases the focal length and thus allows a sharp image to form on the retina. This also allows a larger aperture for a given sharpness of image, allowing more light to enter the lens; and a flatter lens, reducing spherical aberration . Such a non-homogeneous lens is necessary for the focal length to drop from about 4 times the lens radius, to 2.5 radii. So-called under-focused lens eyes, found in gastropods and polychaete worms, have eyes that are intermediate between lens-less cup eyes and real camera eyes. Also box jellyfish have eyes with

870-403: A lens focusing light from one direction on the rhabdom, while light from other directions is absorbed by the dark wall of the ommatidium . The second type is named the superposition eye. The superposition eye is divided into three types: The refracting superposition eye has a gap between the lens and the rhabdom, and no side wall. Each lens takes light at an angle to its axis and reflects it to

957-447: A limit on the possible resolution that can be obtained (assuming that they do not function as phased arrays ). This can only be countered by increasing lens size and number. To see with a resolution comparable to our simple eyes, humans would require very large compound eyes, around 11 metres (36 ft) in radius. Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form

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1044-462: A new 10–12 foot high base, which includes the 2002 Olympic and Paralympic logos and wording memorializing Salt Lake City as the 2002 host of the games. The reconstruction cost $ 3.79 million and was completed under the direction of artist Gordon Huether . The installation does not allow the arch to open and close, but does provide lighting at night. The restoration required newly fabricated pieces to replace those that had been previously stolen. The arch

1131-451: A parabolic mirror to focus the image; it combines features of superposition and apposition eyes. Another kind of compound eye, found in males of Order Strepsiptera , employs a series of simple eyes—eyes having one opening that provides light for an entire image-forming retina. Several of these eyelets together form the strepsipteran compound eye, which is similar to the 'schizochroal' compound eyes of some trilobites . Because each eyelet

1218-539: A patient's systemic health. Practitioners match their observations to "iris charts", which divide the iris into zones corresponding to specific parts of the human body. Iridologists see the eyes as "windows" into the body's state of health. Iridology is not supported by quality research studies, and is considered pseudoscience. Eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It

1305-420: A pit to reduce the angles of light that enters and affects the eye-spot, to allow the organism to deduce the angle of incoming light. Found in about 85% of phyla, these basic forms were probably the precursors to more advanced types of "simple eyes". They are small, comprising up to about 100 cells covering about 100 μm. The directionality can be improved by reducing the size of the aperture, by incorporating

1392-498: A reflective layer behind the receptor cells, or by filling the pit with a refractile material. Pit vipers have developed pits that function as eyes by sensing thermal infra-red radiation, in addition to their optical wavelength eyes like those of other vertebrates (see infrared sensing in snakes ). However, pit organs are fitted with receptors rather different from photoreceptors, namely a specific transient receptor potential channel (TRP channels) called TRPV1 . The main difference

1479-466: A refractive cornea: these have a negative lens, enlarging the observed image by up to 50% over the receptor cells, thus increasing their optical resolution. In the eyes of most mammals , birds , reptiles, and most other terrestrial vertebrates (along with spiders and some insect larvae) the vitreous fluid has a higher refractive index than the air. In general, the lens is not spherical. Spherical lenses produce spherical aberration. In refractive corneas,

1566-617: A resolution better than 1°. Also, superposition eyes can achieve greater sensitivity than apposition eyes , so are better suited to dark-dwelling creatures. Eyes also fall into two groups on the basis of their photoreceptor's cellular construction, with the photoreceptor cells either being ciliated (as in the vertebrates) or rhabdomeric . These two groups are not monophyletic; the Cnidaria also possess ciliated cells, and some gastropods and annelids possess both. Some organisms have photosensitive cells that do nothing but detect whether

1653-405: A sharp image. Ocelli (pit-type eyes of arthropods) blur the image across the whole retina, and are consequently excellent at responding to rapid changes in light intensity across the whole visual field; this fast response is further accelerated by the large nerve bundles which rush the information to the brain. Focusing the image would also cause the sun's image to be focused on a few receptors, with

1740-499: A single erect image. Compound eyes are common in arthropods, annelids and some bivalved molluscs. Compound eyes in arthropods grow at their margins by the addition of new ommatidia. Apposition eyes are the most common form of eyes and are presumably the ancestral form of compound eyes. They are found in all arthropod groups, although they may have evolved more than once within this phylum. Some annelids and bivalves also have apposition eyes. They are also possessed by Limulus ,

1827-454: A single lens. Jumping spiders have one pair of large simple eyes with a narrow field of view , augmented by an array of smaller eyes for peripheral vision . Some insect larvae , like caterpillars , have a type of simple eye ( stemmata ) which usually provides only a rough image, but (as in sawfly larvae) can possess resolving powers of 4 degrees of arc, be polarization-sensitive, and capable of increasing its absolute sensitivity at night by

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1914-408: A spherical lens, cornea and retina, but the vision is blurry. Heterogeneous eyes have evolved at least nine times: four or more times in gastropods , once in the copepods , once in the annelids , once in the cephalopods , and once in the chitons , which have aragonite lenses. No extant aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens

2001-415: A transparent humour that optimised colour filtering, blocked harmful radiation, improved the eye's refractive index , and allowed functionality outside of water. The transparent protective cells eventually split into two layers, with circulatory fluid in between that allowed wider viewing angles and greater imaging resolution, and the thickness of the transparent layer gradually increased, in most species with

2088-411: Is 36 feet (11 m) tall, 72 feet (22 m) feet wide, and weighs 31,000 pounds (14,000 kg) It is made up of 4,000 individual pieces put together as 96 connected panels and are connected with 13,000 steel rivets. The 96 panels vary in size, but the largest are 9 feet (2.7 m) tall and 5 feet (1.5 m) wide. The panels are also translucent which allowed light from behind to be seen and echoed

2175-400: Is a combination of inputs from the numerous ommatidia (individual "eye units"), which are located on a convex surface, thus pointing in slightly different directions. Compared with simple eyes, compound eyes possess a very large view angle, and can detect fast movement and, in some cases, the polarisation of light. Because the individual lenses are so small, the effects of diffraction impose

2262-412: Is a simple eye, it produces an inverted image; those images are combined in the brain to form one unified image. Because the aperture of an eyelet is larger than the facets of a compound eye, this arrangement allows vision under low light levels. Good fliers such as flies or honey bees, or prey-catching insects such as praying mantis or dragonflies , have specialised zones of ommatidia organised into

2349-470: Is actually the color of one's iris, the cornea being transparent and the white sclera entirely outside the area of interest. Melanin is yellowish to dark hazel in the stromal pigment cells, and black in the iris pigment epithelium , which lies in a thin but very opaque layer across the back of the iris. Most human irises also show a condensation of the brownish stromal melanin in the thin anterior border layer, which by its position has an overt influence on

2436-404: Is advantageous to have a convex eye-spot, which gathers more light than a flat or concave one. This would have led to a somewhat different evolutionary trajectory for the vertebrate eye than for other animal eyes. The thin overgrowth of transparent cells over the eye's aperture, originally formed to prevent damage to the eyespot, allowed the segregated contents of the eye chamber to specialise into

2523-430: Is common in mammals, including humans. The simplest eyes are pit eyes. They are eye-spots which may be set into a pit to reduce the angle of light that enters and affects the eye-spot, to allow the organism to deduce the angle of incoming light. Eyes enable several photo response functions that are independent of vision. In an organism that has more complex eyes, retinal photosensitive ganglion cells send signals along

2610-542: Is considered a key factor in this. The majority of the advancements in early eyes are believed to have taken only a few million years to develop, since the first predator to gain true imaging would have touched off an "arms race" among all species that did not flee the photopic environment. Prey animals and competing predators alike would be at a distinct disadvantage without such capabilities and would be less likely to survive and reproduce. Hence multiple eye types and subtypes developed in parallel (except those of groups, such as

2697-401: Is great enough for this stage to be quickly "outgrown". This eye creates an image that is sharp enough that motion of the eye can cause significant blurring. To minimise the effect of eye motion while the animal moves, most such eyes have stabilising eye muscles. The ocelli of insects bear a simple lens, but their focal point usually lies behind the retina; consequently, those can not form

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2784-410: Is little difference in refractive index between the vitreous fluid and the surrounding water. Hence creatures that have returned to the water—penguins and seals, for example—lose their highly curved cornea and return to lens-based vision. An alternative solution, borne by some divers, is to have a very strongly focusing cornea. A unique feature of most mammal eyes is the presence of eyelids which wipe

2871-413: Is part of an organism's visual system . In higher organisms, the eye is a complex optical system that 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 neural pathways that connect

2958-415: Is seen; if not, they will remain blue or gray. All the contributing factors towards eye color and its variation are not fully understood. Autosomal recessive/dominant traits in iris color are inherent in other species, but coloration can follow a different pattern. Heterochromia (also known as a heterochromia iridis or heterochromia iridum) is an ocular condition in which one iris is a different color from

3045-472: Is that photoreceptors are G-protein coupled receptors but TRP are ion channels . The resolution of pit eyes can be greatly improved by incorporating a material with a higher refractive index to form a lens, which may greatly reduce the blur radius encountered—hence increasing the resolution obtainable. The most basic form, seen in some gastropods and annelids, consists of a lens of one refractive index. A far sharper image can be obtained using materials with

3132-414: Is two cells thick (the iris pigment epithelium), but the front surface has no epithelium. This anterior surface projects as the dilator muscles. The high pigment content blocks light from passing through the iris to the retina, restricting it to the pupil. The outer edge of the iris, known as the root, is attached to the sclera and the anterior ciliary body . The iris and ciliary body together are known as

3219-449: The Iliad , because of the many colours of this eye part. The iris consists of two layers: the front pigmented fibrovascular layer known as a stroma and, behind the stroma, pigmented epithelial cells. The stroma is connected to a sphincter muscle ( sphincter pupillae ), which contracts the pupil in a circular motion, and a set of dilator muscles ( dilator pupillae ), which pull

3306-609: The Dave Matthews Band . Following the Olympics, plans to install the arch at a park setting were developed. Some people wanted the arch to be used in an amphitheater or some kind of concert venue, possibly at downtown's Gallivan Center or Pioneer Park. But because the arch was a symbol of the 2002 games, the United States Olympic Committee put restrictions on possible future locations for

3393-442: The arthropods are composed of many simple facets which, depending on anatomical detail, may give either a single pixelated image or multiple images per eye. Each sensor has its own lens and photosensitive cell(s). Some eyes have up to 28,000 such sensors arranged hexagonally, which can give a full 360° field of vision. Compound eyes are very sensitive to motion. Some arthropods, including many Strepsiptera , have compound eyes of only

3480-440: The copepod Pontella has three. The outer has a parabolic surface, countering the effects of spherical aberration while allowing a sharp image to be formed. Another copepod, Copilia , has two lenses in each eye, arranged like those in a telescope. Such arrangements are rare and poorly understood, but represent an alternative construction. Multiple lenses are seen in some hunters such as eagles and jumping spiders, which have

3567-406: The depth of field . Very few humans possess the ability to exert direct voluntary control over their iris muscles, which grants them the ability to dilate and constrict their pupils on command. However, there is no clear purpose or advantage to this. The iris is usually strongly pigmented , with the color typically ranging between brown, hazel, green, gray, and blue. Occasionally, the color of

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3654-452: The eye in most mammals and birds that is responsible for controlling the diameter and size of the pupil , and thus the amount of light reaching the retina . In optical terms, the pupil is the eye's aperture , while the iris is the diaphragm . Eye color is defined by the iris. The word "iris" is derived from the Greek word for " rainbow ", also its goddess plus messenger of the gods in

3741-420: The incident light , while those to one side reflect it. There are some exceptions from the types mentioned above. Some insects have a so-called single lens compound eye, a transitional type which is something between a superposition type of the multi-lens compound eye and the single lens eye found in animals with simple eyes. Then there is the mysid shrimp, Dioptromysis paucispinosa . The shrimp has an eye of

3828-414: The retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment and to the pretectal area to control the pupillary light reflex . Complex eyes distinguish shapes and colours . The visual fields of many organisms, especially predators, involve large areas of binocular vision for depth perception . In other organisms, particularly prey animals, eyes are located to maximise

3915-492: The 2002 Olympic theme Light the Fire Within . Two 30-horsepower motors controlled eight separate cables which pulled the mechanical curtain open in about 20 seconds. When the arch was fully opened it had folded up into a 6 feet (1.8 m) ring, which framed the stage. It was designed to open and close like the iris of an eye. During the Olympics, it was included in the every evening medal ceremony and when opened, revealed

4002-533: The First was dubbed dikoros (having two irises) for his patent heterochromia since his right iris had a darker color than the left one. In contrast, heterochromia and variegated iris patterns are common in veterinary practice. Siberian Husky dogs show heterochromia, possibly analogous to the genetically determined Waardenburg syndrome of humans. Some white cat fancies (e.g., white Turkish Angora or white Turkish Van cats) may show striking heterochromia, with

4089-413: The anterior uvea . Just in front of the root of the iris is the region referred to as the trabecular meshwork , through which the aqueous humour constantly drains out of the eye, with the result that diseases of the iris often have important effects on intraocular pressure and indirectly on vision. The iris along with the anterior ciliary body provide a secondary pathway for aqueous humour to drain from

4176-599: The arch to expand and contract, were fabricated by Hudson Machine Works in Brewster, NY . These were paired with the arch's structural components and pieced together in its entirety by Scenic Technologies of New Windsor, New York , who spent an additional four months in constructing the arch in their warehouse in New York. It was then disassembled and then trucked to Utah , being reassembled in January 2002, and unveiled to

4263-554: The arch was lit with multicolored lights. The arch was removed from the park in August 2014. On December 6, 2014, 29 pieces of the arch were stolen from an impound lot where it had been stored. after which the remaining pieces were moved to an undisclosed location. On August 29, 2023, the restored arch was unveiled alongside the exit road of the Salt Lake City International Airport . The arch sits atop

4350-474: The arch. Because of these restrictions, and a lack of consensus on other options, SLOC announced plans, on December 5, 2002, to install the arch at the Salt Lake 2002 Olympic Cauldron Park . On July 30, 2003, the arch was lifted onto its new base at the park using 3 cranes. The arch was located just outside the park's southern fence and was partly open which allowed visitors to walk through it, while at night

4437-619: The blood vessels, collagen in the vessel and stroma) is the most important element. Rayleigh scattering and Tyndall scattering , (which also happen in the sky) and diffraction also occur. Raman scattering , and constructive interference , as in the feathers of birds, do not contribute to the color of the eye, but interference phenomena are important in the brilliantly colored iris pigment cells ( iridophores ) in many animals. Interference effects can occur at both molecular and light-microscopic scales, and are often associated (in melanin-bearing cells) with quasicrystalline formations, which enhance

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4524-399: The cells of the dilator muscle. The vitreous is the transparent, colourless, gelatinous mass that fills the space between the lens of the eye and the retina lining the back of the eye. It is produced by certain retinal cells. It is of rather similar composition to the cornea, but contains very few cells (mostly phagocytes which remove unwanted cellular debris in the visual field, as well as

4611-399: The directionality of light by the shadow cast by its opaque body. The ciliary body is triangular in horizontal section and is coated by a double layer, the ciliary epithelium. The inner layer is transparent and covers the vitreous body, and is continuous from the neural tissue of the retina. The outer layer is highly pigmented, continuous with the retinal pigment epithelium, and constitutes

4698-588: The edge of its shell. It detects moving objects as they pass successive lenses. There is at least one vertebrate, the spookfish , whose eyes include reflective optics for focusing of light. Each of the two eyes of a spookfish collects light from both above and below; the light coming from above is focused by a lens, while that coming from below, by a curved mirror composed of many layers of small reflective plates made of guanine crystals . A compound eye may consist of thousands of individual photoreceptor units or ommatidia ( ommatidium , singular). The image perceived

4785-422: The eye allows light to enter and project onto a light-sensitive layer of cells known as the retina . The cone cells (for colour) and the rod cells (for low-light contrasts) in the retina detect and convert light into neural signals which are transmitted to the brain via the optic nerve to produce vision. Such eyes are typically spheroid, filled with the transparent gel-like vitreous humour , possess

4872-412: The eye and spread tears across the cornea to prevent dehydration. These eyelids are also supplemented by the presence of eyelashes , multiple rows of highly innervated and sensitive hairs which grow from the eyelid margins to protect the eye from fine particles and small irritants such as insects. An alternative to a lens is to line the inside of the eye with "mirrors", and reflect the image to focus at

4959-428: 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, classified into compound eyes and non-compound eyes. Compound eyes are made up of multiple small visual units, and are common on insects and crustaceans . Non-compound eyes have a single lens and focus light onto the retina to form a single image. This type of eye

5046-450: The eye. Photoreception is phylogenetically very old, with various theories of phylogenesis. The common origin ( monophyly ) of all animal eyes is now widely accepted as fact. This is based upon the shared genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 650-600 million years ago, and the PAX6 gene

5133-403: The eye. The iris is divided into two major regions: The collarette is the thickest region of the iris, separating the pupillary portion from the ciliary portion. The collarette is a vestige of the coating of the embryonic pupil. It is typically defined as the region where the sphincter muscle and dilator muscle overlap. Radial ridges extend from the periphery to the pupillary zone, to supply

5220-490: The field of view, such as in rabbits and horses , which have monocular vision . The first proto-eyes evolved among animals 600  million years ago about the time of the Cambrian explosion . The last common ancestor of animals possessed the biochemical toolkit necessary for vision, and more advanced eyes have evolved in 96% of animal species in six of the ~35 main phyla . In most vertebrates and some molluscs ,

5307-481: The geometry of cephalopod and most vertebrate eyes creates the impression that the vertebrate eye evolved from an imaging cephalopod eye , but this is not the case, as the reversed roles of their respective ciliary and rhabdomeric opsin classes and different lens crystallins show. The very earliest "eyes", called eye-spots, were simple patches of photoreceptor protein in unicellular animals. In multicellular beings, multicellular eyespots evolved, physically similar to

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5394-449: The horseshoe crab, and there are suggestions that other chelicerates developed their simple eyes by reduction from a compound starting point. (Some caterpillars appear to have evolved compound eyes from simple eyes in the opposite fashion.) Apposition eyes work by gathering a number of images, one from each eye, and combining them in the brain, with each eye typically contributing a single point of information. The typical apposition eye has

5481-561: The hot vents, allowing the creatures to avoid being boiled alive. There are ten different eye layouts. Eye types can be categorised into "simple eyes", with one concave photoreceptive surface, and "compound eyes", which comprise a number of individual lenses laid out on a convex surface. "Simple" does not imply a reduced level of complexity or acuity. Indeed, any eye type can be adapted for almost any behaviour or environment. The only limitations specific to eye types are that of resolution—the physics of compound eyes prevents them from achieving

5568-425: The hyalocytes of Balazs of the surface of the vitreous, which reprocess the hyaluronic acid ), no blood vessels, and 98–99% of its volume is water (as opposed to 75% in the cornea) with salts, sugars, vitrosin (a type of collagen), a network of collagen type II fibres with the mucopolysaccharide hyaluronic acid, and also a wide array of proteins in micro amounts. Amazingly, with so little solid matter, it tautly holds

5655-405: The iris is due to a lack of pigmentation, as in the pinkish-white of oculocutaneous albinism , or to obscuration of its pigment by blood vessels, as in the red of an abnormally vascularised iris. Despite the wide range of colors, the only pigment that contributes substantially to normal iris color is the dark pigment melanin . The quantity of melanin pigment in the iris is one factor in determining

5742-423: The iris radially to enlarge the pupil, pulling it in folds. The sphincter pupillae is the opposing muscle of the dilator pupillae. The pupil's diameter, and thus the inner border of the iris, changes size when constricting or dilating. The outer border of the iris does not change size. The constricting muscle is located on the inner border. The back surface is covered by a heavily pigmented epithelial layer that

5829-408: The iris with blood vessels. The root of the iris is the thinnest and most peripheral. The muscle cells of the iris are smooth muscle in mammals and amphibians, but are striated muscle in reptiles (including birds). Many fish have neither, and, as a result, their irises are unable to dilate and contract, so that the pupil always remains of a fixed size. From anterior (front) to posterior (back),

5916-419: The layers of the iris are: The stroma and the anterior border layer of the iris are derived from the neural crest , and behind the stroma of the iris, the sphincter pupillae and dilator pupillae muscles, as well as the iris epithelium, develop from optic cup neuroectoderm. The iris controls the size of the pupil by means of contracting the iris sphincter muscle and/or the iris dilator muscle . The size of

6003-424: The lens tissue is corrected with inhomogeneous lens material (see Luneburg lens ), or with an aspheric shape. Flattening the lens has a disadvantage; the quality of vision is diminished away from the main line of focus. Thus, animals that have evolved with a wide field-of-view often have eyes that make use of an inhomogeneous lens. As mentioned above, a refractive cornea is only useful out of water. In water, there

6090-452: The most common pattern being one uniformly blue, the other copper, orange, yellow, or green. Striking variation within the same iris is also common in some animals, and is the norm in some species. Several herding breeds, particularly those with a blue merle coat color (such as Australian Shepherds and Border Collies ) may show well-defined blue areas within a brown iris, as well as separate blue and darker eyes. Some horses (usually within

6177-446: The optical effects. Interference is recognised by characteristic dependence of color on the angle of view, as seen in eyespots of some butterfly wings , although the chemical components remain the same. White babies are usually born blue-eyed since no pigment is in the stroma, and their eyes appear blue due to scattering and selective absorption from the posterior epithelium. If melanin is deposited substantially, brown or black color

6264-410: The other iris (complete heterochromia), or where the part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). Uncommon in humans, it is often an indicator of ocular disease, such as chronic iritis or diffuse iris melanoma, but may also occur as a normal variant. Sectors or patches of strikingly different colors in the same iris are less common. Anastasius

6351-414: The other stromal components. Sometimes, lipofuscin , a yellow "wear and tear" pigment, also enters into the visible eye color, especially in aged or diseased green eyes. The optical mechanisms by which the nonpigmented stromal components influence eye color are complex, and many erroneous statements exist in the literature. Simple selective absorption and reflection by biological molecules ( hemoglobin in

6438-510: The overall color. The degree of dispersion of the melanin, which is in subcellular bundles called melanosomes , has some influence on the observed color, but melanosomes in the iris of humans and other vertebrates are not mobile, and the degree of pigment dispersion cannot be reversed. Abnormal clumping of melanosomes does occur in disease and may lead to irreversible changes in iris color (see heterochromia , below). Colors other than brown or black are due to selective reflection and absorption from

6525-459: The parabolic superposition compound eye type, seen in arthropods such as mayflies , the parabolic surfaces of the inside of each facet focus light from a reflector to a sensor array. Long-bodied decapod crustaceans such as shrimp , prawns , crayfish and lobsters are alone in having reflecting superposition eyes, which also have a transparent gap but use corner mirrors instead of lenses. This eye type functions by refracting light, then using

6612-413: The phenotypic eye color of an organism. Structurally, this huge molecule is only slightly different from its equivalent found in skin and hair . Iris color is due to variable amounts of eumelanin (brown/black melanins) and pheomelanin (red/yellow melanins) produced by melanocytes. More of the former is found in brown-eyed people and of the latter in blue- and green-eyed people. The limbal ring appears as

6699-461: The possibility of damage under the intense light; shielding the receptors would block out some light and thus reduce their sensitivity. This fast response has led to suggestions that the ocelli of insects are used mainly in flight, because they can be used to detect sudden changes in which way is up (because light, especially UV light which is absorbed by vegetation, usually comes from above). Some marine organisms bear more than one lens; for instance

6786-598: The public and media by the Salt Lake Organizing Committee (SLOC), on January 25, 2002. When installed at the medal plaza, it would open to reveal a large 3D sculpture of the 2002 Olympic logo and a second Olympic cauldron, known as the Hero's Cauldron . The stage hosted award ceremonies, where the athletes received their medals, and was used as a concert venue during the Olympics, hosting many performing artists including Creed , Brooks & Dunn and

6873-401: The pupils is dependent on many factors (including light, emotional state, cognitive load, arousal, stimulation), and can range from less than 2 mm in diameter, to as large as 9 mm in diameter. However, there is considerable variation in maximal pupil diameter by individual humans, and decreases with age. The irises also contract the pupils when accommodation is initiated, to increase

6960-438: The receptor patches for taste and smell. These eyespots could only sense ambient brightness: they could distinguish light and dark, but not the direction of the light source. Through gradual change, the eye-spots of species living in well-lit environments depressed into a shallow "cup" shape. The ability to slightly discriminate directional brightness was achieved by using the angle at which the light hit certain cells to identify

7047-514: The refracting superposition type, in the rear behind this in each eye there is a single large facet that is three times in diameter the others in the eye and behind this is an enlarged crystalline cone. This projects an upright image on a specialised retina. The resulting eye is a mixture of a simple eye within a compound eye. Another version is a compound eye often referred to as "pseudofaceted", as seen in Scutigera . This type of eye consists of

7134-416: The same angle on the other side. The result is an image at half the radius of the eye, which is where the tips of the rhabdoms are. This type of compound eye, for which a minimal size exists below which effective superposition cannot occur, is normally found in nocturnal insects, because it can create images up to 1000 times brighter than equivalent apposition eyes, though at the cost of reduced resolution. In

7221-414: The second Olympic cauldron. Because of the potential of strong storms during the games, the arch was built to operate in extreme weather, including up to 70-mile (110 km) -per-hour winds. 40°46′27″N 111°59′13″W  /  40.77417°N 111.98694°W  / 40.77417; -111.98694 Iris (anatomy) The iris ( pl. : irides or irises ) is a thin, annular structure in

7308-404: The source. The pit deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an effective pinhole camera that was capable of dimly distinguishing shapes. However, the ancestors of modern hagfish , thought to be the protovertebrate, were evidently pushed to very deep, dark waters, where they were less vulnerable to sighted predators, and where it

7395-622: The surroundings are light or dark , which is sufficient for the entrainment of circadian rhythms . These are not considered eyes because they lack enough structure to be considered an organ, and do not produce an image. Every technological method of capturing an optical image that humans commonly use occurs in nature, with the exception of zoom and Fresnel lenses . Simple eyes are rather ubiquitous, and lens-bearing eyes have evolved at least seven times in vertebrates , cephalopods , annelids , crustaceans and Cubozoa . Pit eyes, also known as stemmata , are eye-spots which may be set into

7482-469: The vertebrates, that were only forced into the photopic environment at a late stage). Eyes in various animals show adaptation to their requirements. For example, the eye of a bird of prey has much greater visual acuity than a human eye , and in some cases can detect ultraviolet radiation. The different forms of eye in, for example, vertebrates and molluscs are examples of parallel evolution , despite their distant common ancestry. Phenotypic convergence of

7569-446: The white, spotted, palomino, or cremello groups of breeds) may show amber, brown, white and blue all within the same eye, without any sign of eye disease. One eye with a white or bluish-white iris is also known as a "walleye". Iridology (also known as iridodiagnosis) is an alternative medicine technique whose proponents believe that patterns, colors, and other characteristics of the iris can be examined to determine information about

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