See also

• Arthropod eye
• Human eye
• Mammalian eye

References

1. The precise number depends on the author



2. In most vertebrates and some mollusks, the eye works by allowing light to enter it and project onto a light-sensitive panel of cells, known as the retina, at the rear of the eye. The cone cells (for color) and the rod cells (for low-light contrasts) in the retina detect and convert light into neural signals. The visual signals are then transmitted to the brain via the optic nerve. Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris; the relaxing or tightening of the muscles around the iris change the size of the pupil, thereby regulating the amount of light that enters the eye, BOOK, Nairne, James, Psychology, Wadsworth Publishing, Belmont, 2005, 049503150x,weblink 61361417,



3. BOOK, Visual Perception: Physiology, Psychology and Ecology, Vicki Bruce, Patrick R. Green, and Mark A. Georgeson, Psychology Press, 1996, 0863774504, 20,weblink



4. BioMedia Associates Educational Biology Site: What animal has a more sophisticated eye, Octopus or Insect?



5. Who You Callin' "Shrimp"? – National Wildlife Magazine




6. Multiple lenses

   Some marine organisms bear more than one lens; for instance the copeopod Pontella has three. The outer has a parabolic surface, countering the effects of spherical aberration while allowing a sharp image to be formed. Copilla's eyes have two lenses, which move in and out like a telescope. Such arrangements are rare and poorly understood, but represent an interesting alternative construction. An interesting use of multiple lenses is seen in some hunters such as eagles and jumping spiders, which have a refractive cornea (discussed next): these have a negative lens, enlarging the observed image by up to 50% over the receptor cells, thus increasing their optical resolution.

   Refractive cornea

   {{See|Mammalian eye}}In the eyes of most terrestrial vertebrates (along with spiders and some insect larvae) the vitreous fluid has a higher refractive index than the air, relieving the lens of the function of reducing the focal length. This has freed it up for fine adjustments of focus, allowing a very high resolution to be obtained. As with spherical lenses, the problem of spherical aberration caused by the lens can be countered either by using an inhomogeneous lens material, or by flattening the lens. Flattening the lens has a disadvantage: the quality of vision is diminished away from the main line of focus, meaning that animals requiring all-round vision are detrimented. Such animals often display an inhomogeneous lens instead.As mentioned above, a refractive cornea is only useful out of water; in water, there is no difference in refractive index between the vitreous fluid and the surrounding water. Hence creatures which have returned to the water — penguins and seals, for example — lose their refractive cornea and return to lens-based vision. An alternative solution, borne by some divers, is to have a very strong cornea.

   Reflector eyes

   An alternative to a lens is to line the inside of the eye with "mirrors", and reflect the image to focus at 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, copeopods and platyhelminths 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 millimeter-scale reflector eyes fringing the edge of its shell. It detects moving objects as they pass successive lenses.

   Compound eyes{{anchor|Compound eyes}}

   missing image!
   - Superposkils.gif -
   Cross-section of a compound eye — top: low light condition, with separating pigments contracted, so that the light reaching multiple ommatidia focus on a single nerve; bottom: bright condition, with pigments expanded
   A compound eye may consist of thousands of individual photoreception units. The image perceived is a combination of inputs from the numerous ommatidia (individual "eye units"), which are located on a convex surface, thus point 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 polarization of light.JOURNAL,weblink doi = 10.1016/S0167-9317(03)00102-3, Miniaturized imaging systems, June, 2003, Völkel, R.; Eisner, M.; Weible, K. J., Microelectronic Engineering, 67-68, 1, 461–472,



7. JOURNAL, Gaten, Edward, Optics and phylogeny: is there an insight? The evolution of superposition eyes in the Decapoda (Crustacea), 1998, Contributions to Zoology, 67, 4, 223–236,weblink



8. JOURNAL, Ritchie, Alexander, Ainiktozoon loganense Scourfield, a protochordate? from the Silurian of Scotland, 1985, Alcheringa, 9, 137,



9. {{citation| last = Mayer| first = G.| year = 2006| title = Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods? | journal = Arthropod Structure and Development| volume = 35| issue = 4| pages = 231–245| doi = 10.1016/j.asd.2006.06.003}}



10. On the other hand, the eyes of organisms which operate in low light levels, such as around dawn and dusk or in deep water, tend to be larger to increase the amount of light that can be captured.It is not only the shape of the eye that may be affected by lifestyle. Eyes can be the most visible parts of organisms, and this can act as a pressure on organisms to have more transparent eyes at the cost of function.

    Acuity

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    A hawk's eye
    Visual acuity is often measured in cycles per degree (CPD), which measures an angular resolution, or how much an eye can differentiate one object from another in terms of visual angles. Resolution in CPD can be measured by bar charts of different numbers of white — black stripe cycles. For example, if each pattern is 1.75 cm wide and is placed at 1 m distance from the eye, it will subtend an angle of 1 degree, so the number of white — black bar pairs on the pattern will be a measure of the cycles per degree of that pattern. The highest such number that the eye can resolve as stripes, or distinguish from a gray block, is then the measurement of visual acuity of the eye.For a human eye with excellent acuity, the maximum theoretical resolution would be 50 CPDBOOK, The Image Processing Handbook, John C. Russ, CRC Press, 2006, 0849372542,weblink The upper limit (finest detail) visible with the human eye is about 50 cycles per degree,… (Fifth Edition, 2007, Page 94), 156223054,



11. BOOK, Casarett and Doull's Toxicology: The Basic Science of Poisons, Curtis D. Klaassen, McGraw-Hill Professional, 2001, 0071347216,weblink 47965382,



12. BOOK, Optical System Design, McGraw-Hill Professional, 2000, 0071349162,weblink Robert E. Fischer; Biljana Tadic-Galeb. With contributions by Rick Plympton…, 247851267, Steve Chapman (editor),



13. {{rp|309}}

    Rods and cones

    The retina contains two major types of light-sensitive photoreceptor cells: the rods and the cones.Rods cannot distinguish colors, but are responsible for low-light black-and-white (scotopic) vision; they work well in dim light as they contain a pigment, visual purple, which is sensitive at low light intensity, but saturates at higher (photopic) intensities. Rods are distributed throughout the retina but there are none at the fovea and none at the blind spot. Rod density is greater in the peripheral retina than in the central retina.Cones are responsible for color vision. They require brighter light to function than rods require. There are three types of cones, maximally sensitive to long-wavelength, medium-wavelength, and short-wavelength light (often referred to as red, green, and blue, respectively, though the sensitivity peaks are not actually at these colors). The color seen is the combined effect of stimuli to, and responses from, these three types of cone cells. Cones are mostly concentrated in and near the fovea. Only a few are present at the sides of the retina. Objects are seen most sharply in focus when their images fall on this spot, as when one looks at an object directly. Cone cells and rods are connected through intermediate cells in the retina to nerve fibers of the optic nerve. When rods and cones are stimulated by light, the nerves send off impulses through these fibers to the brain.

    Pigment

    The pigment molecules used in the eye are various, but can be used to define the evolutionary distance between different groups, and can also be an aid in determining which are closely related – although problems of convergence do exist.The opsin protein group evolved long before the last common ancestor of animals, and has continued to diversify since.

    Human consumption

    Eyes can be eaten by humans: seal eyes are a source of zinc for the Inuit.weblink

• WEB,weblink Anatomy, History of Ophthalmology, 23 April, 2005,
• Kandel ER, Schwartz JH, Jessell TM. Principles of Neural Science, 4th ed. McGraw-Hill, New York (2000). ISBN 0-8385-7701-6

External links

• A human eye
• Evolution of the eye
• Diagram of the eye
• Webvision. The organisation of the retina and visual system. An in-depth treatment of retinal function, open to all but geared most toward graduate students.
• Eye strips images of all but bare essentials before sending visual information to brain, UC Berkeley research shows