HELIUM

DEFINITION: a colourless, odourless chemical element, one of the noble gases, having the lowest known boiling and melting points: it is used in low-temperature work, as a diluent for oxygen, in deep-sea breathing systems, for inflating balloons, etc.: symbol, He; at. wt., 4.0026; at. no., 2; density, 0.1785 g/l (0 C); melt. pt., -272.2 C; boil. pt., -268.9 C

Lockyer, Joseph Norman (1836-1920), British astronomer and physicist, born in Rugby, England; pioneer in application of spectroscope to sun and stars; explained sunspots; between 1870 and 1905 conducted eight British expeditions for observing total solar eclipses ('The Sun's Place in Nature'; 'Recent and Coming Eclipses'; 'The Chemistry of the Sun'; 'Inorganic Evolution').

Before its presence was known on Earth, helium was identified in the sun. In 1868 a British astronomer, Joseph Norman Lockyer, used spectral analysis to isolate helium in the sun's spectrum. Thus helium got its name from the Greek word helios, meaning "sun."

Helium is a colourless, odourless, gaseous element. It is chemically inert; hence it will not burn or react with other materials. Helium has the lowest boiling point of any element. Next to hydrogen, it is the lightest known gas and the second most abundant element in the universe. It is found in great abundance in the stars, where it is synthesized by nuclear fusion of hydrogen. In the Earth's atmosphere, helium is present only in about 1 part per 186,000 because the Earth's gravity is not strong enough to prevent its gradual escape into space. The helium present in the Earth's atmosphere has been generated by the radioactive decay of heavy substances.

Decompression sickness (or bends, or caisson disease), physiological problem associated with underwater diving; results from differences in underwater pressure and air pressure.

Most of the world's helium occurs in natural-gas deposits in the United States. Smaller supplies have been discovered in Canada, South Africa, the Sahara Desert, and elsewhere. Its lightness and non flammability make helium ideal for use in the inflation of lighter-than-air craft. Because helium's boiling point is close to absolute zero, it is widely used in low-temperature research. It is also vital to the study of superconductivity.

Helium is used in arc welding. Deep-sea divers breathe a helium-oxygen mixture to prevent decompression sickness, also called the bends.

TERMITES

DEFINITION: any of an order (Isoptera) of pale-coloured social insects having a soft body and living in colonies composed of winged forms that mate and wingless workers and soldiers that are usually sterile or immature: they are very destructive to wooden structures and are found in the temperate zones and esp. in the tropics.

Although they are closely related to cockroaches, termites are sometimes called "white ants" because their general appearance and social organization are like those of the ants.

Termites, however, are distinguished from ants by their soft bodies and lighter colour Ants have hard bodies and are usually dark. Furthermore, the termite's mid body segment, or thorax, is broadly attached to the rear segment, or abdomen, whereas ants have a constriction where the thorax joins the abdomen. Termites belong to the insect order Isoptera.

Ground Nesters

More than 2,000 species of termite have been described, most of which live in the tropics. More than 40 species live in the United States. A typical colony lives underground in a damp, chamber like nest. The colony is organized into a caste system with four different adult forms: royalty, nobility, soldiers, and workers. The royalty consists of the kings and queens, which carry on the work of reproduction. They have well-developed wings and eyes. The kings are usually smaller than the queens, which may reach a length of 4.3 inches (11 centimetres) in some species.

Once a year pairs of young kings and queens depart from the parent nest, leaving the ruling king and queen behind. Each pair starts a new colony nearby. They then shed their wings. Within a short time the young queens may begin laying eggs at the rate of 3,000 to 5,000 a day. The nobility consists of wingless or short-winged adults. They take over the work of reproduction if a king or queen should die.

The soldiers and workers grey-white, wingless, usually blind, and less than 0.4 inch (1 centimetre) long are the most populous members of the colony. Both male and female soldiers and workers are sterile, so they cannot reproduce. The soldiers, which have large heads and jaws, guard the nest against insect enemies, chiefly ants. The workers keep the colony supplied with food, and they actually feed the queens, soldiers, and young termites.

Cellulose, complex carbohydrate consisting of 3,000 or more glucose units; basic structural component of plant cell walls; 90% of cotton and 50% of wood is cellulose; most abundant of all naturally occurring organic compounds; indigestible by humans; can be digested by herbivores, such as cows and horses, because they retain it long enough for digestion by micro-organisms present in their digestive systems; also digestible by termites; processed to produce papers and fibres; chemically modified to yield plastics, photographic film, and rayon; other derivatives used as adhesives, explosives, thickening agents, and in moisture-proof coatings.

Termites feed primarily on wood fibre, or cellulose, which they get from dead trees, rotting plant material in the soil, fence posts, house timbers, or furniture. Although some kinds of termites can destroy human dwellings, they serve a vital function in the food web by recycling the nutrients in dead wood so that the nutrients can be used by bacteria and plants.

Cellulose is indigestible to nearly all animals, large or small, including termites. The termite workers, however, have formed a remarkable partnership, or symbiosis, with micro-organisms called protozoans. The workers harbour the protozoans in their intestines. As they chew and swallow the wood fibre, the protozoans transform it into a product that termites can digest. Soldiers also have symbiotic protozoa, and they can digest cellulose after the workers have chewed it up for them. The soldiers' enormous fighting jaws prevent them from gathering this fibre for themselves. Royalty and nobility lack protozoans and are fed on digested cellulose secreted by workers.

Worker termites may eat wood that is above ground by entering the wood where the timbers touch the ground. If a house has a stone foundation, the termites may build tubular, earthen passages over the foundation and up to the house beams. The termites thus maintain their contact with the ground and the necessary moisture. Under a porch they may erect towers more than 1 foot (0.3 meter) high to reach the wooden floor.

Once inside the woodwork of a building, they tunnel in all directions, with no openings showing on the surface. Houses may be inspected for signs of termite problems by searching for hollow timbers, termite nests at the base of wood, or the insects themselves. Unfortunately, the first sign of their presence may be the collapse of a wall or some other wooden structure. Termites work in large numbers as many as 4,000 have been counted in 1 cubic foot (0.03 cubic meter) of wood.

To rid an area of termites would require the destruction of all the nests. It is more practical to "insulate" a building against the insects by treating woodwork with chemicals or by covering all possible points of attack with metal.

Ground-nesting termite, a termite (Reticulitermes flavipes) of the order Isoptera.

The scientific name of the common ground-nesting termite of eastern North America is Reticulitermes flavipes. Besides the ground-nesting termites there are dry wood, damp wood, and powder post termites. Many of these species live not in soil but in the wood that they attack. They do not require moisture from the soil because they can conserve water in their bodies. These termites can also be eliminated from infested sites by the use of chemicals.

Mound Builders

Mound-building termites live in South America, Africa, and Australia. Their brown mounds, or termitaries, often crowd together in a close group of slender towers. They are built of saliva-soaked soil particles and are as hard as concrete. Some termitaries are decades old and are more than 23 feet (7 meters) high and 43 feet (13 meters) wide at the base. The bases of some termitaries are oval, with the long axis pointing north and south, presumably so that the sun can reach both of the broad outside walls and keep them warm and dry.

Inside the walls of each termitary the social order is the same as that of the ground-nesting termites. The king and queen occupy the royal chamber. The king is small, but the queen is large and may carry as many as 75,000 eggs. Some of the larger queens lay one egg each second, 24 hours a day, during their reproductive life, which may last up to 10 years. After being laid the eggs are taken by nurses, washed with saliva to prevent mould, then carried to the hatchery, which is kept warm by decaying vegetation.

The sightless soldiers, with their strong scissors-like mandibles, guard every turn of the galleries inside the nest. Other soldiers, equipped with tough "helmets" to check any onrush of ants, guard the entrances from the outside world. The soldiers of some species have snouts through which they spray a sticky liquid that entangles the legs of their enemies and that also stupefies them. The worker caste gathers bits of wood to feed the entire community. Some termite colonies grow small mushrooms in fungus gardens for their food. Some have community "cows" small beetles called termitophiles that live only in termite nests and secrete a fluid relished by the termites.

Assisted by J. Whitfield Gibbons

RAINBOWS

DEFINITION: an arc or ring containing the colours of the spectrum in consecutive bands, formed in the sky by the refraction, reflection, and dispersion of light in rain or fog.

When light from a distant source, such as the sun, strikes a collection of water drops such as rain, spray, or fog a rainbow may appear. It appears as a multicoloured arc whose "ends" seem to touch the Earth. Rainbows are seen only when the observer is between the sun and the water drops, so rainbows appear in the part of the sky opposite the sun.

The centre of the rainbow's arc is located on an imaginary line extending from the light source through the observer's eye to the area of the water drops.

Rainbows are most commonly seen when the sun's rays strike raindrops falling from distant rain clouds. Generally, this is only in the early morning or late afternoon. When the sun is too far above the horizon no rainbow can be seen. When the sun is lower in the sky, however, part of the arc becomes visible. In fact, if the sun is low enough and the observer is located in a place that is high enough, such as on a mountain, in an aeroplane or a spaceship, the observer may see a circular rainbow.

The most brilliant and most commonly seen rainbow is called the primary rainbow. The arcs of colour in a rainbow are caused by the refraction, or bending, and internal reflection of light rays that enter the raindrops. A ray of white sunlight is actually composed of all the colours of the spectrum. Inside the drop the ray of white light is separated into the colours that make it up and reflected back toward the observer. In the primary rainbow the colours are, from inside to outside, violet, blue, green, yellow, orange, and red. The red band makes an angle of about 42 degrees with the sun's rays, and the other coloured bands make successively smaller angles. Sometimes another less intense rainbow may also be seen; this is called the secondary bow. The secondary bow, when visible, is seen outside the primary bow and with its colour sequence reversed. It is produced by light that has been reflected from two different points on the back of the drop before emerging into the air. Higher-order rainbows are very weak and so are rarely seen.

Occasionally, faintly coloured rings are seen just inside the primary bow. These are called spurious, or supernumerary, bows. When raindrops are extremely fine, an almost white bow, called a fog bow, is produced. A fog bow at night, sometimes called a lunar rainbow, is made by sunlight reflected from the moon and appears as a ring around the moon.

CATERPILLARS

DEFINITION: the worm like larva of various insects, esp. of a butterfly or moth [C-] trademark for a tractor equipped on each side with a continuous roller belt over cogged wheels, for moving over rough or muddy ground.

The larvae, or young, of butterflies and moths are called caterpillars, from the Latin catta pilosa, meaning "hairy cat." Although people usually recognize the hairy kinds, many caterpillars with bare skins are popularly called worms, such as the cabbage worm and army worm

A caterpillar's body consists of a head followed by 12 or 13 segments. Like all insects, it also has three pairs of permanent or "true" legs one pair on each of the first three segments directly behind the head. These true legs are usually hard, jointed, and tipped with tiny claws, but in a few caterpillars they are not developed. To support the rest of its long body, the caterpillar also has from two to five pairs of soft, thick prolegs that disappear when it changes into a moth or butterfly.

A caterpillar has six eyes like tiny beads on each side of the head, just above the strong upper jaws. It breathes through nine pore like openings, called tracheae, on each side of its body.

When a caterpillar hatches from the egg laid by a female butterfly or moth, it is usually very small. But it grows rapidly and soon gets too large for its skin. Thereupon the old skin splits, and the caterpillar wriggles out of it, revealing a soft new covering. This skin-shedding is called moulting and occurs four or five times. Some caterpillars eat their old skins. The hawk moth caterpillar, one of the largest, may grow 4 inches (10 centimetres) long; the clothes moth caterpillar, one of the smallest, seldom exceeds a quarter of an inch. Some caterpillars may take only a few days before they turn into butterflies or moths, but most last throughout the warm season. A very few may live as long as four years in the caterpillar form before they change.

Cocoon, envelope, often largely of silk, which an insect larva forms around itself.

The change that caterpillars undergo is called metamorphosis. The first step for many moth caterpillars is to build cocoons. They spin them with threads of sticky fluid that flows from an opening in the lower lip and hardens in the air.

Some caterpillars form bags of silk that entirely enclose them. Others roll up a leaf, fastening the edges with the silk. Many of the hairy kinds pad the cocoons with their own hair.

Some caterpillars do not build cocoons. Many of the moth caterpillars take shelter simply by burrowing in the ground or under a stone or fallen leaf. Butterfly caterpillars may suspend themselves from leaves or twigs by their tails, or spin a button of silk on a twig or leaf and hang from it by a silk girdle.

Pupa, quiescent stage between larva and adult in insect metamorphosis.

Whether protected by a cocoon or not, the caterpillar becomes ready to shed its last skin, and in place of it grows a tough flexible shell or case. When this happens it has become a pupa. The moth pupa is usually dull brown and mummy like The butterfly pupa, sometimes called a chrysalis, is shiny and often brilliantly coloured

Inside the pupal, or chrysalids, case, the rudimentary wings and other organs enlarge to make the moth or butterfly. This transformation from the larval to the adult stage may be completed in a few days or take several months.

To grow and prepare for this period of change, caterpillars eat enormously, causing widespread damage to trees, flowers, and crops. The larva of the Polyphemus moth, a species of the American silkworm, has been estimated to eat as much as 86,000 times its own weight during its 56 days as a caterpillar.

Caterpillars are the prey of many birds and insects, especially parasites. To avoid their attacks, caterpillars have various natural protections. Some are coloured to blend with their surroundings. Others have gaudy dots or stripes to make them look fierce or very large. A few give off unpleasant smells, and a very few grow poisonous nettle like hairs.

COLOURS (Part 1 of 3)

DEFINITION: 1 the sensation resulting from stimulation of the retina of the eye by light waves of certain lengths 2 the property of reflecting light of a particular wavelength: the distinct colours of the spectrum are red, orange, yellow, green, blue, indigo, and violet, each of these shading into the next; the primary colours of the spectrum are red, green, and blue, the light beams of which variously combined can produce any of the colours 3 any colouring matter; dye; pigment; paint: the primary colours of paints, pigments, etc. are red, yellow, and blue, which, when mixed in various ways, produce the secondary colours (green, orange, purple, etc.): black, white, and grey are often called colours (achromatic colours ), although black is caused by the complete absorption of light rays, white by the reflection of all the rays that produce colour, and grey by an imperfect absorption of all these rays 4 any colour other than black, white, or grey; chromatic colour: colour is distinguished by the qualities of hue (as red, brown, yellow, etc.), lightness (for pigmented surfaces) or brightness (for light itself), and saturation (the degree of intensity of a hue) 5 colour of the face; esp., a healthy rosiness or a blush 6 the colour of a person's skin 7 skin pigmentation of a particular people or racial group, esp. when other than white 8 [pl. ] a coloured badge, ribbon, costume, etc. that identifies the wearer 9 [pl. ] a) a flag or banner of a country, regiment, etc. b) the armed forces of a country, symbolized by the flag [to serve with the colours ] 10 [pl. ] the side that a person is on; position or opinion [stick to your colours ] 11 outward appearance or semblance; plausibility 12 appearance of truth, likelihood, validity, or right; justification [the circumstances gave colour to his contention] 13 general nature; character [the colour of his mind] 14 vivid quality or character, as in a personality, literary work, etc. 15 Art the way of using colour, esp. to gain a total effect 16 Law an apparent or prima-facie right 17 Mining a trace of gold found in panning 18 Music a) timbre, as of a voice or instrument; tone colour b) elaborate ornamentation 19 Particle Physics a unique, hypothetical force or charge on each type of quark that controls how quarks combine to form hadrons: although called red, green, and blue, they are not related to visual colours 20 Photog., TV, etc. reproduction of images in chromatic colours rather than in black, white, and grey

One of the most striking features of the visible world is the abundance of colour The most extensive parts of the Earth and its atmosphere air, soil, and water are usually coloured The sky can be blue or black or grey and even reddish or purplish. Soils can be black or brown or grey and even red. Bodies of water look blue or green. One of the important ways people obtain information about the world is by looking at the colours of things.

When the green leaves of a plant turn brown, it may be a sign that the plant is sick. It can also be a sign of the season of year, since in the autumn the leaves of many trees turn brown.

The colour of a fruit can reveal whether it is ripe. A green banana is unripe, a yellow one is ripe, and a yellow banana with brown and black spots is overripe. A green tomato is unripe, but a red one is ripe. Colour can also indicate the flavour of foods. Brown rice has a different flavour from that of white rice.

What does it mean to say that a tomato is red? Is colour part of the tomato in the same way that shape is? A tomato examined in the dark is still perceived as round but not as being red. It has no colour at all. Moreover, if a bright blue light is shined only on the tomato, it does not look red but black. So colour, unlike shape, depends on light. In fact, it cannot exist apart from light. Yet in a sense the tomato can be described as red.

Somehow, if the right kind of light shines on it, the tomato looks red. The colour of the tomato has something to do with the way light interacts with it.

But colour also has something to do with the persons and animals who see it. For the tomato to be red, viewers able to perceive colour are needed. Many kinds of animals cannot distinguish colours They see only in black, white, and greys A guinea pig looking at a tomato sees only a grey object. Colour exists the tomato is red because something happens in the eyes and the brains of certain persons and animals that enables them to perceive colour

It is possible to study colour from many points of view. Chemists and physicists, for example, have a special interest in colour Sometimes the molecular structure of chemicals or the physical arrangement of their atoms may reveal why they reflect only certain kinds of coloured light. Physicists who study optics a branch of physics have developed theories of colour Biologists and psychologists use many interesting techniques to find out what enables people's eyes and brains to perceive colour

Light from the noon-time sun looks white. But if a ray of white light is aimed at a prism, a broad band of different colours looking like a rainbow emerges. This colour array is called the visible spectrum.

In the 17th century Isaac Newton discovered that a second prism could not add more colour to light that had already passed through a prism. Red stayed red, green stayed green, and so on. But he observed that the second prism could spread the colours of the spectrum farther apart. A narrow red beam entering the second prism would emerge as a wider band of red. Newton also found that if he turned the second prism upside down so that the entire coloured band coming from the first prism entered it, white light would emerge. From these experiments he concluded that white light is a mixture of many different colours and that a prism is somehow able to bend it in such a way that the individual colours separate.

In the late 19th century the theory that light travels in the form of electromagnetic waves won acceptance. Waves are described by their speed, their wavelength, and their frequency. In a given medium, such as air or a vacuum, all light waves travel at the same speed, but they differ in wavelength and frequency. Wavelength and frequency are inversely proportional to each other the longer the wavelength, the lower the frequency.

For the visible light spectrum, scientists commonly specify only the wavelength.

Each colour is associated with a range of wavelengths. The name green or red does not apply to just one colour A wide segment of the spectrum contains colours that are called green. These include blue-green, apple green, and chartreuse, as well as many intermediate greens. Another wide segment contains colours that are called red. Colours of nearly the same wavelength look exactly alike to the human eye.

The colours of the spectrum range, in order, from violet, through blue, green, yellow, and orange, to red. The wavelengths of violet are the shortest, ranging from 380 to about 450 nanometres (A nanometre is one billionth of a meter long.) Wavelengths of red are the longest, ranging from about 630 to 760 nanometres Wavelengths shorter than those of violet are called ultraviolet radiation; wavelengths longer than those of red are infra-red radiation. They produce no sensation of colour in humans. "Black" is the absence of colour

Additive Mixing with Coloured Light

Newton discovered that by mixing two differently coloured rays of light he could produce other colours When he projected light beams from different prisms onto a white background, he found that sometimes the new colour looked like one of the other colours of the spectrum.

Red and yellow, for example, could be mixed to look like the orange of the spectrum. But colours could also be created in this way that did not look like any of the spectral colours Thus red and violet could form purples that did not match any colour in the spectrum.

Newton also observed that as certain coloured lights were combined, a grey or white patch of light was produced. He found that he could often obtain white light by mixing the beams of three different colours

Almost all colours can be matched by three beams of differently coloured light. The greatest number of different colours can be produced when the three colours are chosen from the middle and the two ends of the spectrum. In other words, a combination of one of the reds, a green, and a blue or violet will produce the greatest range of colours For this reason red, green, and deep blue are called the primaries for additive colour mixing, or additive primaries. These three colours are used more than any other combination of colours to mix coloured light beams.

When only two of the additive primaries are mixed in a certain amount, the resulting colour is called the complementary colour, or complement, of the third additive primary.

When red and green light beams are mixed, the resulting colour is yellow, the complementary colour of blue. A mixture of red and blue makes a purplish colour called magenta, the complement of green. And green and blue mixed together form cyan, the complement of red.

When the additive primaries are mixed in other amounts, intermediate colours are formed. This fact is the basis of the science of colorimetry, or colour measurement. Once the three primary colours are agreed upon, most other colours can be defined by the amounts of the three primary colours that, mixed together, match the new colour

Subtractive Colour Mixing

When light strikes an object, it may be transmitted, absorbed, or reflected. A windowpane, for example, transmits almost all the light that strikes it. Since it does not change the light, the pane looks colourless, or clear. A blackboard free of chalk dust, on the other hand, absorbs almost all the light that strikes it and therefore since blackness is the absence of light looks dull and black. A plaster wall both reflects and absorbs light. If the wall is white, it reflects almost all the light that falls on it.

Sometimes a substance absorbs some but not all the colours that reach it. For example, a red tomato absorbs all wavelengths but those of red, which, after bouncing from molecule to molecule within the top layers of the tomato, are redirected outward. When blue light (which does not contain red wavelengths) shines on a tomato, the blue wavelengths are absorbed. The tomato then looks black because no light is reflected from it.

Transparent red objects such as red cellophane, red plastic, or red glass absorb all wavelengths but red ones, which they partly transmit and partly reflect. Such transparent objects are called colour filters because when white light strikes them they filter out all colours except their own, which can pass through them easily.

Colour filters are the basis of subtractive colour mixing, just as coloured beams of light are the basis of additive mixing. Subtractive colour mixing is a complicated procedure because the different dye molecules in two different filters may produce the same colour sensation yet absorb different wavelengths of light. The description of subtractive colour mixing that follows assumes that ideal filters are used.

When a beam of white light strikes a yellow filter, the wavelengths that make up yellow can pass through the filter while all other wavelengths are absorbed. Since yellow is a mixture of green and red light, the wavelengths of those colours pass through, but the wavelengths of blue the complement of yellow are absorbed. Yellow is sometimes called minus-blue, since it can filter out blue light. Similarly, a magenta filter allows wavelengths of red and blue to pass but absorbs wavelengths of its complement, green. For this reason, magenta is sometimes called minus-green.

If a yellow filter (minus-blue) is placed on top of a magenta filter (minus-green) and a beam of white light is passed through them, the yellow filter absorbs blue, the magenta filter absorbs green, and only red light emerges.

A cyan filter (minus-red) absorbs its complement, red. If a yellow, a cyan, and a magenta filter are aligned in front of a beam of white light, all three of the additive primaries are absorbed, and no light emerges. This is called subtractive colour mixing because the filters absorb, or subtract, colour from a beam of light.

Pointillism (or divisionism), impressionistic painting process; the chief exponents were French artists Georges Seurat and Paul Signac.

Paint mixtures usually exhibit the complex behaviour of subtractive mixing. A mixture of yellow and cyan watercolours gives one of several greens, depending on what pigments make up the original cyan and yellow paints. If magenta is then added, black or grey results. However, pigments can be combined in additive mixtures by means of special techniques. A famous method is divisionism, sometimes called pointillism, which was used by some post-impressionist painters. They painted tiny dots of pure spectrum colours next to one another so that light reflected by one dot would combine with light reflected by a second dot in an additive mixture. One of the most famous paintings of this school is Georges Seurat's 'Sunday Afternoon on the Island of the Grande Jatte'.

Colours produced by the subtraction of wavelengths, or filtering, often occur in nature. The reds and oranges of a sunset are caused by the filtering action of the sky. The sky scatters light of short wavelengths, such as blue. At midday, when the sun is overhead, the scattered blue light does not have to travel through very much air to reach a viewer.

The sky looks blue because a great deal of blue light is reflected from it. But at sunset the light must travel through much more air on its way to Earth. The blue is soon scattered, and only the colours of longer wavelengths combined to appear orange and red can be seen.

COLOURS (Part 2 of 3)

Colour Classification Systems

People who make, sell, or use nail polish, lipstick, paint, ink, and many other products deal with very small variations in colour Colour classification systems have been developed that enable them to specify and obtain the precise colours they want. Some of these systems show how colours differ in ordinary daylight. Others calculate the wavelengths of light that pass through filters of different colours when a special light source, such as a tungsten lamp, is used.

The Munsell system arranges colour samples according to three qualities hue, value, and chroma. Hue is what is usually meant by the word colour Red, blue, green, and yellow are hues. The Munsell system divides all hues into ten categories: yellows, green-yellows, greens, blue-greens, blues, purple-blues, purples, red-purples, reds, and yellow-reds. Hues are often arranged in a circle. Value is the Munsell term for the lightness of a coloured sample. A yellow material may be light while a blue material may be dark. A series of greys, from black to white, best define value. Chroma defines the amount of hue in a given sample. The word chroma is related to the word chromatic and describes colours ranging from grey to vivid hues. A brick and a ripe tomato, for example, may have the

same red hue and the same value. Their difference in colour is a difference in chroma.

Thus colours can vary in hue, value, and chroma.

In addition to the Munsell system, there are many colour classification systems that relate to colour perception. Two examples are the Optical Society of America Uniform Colour Scales (OSA-UCS) and the Swedish Natural Colour System (NCS). The NCS system arranges colours based on the perceptions of white, black, red, green, yellow, and blue, with only four perceptions for a given colour A purple, for example, may consist of white, black, red, and blue. With experience, one can assign percentages to each perception.

There are also classification systems based on colourant mixtures. These systems are very useful to help visualize how colours mix together. Examples include the Ostwald System and the Pantone Matching System.

Finally, numerical systems have been developed based on the knowledge of the eye's physiology and extensive experimental studies. These numerical systems have been standardized by the Commission Internationale de l'Eclairage (International Commission on Illumination), or CIE. A colour is specified by three numbers relating to the eye's three colour receptors. To specify a colour, one first performs physical measurements on how much a coloured material will reflect or transmit light across the visible spectrum.

Computations follow using this wavelength information to arrive at the three numbers. This system is used to specify the colour of most man-made products and forms the basis for standardizing present and future colour television signals, including high-definition television (HDTV).

Colour Chemistry

Subtractive mixing is based on the way matter affects light. Somehow a beam of white light is changed when it meets certain kinds of matter. Some of the light stays in the matter is absorbed. As a result, the light that emerges is reflected or transmitted has a different colour This happens in part because of the way matter is constructed.

All matter consists of atoms. Each atom contains a dense, heavy centre called a nucleus and one or more electrons that are in continuous motion around the nucleus. According to atomic theory, distinct quantities of energy are available to each of these electrons. An electron can have the quantity of energy dictated by one or another of the atom's energy levels, but it cannot have an intermediate quantity.

Transition element, any of various chemical elements that have valence electrons in two shells instead of one.

Sometimes an atom has two electronic energy levels whose difference is equal to the amount of energy possessed by a light quantum associated with a certain wavelength. This is a characteristic of a series of chemical elements called the transition elements. In chemical combination the atoms of these elements can absorb visible light. The light energy boosts the electrons into higher energy levels. The electrons then dissipate this energy in the form of heat and return to their normal energy levels. The compounds of the transition element cobalt, for example, are known for the brilliant blue that is left after they absorb and dissipate red light.

Many kinds of molecules, or combinations of atoms that form a chemical substance, have electronic energy levels that lie close together, a situation similar to that of the transition elements. The molecules that can absorb and dissipate visible light usually contain many double bonds. These light-sensitive coloured molecules make up a very important group of chemicals. The green pigment chlorophyll, found in the leaves of plants, absorbs light energy that is then converted to food energy. Four types of similar light-sensitive molecules are involved in human vision, each sensitive to a different range of wavelengths.

Dyes are another group of chemicals that often contain many double bonds. The exact structure of a given dye determines the energy levels available to the electrons and, therefore, the wavelengths that the dye will be able to absorb. For example, when the molecules of a substance can be linked chemically with a textile fibre, the substance can be used as a dye.

Colour Perception

In a psychological sense colour can exist without light. People in a completely dark room can "see" colour by shutting their eyes tightly. When they do this, coloured spots called phosphenes seem to appear in front of their eyes. Phosphenes have also been produced by direct stimulation of the brain and by stimulation of the eye with pressure or electricity.

Persistence of vision in another example of colour perception in the absence of a physical stimulus. When people watch a motion picture, they are actually observing a series of rapidly projected still pictures. During the very short interval between pictures, a person retains an image of the preceding picture. This image blends into that of the following picture, giving an impression of continuous motion. The retained image is called a positive after-image Similarly, if people look at a patch of one colour for about 30 seconds and then look at a blank sheet of white or grey paper, they will probably see a patch of colour that is the complement of the original colour This is called a negative after-image

When light reflected from an object enters a human eye, it passes through the cornea, the pupil, and the lens and lands on the retina. The retina contains two kinds of light-detecting cells. These cells are called rods and cones. The cones are colour sensors. The rods make night vision possible.

Young, Thomas (1773-1829), English physicist and physician, born in Milverton, Somerset; discovered interference of light; offered red-green-violet theory of light perception; professor of natural philosophy at Royal Institution and foreign secretary of Royal Society; helped decipher text of Rosetta stone.

In the early 1800s Thomas Young advanced a theory of human vision that was later elaborated by Hermann von Helmholtz. A modern version of the Young-Helmholtz theory states that the eye contains three kinds of colour receptors, or cones. One kind has greatest sensitivity to green light, another to red light, and the third to blue light. According to this theory, any other colour stimulates more than one kind of cone in varying amounts, depending on the mixture of wavelengths in the colour

Four different light-sensitive chemicals are found in the human eye. Rhodopsin, located in the rods, seems to be limited to black-and-white vision. The other three, called iodopsins, are involved in colour vision. However, eye chemistry alone does not account for man's ability to identify colours A part is also played by the brain, which may contain separate colour-detection centres

Hering, Ewald (1834-1918), German physiologist and psychologist, born in Alt-Gersdorf, Saxony; advanced theory of four colours occurring in pairs as opposed to three-colour theory of Helmholtz.

In the 1870s Ewald Hering suggested that there were four primary colours blue, green, yellow, and red. He arranged these on a circle, with red opposite green and blue opposite yellow. The circle could be filled in with intermediate colours Hering considered colours opposite each other to be opponents. He regarded white and black as a special pair of opponents. Hering's theory agrees with the common notion that red and yellow are perceived psychologically as warm colours, while blue and green, their opponents, are regarded as cool colours The three cone receptors hypothesized by Young and Helmholtz form the first stage of colour perception. Electrical signals generated by the cones combine in the retina to yield opponent signals as suggested by Hering. There are three signals: white-black, red-green, and yellow-blue. Thus colour vision begins with a two stage-process. Signals sent to the brain along the optic nerve are coded into these opponent signals.

A series of experiments performed by Edwin H. Land in the 1950s called both theories into question. Land demonstrated that a wide range of colours could be produced from a mixture of only two colours He photographed the same scene twice once with light having long wavelengths, and once with light having medium wavelengths. He called the two types of photographs that resulted the long record (for long wavelengths) and the short record. When Land projected the two records onto the same screen using red light to project the long record and white light to project the short record the image on the screen seemed to have a full range of colours Land suggested that some of the colours seen whose wavelengths were absent were perceived through a process involving the comparison of surrounding colours He surmised that the total combination of colours in a scene played an important part in colour vision. His experiments can also be interpreted as proof for the two-stage theory of colour vision.

Abnormal Colour Vision

Some people suffer from abnormal colour vision, or colour blindness. When asked to pick out chips that have the same colour a process called colour matching such people may pair chips that do not look at all alike to most observers. Abnormal colour vision is usually inherited, but it may also result from a chemical imbalance in the body or from eye injuries.

Scientists classify colour blindness into three major types. The most severe, as well as the most unusual, is mono chromatism, or total colour blindness. A person who is completely colour-blind cannot distinguish individual hues. To him all colours match with greys of the same lightness.

Dichromatism, or partial colour blindness, is a more widespread abnormality. Some dichromatic people confuse red, green, and grey but are able to distinguish blue and yellow. Others cannot see the longest wavelengths of light the red end of the visible spectrum. Rare forms of dichromatism include the inability to distinguish among blue, yellow, and grey and the inability to see light of very short wavelengths the violet end of the spectrum.

Normal colour vision the vision that most people possess is trichromatic. The most common type of colour blindness is a variation of normal colour vision called anomalous trichromatism. The whole range of colours visible to people with normal colour vision is also visible to people with this condition, but they match colours that do not appear the same to people with normal colour vision. For example, in a test in which mixtures of red and green light are varied to match a yellow sample, a person with green-weak vision adds more green light to match the yellow than does a person of normal colour vision. Green-weak and red-weak vision are the most prevalent forms of anomalous trichromatism; the blue-weak form is extremely rare.

COLOURS (Part 3 of 3)

Colour Vision in Animals

Since animals cannot answer questions about the colours they perceive, scientists have had to develop experiments to find out whether animals can be trained to make choices on the basis of colour If an animal's food is always placed under a red square instead of a green square and if the animal consistently looks under the red square when it is hungry, scientists conclude that the animal can distinguish between red and green. Since monkeys and apes can be trained in this way and, in addition, their retinal cells contain colour-sensitive chemicals, researchers are convinced that these primates have colour vision.

Non primate mammals tend to be insensitive to colour differences, but it is not certain whether this means that they cannot perceive them or that they do not regard them as important. Cats, for example, are commonly assumed to be colour-blind, but claims have been made that they can be trained to discriminate between some colours, among them blue and green, by first linking the colours with position. In any case, their sensitivity to colour is not great.

Birds have good colour discrimination, somewhat similar to that of humans. Many of their behaviour patterns for example, the identification of their mates or of their prey are based on the recognition of colour Fish can also discriminate among colours Bees have colour vision similar to human colour vision, except that it includes ultraviolet wavelengths too short for humans to perceive, and it excludes the red end of the spectrum, visible to humans.

Techniques for Reproducing Colour

When early humans painted pictures on cave walls, they used the pigments in coloured earth and clay to give colour to their creations. Red, black, and yellow pigments were the easiest to find. Then humans learned how to create new colours by mixing these three pigments. Other pigments, it was discovered, produced other colours, such as orange, brown, and blue-black.

As humans learned how to make pottery and carvings and to weave fabrics, they also learned how to apply colour to these new objects. Pigments had to be applied to pottery and carvings in a form that would adhere to their surfaces. The materials and techniques originally employed were closely related to those that had been used in painting pictures on flat surfaces. Naturally available pigments were mixed to get new colours

From the beginning, however, making dyes for woven fabrics involved chemical processes. At first, parts of plants were boiled to separate coloured chemicals. The cloth was then soaked in the resulting coloured solutions.

Today chemical reactions are used in various ways to produce new dyes. The changes in colour that result from these reactions are a consequence of changes in chemical structure and cannot be explained by the laws of colour mixing.

Modern techniques for printing pictures in full colour can become quite complicated. They are based on colour separation at one stage and subtractive colour mixing at another and may also include additive colour mixing. The three basic colours of colour printing yellow, cyan, and magenta can be mixed in various proportions to duplicate almost any colour

The first step in colour printing is to scan the original photograph or piece of artwork with a light beam that gets split into three beams after it has passed through, or has been reflected from, the original document. Each beam then strikes a photocell that is covered with a filter coloured to match one of the additive primary colours In this way each area of the original is separated into its three colour components. Four computers are used to correct the colour via electric currents that are fed into the computers from the photocells.

Each computer corresponds to one of the additive primary colours and one computer is reserved for the black component, which is computed from the other three signals. Exposing lights manipulate the modified currents from the computers and then expose the corrected colour separations on film or paper.

In half-tone printing, the colour is applied as an array of tiny dots. Where both cyan and magenta ink are applied, some of the dots will be cyan and some magenta. Printed side by side, both colours are reflected, and they mix additively to form blue. Where yellow and magenta are reflected in equal strength, they mix additively to form red. Intermediate shades can be formed by varying either the size or the number of the dots and by printing the dots on top of one another.

Colour television also works by first separating colours into their additive primaries and then recombining them by additive mixing of coloured dots. In some cameras, mirrors separate the light into three beams. The first beam passes through a blue filter, the second through a green filter, and the third through a red filter. Three camera tubes then record the colour information in the form of electromagnetic signals, which are beamed to the television receiver. Consumer cameras may use a single detector chip, known as a charge-coupled device (CCD). Red, green, and blue filters are affixed directly to the chip.

A colour picture tube contains three electron guns, one for each colour The back of the television screen is coated with tiny dots of chemicals called phosphors. When a phosphor is hit by an electron, it gives off wavelengths of light. Most colour television sets contain three kinds of phosphors that give off blue, green, and red light, respectively. A screen called a shadow mask lies behind the phosphor layer. The electron gun that receives the blue signal fires electrons toward the phosphors. The shadow mask screens the red-emitting and green-emitting phosphors from these electrons so that only the blue-emitting phosphors are activated. Similarly, the electron gun that receives the red signal is screened from all but the red-emitting phosphors, and the one that receives the green signal is screened from all but the green-emitting phosphors. The colours mix additively to form a wide range of colours

Colour photography is a blend of chemistry and the additive and subtractive principles of colour mixing. A typical colour film consists of three layers of chemicals, each of which is sensitive to one of the additive primaries. The top layer contains chemicals that react to blue light only. Below it lies a yellow filter that absorbs all blue light passing through the top layer. Green and red light pass through the top layer and the filter to reach the middle layer, which is sensitive to green and blue. Since the yellow filter has stopped all the blue light, only green light affects the middle layer. Red passes through this layer to the bottom layer, which is sensitive to red and blue and partly to green. Since the blue light and the green light have already been filtered out, only the red light can cause a chemical change in the bottom layer. In this way, three records are made of the scene, each containing information contributed by one of the additive primaries.

The three records may be combined into a colour transparency, which is dyed in appropriate combinations of the subtractive colour primaries magenta, cyan, and yellow. White light may be shined through the transparency and the resulting image projected onto a screen. If the transparency has been dyed with cyan and magenta only, the cyan absorbs red light, the magenta absorbs green light, and the light transmitted and projected onto the screen is blue. If the transparency has been dyed with yellow and a small amount of magenta, no blue can pass through it, and some but not all of the green is absorbed, leaving a mixture of wavelengths that produces a sensation of orange.

A photographic technique called holography uses laser light to produce a three-dimensional image of a subject. A record called a hologram is made of the interference pattern between laser light that has bounced off the subject and light coming directly from the laser. When laser light is shined through the hologram, light deflected by the hologram reconstructs the image of the subject. If an observer walks around the hologram and views it from different angles, he can see the changing perspective just as if the original subject were still there. Techniques are being developed to obtain holographic images using laser light of three different colours, so that a full-colour image can be produced. A problem not yet overcome is that ghost images appear to the side of the actual image.

Ultimately, holography may be used to produce three-dimensional television.

Response to Colour

On the whole, people tend to regard blue and green as cool, quiet colours, while yellow and, especially, red are considered warm colours Individual colour preferences may be based on this general difference in responses to colour Tests have shown that the colour preferences of children tend to shift from warmer to cooler colours as they grow older.

Other tests have shown that blue is the most widely preferred colour, with red, green, violet, orange, and yellow as runners-up. However, variations on these colours have produced different rankings. Yellowish green usually ranks below a relatively pure green.

It has also been shown that many people prefer pure, or saturated, colours

Assisted by Roy S. Berns, Director of the Rochester Institute of Technology's Munsell Colour Science Laboratory in New York.

BIBLIOGRAPHY FOR COLOUR

Anderson, L.W. Light and Color, rev. ed. (Raintree, 1988).

De Grandis, Luigina. Theory and Use of Color (Prentice, 1986).

Falk, D.S. and others. Seeing the Light: Optics in Nature, Photography Color, Vision, and Holography (Harper, 1986).

Hoban, Tana. Of Colors and Things (Greenwillow, 1989).

Kuehni, R.G. Colour, Essence and Logic (Van Nostrand Reinhold, 1983).

McLaren, K. The Colour Science of Dyes and Pigments, 2nd ed. (Hilger, 1986).

Nassau, Kurt. The Physics and Chemistry of Color (Wiley, 1983).

Norman, R.B. Electronic Color: The Art of Color Applied to Graphic Computing (Van Nostrand Reinhold, 1990).

Rossotti, Hazel. Colour (Princeton Univ. Press, 1985).

Thorell, L.G. and Smith, W.J. Using Computer Color Effectively (Prentice, 1990).

Williamson, S.J. and Cummins, H.Z. Light and Color in Nature and Art (Wiley, 1983).