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top 10 words in brain distribution (in article): water light form time animal type surface produce region cause |
top 10 words in brain distribution (in article): build wood city structure form design house wall type floor |
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top 10 words in brain distribution (not in article): ice drink rock lamp river wine flow beer sea soil |
top 10 words in brain distribution (not in article): material paint size surface cell store tree window people frame |
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times more probable under eye 30 20 10 6 4 2.5 1.25 1 1.25 2.5 4 6 10 20 30 times more probable under chimney (words not in the model) | |
| Eyes'" are organs that detect light, and send signals along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system. Image-resolving eyes are present in cnidaria, mollusks, chordates, annelids and arthropods. The simplest "eyes", in even unicellular organisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms. From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment. Overview. Complex eyes can distinguish shapes and colors. The visual fields of many organisms, especially predators, involve large areas of binocular vision to improve depth perception; in other organisms, eyes are located so as to maximise the field of view, such as in rabbits and horses. The first proto-eyes evolved among animals 540 million years ago, about the time of the so-called 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 6 of the thirty-something main phyla. 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 for vision. 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, and reducing aberrations when there is enough light. The eyes of cephalopods, fish, amphibians and snakes usually have fixed lens shapes, and focusing vision is achieved by telescoping the lens — similar to how a camera focuses. Compound eyes are found among the arthropods and are composed of many simple facets which, depending on the details of anatomy, 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, which are arranged hexagonally, and which can give a full 360-degree field of vision. Compound eyes are very sensitive to motion. Some arthropods, including many Strepsiptera, have compound eyes of only a few facets, each with a retina capable of creating an image, creating vision. With each eye viewing a different thing, a fused image from all the eyes is produced in the brain, providing very different, high-resolution images. Possessing detailed hyperspectral color vision, the Mantis shrimp has been reported to have the world's most complex color vision system. Trilobites, which are now extinct, had unique compound eyes. They used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in one eye. In contrast to compound eyes, simple eyes are those that have a single lens. For example, jumping spiders have a large pair of simple eyes with a narrow field of view, supported by an array of other, smaller eyes for peripheral vision. Some insect larvae, like caterpillars, have a different type of simple eye (stemmata) which gives a rough image. Some of the simplest eyes, called ocelli, can be found in animals like some of the snails, which cannot actually "see" in the normal sense. They do have photosensitive cells, but no lens and no other means of projecting an image onto these cells. They can distinguish between light and dark, but no more. This enables snails to keep out of direct sunlight. In organisms dwelling near deep-sea vents, compound eyes have been secondarily simplified and adapted to spot the infra-red light produced by the hot vents in this way the bearers can spot hot springs and avoid being boiled alive. Evolution. Visual pigments appear to have a common ancestor and were probably involved in circadian rhythms or reproductive timing in simple organisms. Complex vision, associated with dedicated visual organs, or eyes, evolved many times in different lineages. Types of eye. Nature has produced ten different eye layouts — indeed every way of capturing an image has evolved at least once in nature, with the exception of zoom and Fresnel lenses. Eye types can be categorized into "simple eyes", with one concave chamber, and "compound eyes", which comprise a number of individual lenses laid out on a convex surface. Note that "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 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 cilliated (as in the vertebrates) or rhabdomic. These two groups are not monophyletic; the cnidaira also possess cilliated cells, Pit eyes. Pit eyes, also known as stemma, are eye-spots which may be set into a pit to reduce the angles of light that enters and affects the eyespot, 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 eye". 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 a reflective layer behind the receptor cells, or by filling the pit with a refractile material. Pinhole eye. The pinhole eye is an "advanced" form of pit eye incorporating these improvements, most notably a small aperture (which may be adjustable) and deep pit. It is only found in the nautiloids. Without a lens to focus the image, it produces a blurry image, and will blur out a point to the size of the aperture. Consequently, nautiloids can't discriminate between objects with an angular separation of less than 11°. Shrinking the aperture would produce a sharper image, but let in less light. Spherical lensed eye. 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, still seen in some gastropods and annelids, consists of a lens of one refractive index. A far sharper image can be obtained using materials with 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 an inhomogeneous lens is necessary in order for the focal length to drop from about 4 times the lens radius, to 2.5 radii. Heterogeneous eyes have evolved at least eight times — four or more times in gastropods, once in the copepods, once in the annelids and once in the cephalopods. No aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens 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 minimize the effect of eye motion while the animal moves, most such eyes have stabilizing eye muscles. The ocelli of insects bear a simple lens, but their focal point always lies behind the retina; consequently they can never form a sharp image. This capitulates the function of the eye. 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. Focussing the image would also cause the sun's image to be focussed on a few receptors, with 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). Weaknesses. One weakness of this eye construction is that chromatic aberration is still quite high — although for organisms without color vision, this is a very minor concern. A weakness of the vertebrate eye is the blind spot which results from a gap in the retina where the optic nerve exits at the back of the eye; the cephalopod eye has no blind spot as the retina is in the opposite orientation. 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 | A chimney'" is a structure for venting hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. Chimneys are typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the stack, or chimney, effect. The space inside a chimney is called a "flue". Chimneys may be found in buildings, steam locomotives and ships. In the US, the term smokestack'" (colloquially, "'stack'") is also used when referring to locomotive chimneys. The term "'funnel'" is generally used for ships' chimneys and sometimes to refer to locomotive chimneys.. Chimneys are tall to increase their draw of air for combustion and to disperse pollutants in the flue gases over a greater area so as to reduce the pollutant concentrations in compliance with regulatory or other limits. History. Romans used tubes inside the walls to draw smoke out of bakeries but real chimneys appeared only in northern Europe in the 12th century. Industrial chimneys became common in the late 18th century. The earliest extant example of an English chimney is at Conisborough Keep in Yorkshire, which dates from 1185 AD. Chimneys have traditionally been built of brick, both in small and large buildings. Early chimneys were of a simple brick construction. Later chimneys were constructed by placing the bricks around tile liners. To control downdrafts venting caps (often called "chimney pots") with a variety of designs are sometimes placed on the top of chimneys. In the eighteenth and nineteenth centuries, the methods used to extract lead from its ore produced large amounts of toxic fumes. In the north of England, long near-horizontal chimneys were built, often more than 3 km (2 mi) long, which typically terminated in a short vertical chimney in a remote location where the fumes would cause less harm. Lead and silver deposits formed on the inside of these long chimneys, and periodically workers would be sent along the chimneys to scrape off these valuable deposits. Construction. Due to brick's limited ability to handle transverse loads, chimneys in houses were often built in a "stack", with a fireplace on each floor of the house sharing a single chimney, often with such a stack at the front and back of the house. Today's central heating systems have made chimney placement less critical, and the use of non-structural gas vent pipe allows a flue gas conduit to be installed around obstructions and through walls. In fact, many modern high-efficiency heating appliances do not require a chimney. Such appliances are typically installed near an outside wall, and a noncombustible wall thimble allows vent pipe to be run directly through the outside wall. Industrial chimneys are commonly referred to as flue gas stacks and are typically external structures, as opposed to being built into the wall of a building. They are generally located adjacent to a steam-generating boiler or industrial furnace and the gases are carried to it with ductwork. Today the use of reinforced concrete has almost entirely replaced brick as a structural component in the construction of industrial chimneys. Refractory bricks are often used as a lining, particularly if the type of fuel being burned generates flue gases containing acids. Modern industrial chimneys sometimes consist of a concrete windshield with a number of flues on the inside. The 300 metre chimney at Sasol Three consists of a 26 metre diameter windshield with four 4.6 metre diameter concrete flues which are lined with refractory bricks built on rings of corbels spaced at 10 metre intervals. The reinforced concrete can be cast by conventional formwork or sliding formwork. The height is to ensure the pollutants are dispersed over a wider area to meet legislative or safety requirements. Chimney tops. A chimney pot is placed on top of the chimney to inexpensively extend the length of the chimney, and to improve the chimney's draft. A chimney with more than one pot on it indicates that there is more than one fireplace on different floors sharing the chimney. A chimney cowl is placed on top of the chimney to prevent birds and squirrels from nesting in the chimney. They often feature a rain guard to keep rain from going down the chimney. A metal wire mesh is often used as a spark arrestor to minimize burning debris from rising out of the chimney and making it onto the roof. Although the masonry inside the chimney can absorb a large amount of moisture which later evaporates, rainwater can collect at the base of the chimney. Sometimes weep holes are placed at the bottom of the chimney to drain out collected water. A chimney cowl or wind directional cap is helmet shaped chimney cap that rotates to align with the wind and prevent a backdraft of smoke and wind back down the chimney. An H-style cap'" (cowl) is a chimney top constructed from chimney pipes shaped like the letter H. It is an age old method to regulate draft in situations where prevailing winds or turbulences cause down draft and backpuffing. Although the "'H cap'" has a distinctive advantage over most other downdraft caps, it fell out of favor because of it bulky looks. It is found mainly in marine use but has been gaining popularity again due to its energy saving functionality. The "'H-cap stabilizes the draft rather than increasing it. Other down draft caps are based on the Venturi effect, solving downdraft problems by increasing the up draft constantly resulting in much higher fuel consumption. A chimney damper is a metal spring door placed at the top of the chimney with a long metal chain that allows you to open and close the chimney from the fireplace. In the late Middle Ages in Western Europe the design of crow-stepped gables arose to allow maintenance access to the chimney top, especially for tall structures such as castles and great manor houses. Chimney draught or draft. When coal, oil, natural gas, wood or any other fuel is combusted in a stove, oven, fireplace, hot water boiler or industrial furnace, the hot combustion product gases that are formed are called flue gases. Those gases are generally exhausted to the ambient outside air through chimneys or industrial flue gas stacks (sometimes referred to as smokestacks). The combustion flue gases inside the chimneys or stacks are much hotter than the ambient outside air and therefore less dense than the ambient air. That causes the bottom of the vertical column of hot flue gas to have a lower pressure than the pressure at the bottom of a corresponding column of outside air. That higher pressure outside the chimney is the driving force that moves the required combustion air into the combustion zone and also moves the flue gas up and out of the chimney. That movement or flow of combustion air and flue gas is called "natural draught draft", "natural ventilation", "chimney effect", or "stack effect". The taller the stack, the more draught or draft is created. Designing chimneys and stacks to provide the correct amount of natural draught or draft involves a number design factors, many of which require trial-and-error reiterative methods. As a "first guess" approximation, the following equation can be used to estimate the natural draught draft flow rate by assuming that the molecular mass (i.e., molecular weight) of the flue gas and the external air are equal and that the frictional pressure and heat losses are negligible: Drawbacks. A characteristic problem of chimneys is they develop deposits of creosote on the walls of the structure when used with wood as a fuel. Some types of wood, such as pine, generate more creosote than others. Deposits of this substance can interfere with the airflow and more importantly, they are flammable and can cause dangerous chimney fires if the deposits ignite in the chimney. Thus, it is recommended — and in some countries even mandatory — that chimneys be inspected annually and cleaned on a regular basis to prevent these problems. The workers who perform this task professionally are called chimney sweeps. In the middle ages in some parts of Europe, a crow-stepped gable design was developed, partially to provide access to chimneys without use of ladders. Masonry (brick) chimneys have also proved particularly susceptible to crumbling during earthquakes. Government housing authorities in quake-prone cities like San Francisco and Los Angeles now recommend building new homes with stud-framed chimneys around a metal flue. Bracing or strapping old masonry chimneys has not proved to be very effective in preventing damage or injury from earthquakes. Perhaps predictably, a new industry provides "faux-brick" facades to cover these modern chimney structures. Other problems include "spalling" brick, in which moisture seeps into the brick and then freezes, cracking and flaking the brick and loosening mortar seals. Dual-use chimneys. Some very high chimneys are used for carrying antennas of mobile phone services and low power FM TV-transmitters. Special attention must be paid to possible corrosion problems if these antennas are near the exhaust of the chimney. In some cases the chimneys of power stations are used also as pylons. However this type of construction is not very common, because of corrosion problems of conductor cables. The Dům Dětí a Mládeže v Modřanech in Prague, Czech Republic is equipped with an observation deck. Cooling tower used as an industrial chimney. At some power stations, which are equipped with plants for the removal of sulfur dioxide and nitrogen oxides, it is possible to use the cooling tower as a chimney. Such cooling towers can be seen in Germany at the Power Station Staudinger Grosskrotzenburg and at the Power Station Rostock. At power stations that are not equipped for removing sulfur dioxide, such usage of cooling towers could result in serious corrosion problems. |