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Color is an important part of human expression.

Color or colour (see spelling differences) is the visual perceptual property corresponding in humans to the categories called red, yellow, blue and others. Color derives from the spectrum of light (distribution of light energy versus wavelength) interacting in the eye with the spectral sensitivities of the light receptors. Color categories and physical specifications of color are also associated with objects, materials, light sources, etc., based on their physical properties such as light absorption, reflection, or emission spectra. By defining a color space, colors can be identified numerically by their coordinates.

Because perception of color stems from the varying sensitivity of different types of cone cells in the retina to different parts of the spectrum, colors may be defined and quantified by the degree to which they stimulate these cells. These physical or physiological quantifications of color, however, do not fully explain the psychophysical perception of color appearance.

The science of color is sometimes called chromatics. It includes the perception of color by the human eye and brain, the origin of color in materials, color theory in art, and the physics of electromagnetic radiation in the visible range (that is, what we commonly refer to simply as light).

Contents

Physics of color

Continuous optical spectrum (designed for monitors with gamma 1.5).
The colors of the visible light spectrum[1]
color wavelength interval frequency interval
red ~ 700–635 nm ~ 430–480 THz
orange ~ 635–590 nm ~ 480–510 THz
yellow ~ 590–560 nm ~ 510–540 THz
green ~ 560–490 nm ~ 540–610 THz
blue ~ 490–450 nm ~ 610–670 THz
violet ~ 450–400 nm ~ 670–750 THz
Color, wavelength, frequency and energy of light
Color \lambda \,\!/nm \nu \,\!/1014 Hz \nu_b \,\!/104 cm−1 E \,\!/eV E \,\!/kJ mol−1
Infrared >1000 <3.00 <1.00 <1.24 <120
Red 700 4.28 1.43 1.77 171
Orange 620 4.84 1.61 2.00 193
Yellow 580 5.17 1.72 2.14 206
Green 530 5.66 1.89 2.34 226
Blue 470 6.38 2.13 2.64 254
Violet 420 7.14 2.38 2.95 285
Near ultraviolet 300 10.0 3.33 4.15 400
Far ultraviolet <200 >15.0 >5.00 >6.20 >598

Electromagnetic radiation is characterized by its wavelength (or frequency) and its intensity. When the wavelength is within the visible spectrum (the range of wavelengths humans can perceive, approximately from 380 nm to 740 nm), it is known as "visible light".

Most light sources emit light at many different wavelengths; a source's spectrum is a distribution giving its intensity at each wavelength. Although the spectrum of light arriving at the eye from a given direction determines the color sensation in that direction, there are many more possible spectral combinations than color sensations. In fact, one may formally define a color as a class of spectra that give rise to the same color sensation, although such classes would vary widely among different species, and to a lesser extent among individuals within the same species. In each such class the members are called metamers of the color in question.

Spectral colors

The familiar colors of the rainbow in the spectrum – named using the Latin word for appearance or apparition by Isaac Newton in 1671 – include all those colors that can be produced by visible light of a single wavelength only, the pure spectral or monochromatic colors. The table at right shows approximate frequencies (in terahertz) and wavelengths (in nanometers) for various pure spectral colors. The wavelengths are measured in vacuum (see refraction).

The color table should not be interpreted as a definitive list – the pure spectral colors form a continuous spectrum, and how it is divided into distinct colors linguistically is a matter of culture and historical contingency (although people everywhere have been shown to perceive colors in the same way[2]). A common list identifies six main bands: red, orange, yellow, green, blue, and violet. Newton's conception included a seventh color, indigo, between blue and violet – but most people do not distinguish it, and most color scientists do not recognize it as a separate color; it is sometimes designated as wavelengths of 420–440 nm.

The intensity of a spectral color may alter its perception considerably; for example, a low-intensity orange-yellow is brown, and a low-intensity yellow-green is olive-green.

For discussion of non-spectral colors, see below.

Color of objects

The upper disk and the lower disk have exactly the same objective color, and are in identical gray surrounds; based on context differences, humans perceive the squares as having different reflectances, and may interpret the colors as different color categories; see same color illusion.

The color of an object depends on both the physics of the object in its environment and the characteristics of the perceiving eye and brain. Physically, objects can be said to have the color of the light leaving their surfaces, which normally depends on the spectrum of the incident illumination and the reflectance properties of the surface, as well as potentially on the angles of illumination and viewing. Some objects not only reflect light, but also transmit light or emit light themselves (see below), which contribute to the color also. And a viewer's perception of the object's color depends not only on the spectrum of the light leaving its surface, but also on a host of contextual cues, so that the color tends to be perceived as relatively constant: that is, relatively independent of the lighting spectrum, viewing angle, etc. This effect is known as color constancy.

Some generalizations of the physics can be drawn, neglecting perceptual effects for now:

  • Light arriving at an opaque surface is either reflected "specularly" (that is, in the manner of a mirror), scattered (that is, reflected with diffuse scattering), or absorbed – or some combination of these.
  • Opaque objects that do not reflect specularly (which tend to have rough surfaces) have their color determined by which wavelengths of light they scatter more and which they scatter less (with the light that is not scattered being absorbed). If objects scatter all wavelengths, they appear white. If they absorb all wavelengths, they appear black.
  • Opaque objects that specularly reflect light of different wavelengths with different efficiencies look like mirrors tinted with colors determined by those differences. An object that reflects some fraction of impinging light and absorbs the rest may look black but also be faintly reflective; examples are black objects coated with layers of enamel or lacquer.
  • Objects that transmit light are either translucent (scattering the transmitted light) or transparent (not scattering the transmitted light). If they also absorb (or reflect) light of varying wavelengths differentially, they appear tinted with a color determined by the nature of that absorption (or that reflectance).
  • Objects may emit light that they generate themselves, rather than merely reflecting or transmitting light. They may do so because of their elevated temperature (they are then said to be incandescent), as a result of certain chemical reactions (a phenomenon called chemoluminescence), or for other reasons (see the articles Phosphorescence and List of light sources).
  • Objects may absorb light and then as a consequence emit light that has different properties. They are then called fluorescent (if light is emitted only while light is absorbed) or phosphorescent (if light is emitted even after light ceases to be absorbed; this term is also sometimes loosely applied to light emitted because of chemical reactions).

For further treatment of the color of objects, see structural color, below.

To summarize, the color of an object is a complex result of its surface properties, its transmission properties, and its emission properties, all of which factors contribute to the mix of wavelengths in the light leaving the surface of the object. The perceived color is then further conditioned by the nature of the ambient illumination, and by the color properties of other objects nearby, via the effect known as color constancy and via other characteristics of the perceiving eye and brain.

Color perception

Normalized typical human cone cell responses (S, M, and L types) to monochromatic spectral stimuli

Development of theories of color vision

Main article: Color theory

Although Aristotle and other ancient scientists had already written on the nature of light and color vision, it was not until Newton that light was identified as the source of the color sensation. In 1810, Goethe published his comprehensive Theory of Colors. In 1801 Thomas Young proposed his trichromatic theory, based on the observation that any color could be matched with a combination of three lights. This theory was later refined by James Clerk Maxwell and Hermann von Helmholtz. As Helmholtz puts it, "the principles of Newton's law of mixture were experimentally confirmed by Maxwell in 1856. Young's theory of color sensations, like so much else that this marvellous investigator achieved in advance of his time, remained unnoticed until Maxwell directed attention to it."[3]

At the same time as Helmholtz, Ewald Hering developed the opponent process theory of color, noting that color blindness and afterimages typically come in opponent pairs (red-green, blue-yellow, and black-white). Ultimately these two theories were synthesized in 1957 by Hurvich and Jameson, who showed that retinal processing corresponds to the trichromatic theory, while processing at the level of the lateral geniculate nucleus corresponds to the opponent theory.[4]

In 1931, an international group of experts known as the Commission internationale de l'éclairage (CIE) developed a mathematical color model, which mapped out the space of observable colors and assigned a set of three numbers to each.

Color in the eye

This image (when viewed in full size, 1000 pixels wide) contains 1 million pixels, each of a different color. The human eye can distinguish about 10 million different colors[5]

The ability of the human eye to distinguish colors is based upon the varying sensitivity of different cells in the retina to light of different wavelengths. The retina contains three types of color receptor cells, or cones. One type, relatively distinct from the other two, is most responsive to light that we perceive as violet, with wavelengths around 420 nm. (Cones of this type are sometimes called short-wavelength cones, S cones, or, misleadingly, blue cones.) The other two types are closely related genetically and chemically. One of them (sometimes called long-wavelength cones, L cones, or, misleadingly, red cones) is most sensitive to light we perceive as yellowish-green, with wavelengths around 564 nm; the other type (sometimes called middle-wavelength cones, M cones, or, misleadingly, green cones) is most sensitive to light perceived as green, with wavelengths around 534 nm.

Light, no matter how complex its composition of wavelengths, is reduced to three color components by the eye. For each location in the visual field, the three types of cones yield three signals based on the extent to which each is stimulated. These values are sometimes called tristimulus values.

The response curve as a function of wavelength for each type of cone is illustrated above. Because the curves overlap, some tristimulus values do not occur for any incoming light combination. For example, it is not possible to stimulate only the mid-wavelength (so-called "green") cones; the other cones will inevitably be stimulated to some degree at the same time. The set of all possible tristimulus values determines the human color space. It has been estimated that humans can distinguish roughly 10 million different colors.[5]

The other type of light-sensitive cell in the eye, the rod, has a different response curve. In normal situations, when light is bright enough to strongly stimulate the cones, rods play virtually no role in vision at all.[6] On the other hand, in dim light, the cones are understimulated leaving only the signal from the rods, resulting in a colorless response. (Furthermore, the rods are barely sensitive to light in the "red" range.) In certain conditions of intermediate illumination, the rod response and a weak cone response can together result in color discriminations not accounted for by cone responses alone.

Color in the brain

The visual dorsal stream (green) and ventral stream (purple) are shown. The ventral stream is responsible for color perception.

While the mechanisms of color vision at the level of the retina are well-described in terms of tristimulus values (see above), color processing after that point is organized differently. A dominant theory of color vision proposes that color information is transmitted out of the eye by three opponent processes, or opponent channels, each constructed from the raw output of the cones: a red-green channel, a blue-yellow channel and a black-white "luminance" channel. This theory has been supported by neurobiology, and accounts for the structure of our subjective color experience. Specifically, it explains why we cannot perceive a "reddish green" or "yellowish blue," and it predicts the color wheel: it is the collection of colors for which at least one of the two color channels measures a value at one of its extremes.

The exact nature of color perception beyond the processing already described, and indeed the status of color as a feature of the perceived world or rather as a feature of our perception of the world, is a matter of complex and continuing philosophical dispute (see qualia).

Nonstandard color perception

Color deficiency

If one or more types of a person's color-sensing cones are missing or less responsive than normal to incoming light, that person can distinguish fewer colors and is said to be color deficient or color blind (though this latter term can be misleading; almost all color deficient individuals can distinguish at least some colors). Some kinds of color deficiency are caused by anomalies in the number or nature of cones in the retina. Others (like central or cortical achromatopsia) are caused by neural anomalies in those parts of the brain where visual processing takes place.

Tetrachromacy

While most humans are trichromatic (having three types of color receptors), many animals, known as tetrachromats, have four types. These include some species of spiders, most marsupials, birds, reptiles, and many species of fish. Other species are sensitive to only two axes of color or do not perceive color at all; these are called dichromats and monochromats respectively. A distinction is made between retinal tetrachromacy (having four pigments in cone cells in the retina, compared to three in trichromats) and functional tetrachromacy (having the ability to make enhanced color discriminations based on that retinal difference). As many as half of all women are retinal tetrachromats.[7] The phenomenon arises when an individual receives two slightly different copies of the gene for either the medium- or long-wavelength cones, which are carried on the x-chromosome. To have two different genes, a person must have two x-chromosomes, which is why the phenomenon only occurs in women.[7] For some of these retinal tetrachromats, color discriminations are enhanced, making them functional tetrachromats.[7]

Synesthesia

In certain forms of synesthesia, perceiving letters and numbers (grapheme–color synesthesia) or hearing musical sounds (music–color synesthesia) will lead to the unusual additional experiences of seeing colors. Behavioral and functional neuroimaging experiments have demonstrated that these color experiences lead to changes in behavioral tasks and lead to increased activation of brain regions involved in color perception, thus demonstrating their reality, and similarity to real color percepts, albeit evoked through a non-standard route.

Afterimages

After exposure to strong light in their sensitivity range, photoreceptors of a given type become desensitized. For a few seconds after the light ceases, they will continue to signal less strongly than they otherwise would. Colors observed during that period will appear to lack the color component detected by the desensitized photoreceptors. This effect is responsible for the phenomenon of afterimages, in which the eye may continue to see a bright figure after looking away from it, but in a complementary color.

Afterimage effects have also been utilized by artists, including Vincent van Gogh.

Color constancy

There is an interesting phenomenon which occurs when an artist uses a limited color palette: the eye tends to compensate by seeing any grey or neutral color as the color which is missing from the color wheel. E.g., in a limited palette consisting of red, yellow, black and white, a mixture of yellow and black will appear as a variety of green, a mixture of red and black will appear as a variety of purple, and pure grey will appear bluish.[citation needed]

The trichromatic theory discussed above is strictly true only if the whole scene seen by the eye is of one and the same color which, of course, is unrealistic. In reality, the brain compares the various colors in a scene to eliminate the effects of the illumination. If a scene is illuminated with one light, and then with another, as long as the difference between the light sources stays within a reasonable range, the colors in the scene appear constant to us. This was studied by Edwin Land in the 1970s and led to his retinex theory of color constancy.

Color naming

Colors vary in several different ways, including hue (red vs. orange vs. blue), saturation, brightness, and gloss. Some color words are derived from the name of an object of that color, such as "orange" or "salmon", while others are abstract, like "red".

Different cultures have different terms for colors, and may also assign some color names to slightly different parts of the spectrum: for instance, the Chinese character 青 (rendered as qīng in Mandarin and ao in Japanese) has a meaning that covers both blue and green; blue and green are traditionally considered shades of "青." South Korea, on the other hand, differentiates between blue and green by using "綠 (녹)" for green and "靑 (청)" for blue.

In the 1969 study Basic Color Terms: Their Universality and Evolution, Brent Berlin and Paul Kay describe a pattern in naming "basic" colors (like "red" but not "red-orange" or "dark red" or "blood red", which are "shades" of red). All languages that have two "basic" color names distinguish dark/cool colors from bright/warm colors. The next colors to be distinguished are usually red and then yellow or green. All languages with six "basic" colors include black, white, red, green, blue and yellow. The pattern holds up to a set of twelve: black, grey, white, pink, red, orange, yellow, green, blue, purple, brown, and azure (distinct from blue in Russian and Italian but not English).

Associations

Individual colors have a variety of cultural associations such as national colors (in general described in individual color articles and color symbolism). The field of color psychology attempts to identify the effects of color on human emotion and activity. Chromotherapy is a form of alternative medicine attributed to various Eastern traditions.

Spectral colors and color reproduction

The CIE 1931 color space chromaticity diagram. The outer curved boundary is the spectral (or monochromatic) locus, with wavelengths shown in nanometers. Note that the colors depicted depend on the color space of the device on which you are viewing the image, and therefore may not be a strictly accurate representation of the color at a particular position, and especially not for monochromatic colors.

Most light sources are mixtures of various wavelengths of light. However, many such sources can still have a spectral color insofar as the eye cannot distinguish them from monochromatic sources. For example, most computer displays reproduce the spectral color orange as a combination of red and green light; it appears orange because the red and green are mixed in the right proportions to allow the eye's red and green cones to respond the way they do to orange.

A useful concept in understanding the perceived color of a non-monochromatic light source is the dominant wavelength, which identifies the single wavelength of light that produces a sensation most similar to the light source. Dominant wavelength is roughly akin to hue.

There are many color perceptions that by definition cannot be pure spectral colors due to desaturation or because they are purples (mixtures of red and violet light, from opposite ends of the spectrum). Some examples of necessarily non-spectral colors are the achromatic colors (black, gray and white) and colors such as pink, tan, and magenta.

Two different light spectra that have the same effect on the three color receptors in the human eye will be perceived as the same color. This is exemplified by the white light emitted by fluorescent lamps, which typically has a spectrum of a few narrow bands, while daylight has a continuous spectrum. The human eye cannot tell the difference between such light spectra just by looking into the light source, although reflected colors from objects can look different. (This is often exploited e.g., to make fruit or tomatoes look more intensely red.)

Similarly, most human color perceptions can be generated by a mixture of three colors called primaries. This is used to reproduce color scenes in photography, printing, television and other media. There are a number of methods or color spaces for specifying a color in terms of three particular primary colors. Each method has its advantages and disadvantages depending on the particular application.

No mixture of colors, though, can produce a fully pure color perceived as completely identical to a spectral color, although one can get very close for the longer wavelengths, where the chromaticity diagram above has a nearly straight edge. For example, mixing green light (530 nm) and blue light (460 nm) produces cyan light that is slightly desaturated, because response of the red color receptor would be greater to the green and blue light in the mixture than it would be to a pure cyan light at 485 nm that has the same intensity as the mixture of blue and green.

Because of this, and because the primaries in color printing systems generally are not pure themselves, the colors reproduced are never perfectly saturated colors, and so spectral colors cannot be matched exactly. However, natural scenes rarely contain fully saturated colors, thus such scenes can usually be approximated well by these systems. The range of colors that can be reproduced with a given color reproduction system is called the gamut. The CIE chromaticity diagram can be used to describe the gamut.

Another problem with color reproduction systems is connected with the acquisition devices, like cameras or scanners. The characteristics of the color sensors in the devices are often very far from the characteristics of the receptors in the human eye. In effect, acquisition of colors that have some special, often very "jagged," spectra caused for example by unusual lighting of the photographed scene can be relatively poor.

Species that have color receptors different from humans, e.g. birds that may have four receptors, can differentiate some colors that look the same to a human. In such cases, a color reproduction system 'tuned' to a human with normal color vision may give very inaccurate results for the other observers.

The different color response of different devices can be problematic if not properly managed. For color information stored and transferred in digital form, color management techniques, such as those based on ICC profiles, can help to avoid distortions of the reproduced colors. Color management does not circumvent the gamut limitations of particular output devices, but can assist in finding good mapping of input colors into the gamut that can be reproduced.

Pigments and reflective media

Pigments are chemicals that selectively absorb and reflect different spectra of light. When a surface is painted with a pigment, light hitting the surface is reflected, minus some wavelengths. This subtraction of wavelengths produces the appearance of different colors. Most paints are a blend of several chemical pigments, intended to produce a reflection of a given color.

Pigment manufacturers assume the source light will be white, or of roughly equal intensity across the spectrum. If the light is not a pure white source (as in the case of nearly all forms of artificial lighting), the resulting spectrum will appear a slightly different color. Red paint, viewed under blue light, may appear black. Red paint is red because it reflects only the red components of the spectrum. Blue light, containing none of these, will create no reflection from red paint, creating the appearance of black.

Structural color

Structural colors are colors caused by interference effects rather than by pigments. Color effects are produced when a material is scored with fine parallel lines, formed of a thin layer or of two or more parallel thin layers, or otherwise composed of microstructures on the scale of the color's wavelength. If the microstructures are spaced randomly, light of shorter wavelengths will be scattered preferentially to produce Tyndall effect colors: the blue of the sky, the luster of opals, and the blue of human irises. If the microstructures are aligned in arrays, for example the array of pits in a CD, they behave as a diffraction grating: the grating reflects different wavelengths in different directions due to interference phenomena, separating mixed "white" light into light of different wavelengths. If the structure is one or more thin layers then it will reflect some wavelengths and transmit others, depending on the layers' thickness.

Structural color is studied in the field of thin-film optics. A layman's term that describes particularly the most ordered or the most changeable structural colors is iridescence. Structural color is responsible for the blues and greens of the feathers of many birds (the blue jay, for example), as well as certain butterfly wings and beetle shells. Variations in the pattern's spacing often give rise to an iridescent effect, as seen in peacock feathers, soap bubbles, films of oil, and mother of pearl, because the reflected color depends upon the viewing angle. Numerous scientists have carried out research in butterfly wings and beetle shells, including Isaac Newton and Robert Hooke. Since 1942, electron micrography has been used, advancing the development of products that exploit structural color, such as "photonic" cosmetics.[8]

Additional terms

An example of natural colorfulness: A sunset in Flushing, Queens lights up the sky over Citi Field
  • Colorfulness, chroma, purity, or saturation: how "intense" or "concentrated" a color is.
  • Hue: the color's direction from white, for example in a color wheel or chromaticity diagram.
  • Shade: a color made darker by adding black.
  • Tint: a color made lighter by adding white.
  • Value, brightness, or lightness: how light or dark a color is.
  • Dichromatism: a phenomenon where the hue is dependent on concentration and/or thickness of the absorbing substance.

See also

References

  1. ^ Craig F. Bohren (2006). Fundamentals of Atmospheric Radiation: An Introduction with 400 Problems. Wiley-VCH. ISBN 3527405038. http://books.google.com/books?visbn=3527405038&id=1oDOWr_yueIC&pg=PA214&lpg=PA214&ots=Jrfi5sPBhk&dq=indigo+spectra+blue+violet+date:1990-2007&sig=Rm2xP5mIgyGJ1a1pbfAt65QSf0I#PPA214,M1. 
  2. ^ Berlin, B. and Kay, P., Basic Color Terms: Their Universality and Evolution, Berkeley: University of California Press, 1969.
  3. ^ Hermann von Helmholtz, Physiological Optics – The Sensations of Vision, 1866, as translated in Sources of Color Science, David L. MacAdam, ed., Cambridge: MIT Press, 1970.
  4. ^ Palmer, S.E. (1999). Vision Science: Photons to Phenomenology, Cambridge, MA: MIT Press. ISBN 0-262-16183-4.
  5. ^ a b Judd, Deane B.; Wyszecki, Günter (1975). Color in Business, Science and Industry. Wiley Series in Pure and Applied Optics (third edition ed.). New York: Wiley-Interscience. p. 388. ISBN 0471452122. 
  6. ^ "Under well-lit viewing conditions (photopic vision), cones  ...are highly active and rods are inactive." Hirakawa, K.; Parks, T.W. (2005). "Chromatic Adaptation and White-Balance Problem". IEEE ICIP. doi:10.1109/ICIP.2005.1530559. http://www.accidentalmark.com/research/papers/Hirakawa05WBICIP.pdf. 
  7. ^ a b c Jameson, K. A., Highnote, S. M., & Wasserman, L. M. (2001). "Richer color experience in observers with multiple photopigment opsin genes." (PDF). Psychonomic Bulletin and Review 8 (2): 244–261. doi:10.1038/351652a0. http://www.klab.caltech.edu/cns186/papers/Jameson01.pdf. 
  8. ^ "Economic and Social Research Council - Science in the Dock, Art in the Stocks". http://www.esrc.ac.uk/ESRCInfoCentre/about/CI/events/FSS/2006/science.aspx?ComponentId=14867&SourcePageId=14865. Retrieved 2007-10-07. 

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Colour
by John Ruskin

173. To-day I must try to complete our elementary sketch of schools of art, by tracing the course of those which were distinguished by faculty of colour, and afterwards to deduce from the entire scheme advisable methods of immediate practice.

You remember that, for the type of the early schools of colour, I chose their work in glass; as for that of the early schools of chiaroscuro, I chose their work in clay.

I had two reasons for this. First, that the peculiar skill of colourists is seen most intelligibly in their work in glass or in enamel; secondly, that Nature herself produces all her loveliest colours in some kind of solid or liquid glass or crystal. The rainbow is painted on a shower of melted glass, and the colours of the opal are produced in vitreous flint mixed with water; the green and blue, and golden or amber brown of flowing water is in surface glassy, and in motion "splendidior vitro." And the loveliest colours ever granted to human sight--those of morning and evening clouds before or after rain--are produced on minute particles of finely-divided water, or perhaps sometimes ice. But more than this. If you examine with a lens some of the richest colours of flowers, as, for instance, those of the gentian and dianthus, you will find their texture is produced by a crystalline or sugary frost-work upon them. In the lychnis of the high Alps, the red and white have a kind of sugary bloom, as rich as it is delicate. It is indescribable; but if you can fancy very powdery and crystalline snow mixed with the softest cream, and then dashed with carmine, it may give you some idea of the look of it. There are no colours, either in the nacre of shells, or the plumes of birds and insects, which are so pure as those of clouds, opal, or flowers; but the _force_ of purple and blue in some butterflies, and the methods of clouding, and strength of burnished lustre, in plumage like the peacock's, give them more universal interest; in some birds, also, as in our own kingfisher, the colour nearly reaches a floral preciousness. The lustre in most, however, is metallic rather than vitreous; and the vitreous always gives the purest hue. Entirely common and vulgar compared with these, yet to be noticed as completing the crystalline or vitreous system, we have the colours of gems. The green of the emerald is the best of these; but at its best is as vulgar as house-painting beside the green of bird's plumage or of clear water. No diamond shows colour so pure as a dewdrop; the ruby is like the pink of an ill-dyed and half-washed-out print, compared to the dianthus; and the carbuncle is usually quite dead unless set with a foil, and even then is not prettier than the seed of a pomegranate. The opal is, however, an exception. When pure and uncut in its native rock, it presents the most lovely colours that can be seen in the world, except those of clouds.

We have thus in nature, chiefly obtained by crystalline conditions, a series of groups of entirely delicious hues; and it is one of the best signs that the bodily system is in a healthy state when we can see these clearly in their most delicate tints, and enjoy them fully and simply, with the kind of enjoyment that children have in eating sweet things.


174. Now, the course of our main colour schools is briefly this:--First we have, returning to our hexagonal scheme, line; then _spaces_ filled with pure colour; and then _masses_ expressed or rounded with pure colour. And during these two stages the masters of colour delight in the purest tints, and endeavour as far as possible to rival those of opals and flowers. In saying "the purest tints," I do not mean the simplest types of red, blue, and yellow, but the most pure tints obtainable by their combinations.


175. You remember I told you, when the colourists painted masses or projecting spaces, they, aiming always at colour, perceived from the first and held to the last the fact that shadows, though of course darker than the lights with reference to which they _are_ shadows, are not therefore necessarily less vigorous colours, but perhaps more vigorous. Some of the most beautiful blues and purples in nature, for instance, are those of mountains in shadow against amber sky; and the darkness of the hollow in the centre of a wild rose is one glow of orange fire, owing to the quantity of its yellow stamens. Well, the Venetians always saw this, and all great colourists see it, and are thus separated from the non-colourists or schools of mere chiaroscuro, not by difference in style merely, but by being right while the others are wrong. It is an absolute fact that shadows are as much colours as lights are; and whoever represents them by merely the subdued or darkened tint of the light, represents them falsely. I particularly want you to observe that this is no matter of taste, but fact. If you are especially sober-minded, you may indeed choose sober colours where Venetians would have chosen gay ones; that is a matter of taste; you may think it proper for a hero to wear a dress without patterns on it, rather than an embroidered one; that is similarly a matter of taste: but, though you may also think it would be dignified for a hero's limbs to be all black, or brown, on the shaded side of them, yet, if you are using colour at all, you cannot so have him to your mind, except by falsehood; he never, under any circumstances, could be entirely black or brown on one side of him.


176. In this, then, the Venetians are separate from other schools by rightness, and they are so to their last days. Venetian painting is in this matter always right. But also, in their early days, the colourists are separated from other schools by their contentment with tranquil cheerfulness of light: by their never wanting to be dazzled. None of their lights are flashing or blinding; they are soft, winning, precious; lights of pearl, not of lime: only, you know, on this condition they cannot have sunshine: their day is the day of Paradise; they need no candle, neither light of the sun, in their cities; and everything is seen clear, as through crystal, far or near.

This holds to the end of the fifteenth century. Then they begin to see that this, beautiful as it may be, is still a make-believe light; that we do not live in the inside of a pearl; but in an atmosphere through which a burning sun shines thwartedly, and over which a sorrowful night must far prevail. And then the chiaroscurists succeed in persuading them of the fact that there is a mystery in the day as in the night, and show them how constantly to see truly, is to see dimly. And also they teach them the brilliancy of light, and the degree in which it is raised from the darkness; and instead of their sweet and pearly peace, tempt them to look for the strength of flame and coruscation of lightning, and flash of sunshine on armour and on points of spears.


177. The noble painters take the lesson nobly, alike for gloom or flame. Titian with deliberate strength, Tintoret with stormy passion, read it, side by side. Titian deepens the hues of his Assumption, as of his Entombment, into a solemn twilight; Tintoret involves his earth in coils of volcanic cloud, and withdraws, through circle flaming above circle, the distant light of Paradise. Both of them, becoming naturalist and human, add the veracity of Holbein's intense portraiture to the glow and dignity they had themselves inherited from the Masters of Peace: at the same moment another, as strong as they, and in pure felicity of art-faculty, even greater than they, but trained in a lower school,--Velasquez,--produced the miracles of colour and shadow-painting, which made Reynolds say of him, "What we all do with labour, he does with ease;" and one more, Correggio, uniting the sensual element of the Greek schools with their gloom, and their light with their beauty, and all these with the Lombardic colour, became, as since I think it has been admitted without question, the captain of the painter's art as such. Other men have nobler or more numerous gifts, but as a painter, master of the art of laying colour so as to be lovely, Correggio is alone.


178. I said the noble men learned their lesson nobly. The base men also, and necessarily, learn it basely. The great men rise from colour to sunlight. The base ones fall from colour to candlelight. To-day, "non ragioniam di lor," but let us see what this great change which perfects the art of painting mainly consists in, and means. For though we are only at present speaking of technical matters, every one of them, I can scarcely too often repeat, is the outcome and sign of a mental character, and you can only understand the folds of the veil, by those of the form it veils.


179. The complete painters, we find, have brought dimness and mystery into their method of colouring. That means that the world all round them has resolved to dream, or to believe, no more; but to know, and to see. And instantly all knowledge and sight are given, no more as in the Gothic times, through a window of glass, brightly, but as through a telescope-glass, darkly. Your cathedral window shut you from the true sky, and illumined you with a vision; your telescope leads you to the sky, but darkens its light, and reveals nebula beyond nebula, far and farther, and to no conceivable farthest--unresolvable. That is what the mystery means.


180. Next, what does that Greek opposition of black and white mean?

In the sweet crystalline time of colour, the painters, whether on glass or canvas, employed intricate patterns, in order to mingle hues beautifully with each other, and make one perfect melody of them all. But in the great naturalist school, they like their patterns to come in the Greek way, dashed dark on light,--gleaming light out of dark. That means also that the world round them has again returned to the Greek conviction, that all nature, especially human nature, is not entirely melodious nor luminous; but a barred and broken thing: that saints have their foibles, sinners their forces; that the most luminous virtue is often only a flash, and the blackest-looking fault is sometimes only a stain: and, without confusing in the least black with white, they can forgive, or even take delight in things that are like the [Greek: nebris], dappled.


181. You have then--first, mystery. Secondly, opposition of dark and light. Then, lastly, whatever truth of form the dark and light can show.

That is to say, truth altogether, and resignation to it, and quiet resolve to make the best of it. And therefore portraiture of living men, women, and children,--no more of saints, cherubs, or demons. So here I have brought for your standards of perfect art, a little maiden of the Strozzi family, with her dog, by Titian; and a little princess of the house of Savoy, by Vandyke; and Charles the Fifth, by Titian; and a queen, by Velasquez; and an English girl in a brocaded gown, by Reynolds; and an English physician in his plain coat, and wig, by Reynolds: and if you do not like them, I cannot help myself, for I can find nothing better for you.


182. Better?--I must pause at the word. Nothing stronger, certainly, nor so strong. Nothing so wonderful, so inimitable, so keen in unprejudiced and unbiassed sight.

Yet better, perhaps, the sight that was guided by a sacred will; the power that could be taught to weaker hands; the work that was faultless, though not inimitable, bright with felicity of heart, and consummate in a disciplined and companionable skill. You will find, when I can place in your hands the notes on Verona, which I read at the Royal Institution, that I have ventured to call the aera of painting represented by John Bellini, the time "of the Masters." Truly they deserved the name, who did nothing but what was lovely, and taught only what was right. These mightier, who succeeded them, crowned, but closed, the dynasties of art, and since their day, painting has never flourished more.


183. There were many reasons for this, without fault of theirs. They were exponents, in the first place, of the change in all men's minds from civil and religious to merely domestic passion; the love of their gods and their country had contracted itself now into that of their domestic circle, which was little more than the halo of themselves. You will see the reflection of this change in painting at once by comparing the two Madonnas (S. 37, John Bellini's, and Raphael's, called "della Seggiola"). Bellini's Madonna cares for all creatures through her child; Raphael's, for her child only.

Again, the world round these painters had become sad and proud, instead of happy and humble;--its domestic peace was darkened by irreligion, its national action fevered by pride. And for sign of its Love, the Hymen, whose statue this fair English girl, according to Reynolds' thought, has to decorate (S. 43), is blind, and holds a coronet.

Again, in the splendid power of realisation, which these greatest of artists had reached, there was the latent possibility of amusement by deception, and of excitement by sensualism. And Dutch trickeries of base resemblance, and French fancies of insidious beauty, soon occupied the eyes of the populace of Europe, too restless and wretched now to care for the sweet earth-berries and Madonna's ivy of Cima, and too ignoble to perceive Titian's colour, or Correggio's shade.


184. Enough sources of evil were here, in the temper and power of the consummate art. In its practical methods there was another, the fatallest of all. These great artists brought with them mystery, despondency, domesticity, sensuality: of all these, good came, as well as evil. One thing more they brought, of which nothing but evil ever comes, or can come--LIBERTY.

By the discipline of five hundred years they had learned and inherited such power, that whereas all former painters could be right only by effort, they could be right with ease; and whereas all former painters could be right only under restraint, they could be right, free. Tintoret's touch, Luini's, Correggio's, Reynolds', and Velasquez's, are all as free as the air, and yet right. "How very fine!" said everybody. Unquestionably, very fine. Next, said everybody, "What a grand discovery! Here is the finest work ever done, and it is quite free. Let us all be free then, and what fine things shall we not do also!" With what results we too well know.

Nevertheless, remember you are to delight in the freedom won by these mighty men through obedience, though you are not to covet it. Obey, and you also shall be free in time; but in these minor things, as well as in great, it is only right service which is perfect freedom.


185. This, broadly, is the history of the early and late colour-schools. The first of these I shall call generally, henceforward, the school of crystal; the other that of clay: potter's clay, or human, are too sorrowfully the same, as far as art is concerned. But remember, in practice, you cannot follow both these schools; you must distinctly adopt the principles of one or the other. I will put the means of following either within your reach; and according to your dispositions you will choose one or the other: all I have to guard you against is the mistake of thinking you can unite the two. If you want to paint (even in the most distant and feeble way) in the Greek School, the school of Lionardo, Correggio, and Turner, you cannot design coloured windows, nor Angelican paradises. If, on the other hand, you choose to live in the peace of paradise, you cannot share in the gloomy triumphs of the earth.


186. And, incidentally note, as a practical matter of immediate importance, that painted windows have nothing to do with chiaroscuro.[14] The virtue of glass is to be transparent everywhere. If you care to build a palace of jewels, painted glass is richer than all the treasures of Aladdin's lamp; but if you like pictures better than jewels, you must come into broad daylight to paint them. A picture in coloured glass is one of the most vulgar of barbarisms, and only fit to be ranked with the gauze transparencies and chemical illuminations of the sensational stage.

[Footnote 14: There is noble chiaroscuro in the variations of their colour, but not as representative of solid form.]

Also, put out of your minds at once all question about difficulty of getting colour; in glass we have all the colours that are wanted, only we do not know either how to choose, or how to connect them; and we are always trying to get them bright, when their real virtues are to be deep, mysterious, and subdued. We will have a thorough study of painted glass soon: mean while I merely give you a type of its perfect style, in two windows from Chalons-sur-Marne (S. 141).


187. But for my own part, with what poor gift and skill is in me, I belong wholly to the chiaroscurist school; and shall teach you therefore chiefly that which I am best able to teach: and the rather, that it is only in this school that you can follow out the study either of natural history or landscape. The form of a wild animal, or the wrath of a mountain torrent, would both be revolting (or in a certain sense invisible) to the calm fantasy of a painter in the schools of crystal. He must lay his lion asleep in St. Jerome's study beside his tame partridge and easy slippers; lead the appeased river by alternate azure promontories, and restrain its courtly little streamlets with margins of marble. But, on the other hand, your studies of mythology and literature may best be connected with these schools of purest and calmest imagination; and their discipline will be useful to you in yet another direction, and that a very important one. It will teach you to take delight in little things, and develop in you the joy which all men should feel in purity and order, not only in pictures but in reality. For, indeed, the best art of this school of fantasy may at last be in reality, and the chiaroscurists, true in ideal, may be less helpful in act. We cannot arrest sunsets nor carve mountains, but we may turn every English homestead, if we choose, into a picture by Cima or John Bellini, which shall be "no counterfeit, but the true and perfect image of life indeed."


188. For the present, however, and yet for some little time during your progress, you will not have to choose your school. For both, as we have seen, begin in delineation, and both proceed by filling flat spaces with an even tint. And therefore this following will be the course of work for you, founded on all that we have seen.

Having learned to measure, and draw a pen line with some steadiness (the geometrical exercises for this purpose being properly school, not University work), you shall have a series of studies from the plants which are of chief importance in the history of art; first from their real forms, and then from the conventional and heraldic expressions of them; then we will take examples of the filling of ornamental forms with flat colour in Egyptian, Greek, and Gothic design; and then we will advance to animal forms treated in the same severe way, and so to the patterns and colour designs on animals themselves. And when we are sure of our firmness of hand and accuracy of eye, we will go on into light and shade.


189. In process of time, this series of exercises will, I hope, be sufficiently complete and systematic to show its purpose at a glance. But during the present year, I shall content myself with placing a few examples of these different kinds of practice in your rooms for work, explaining in the catalogue the position they will ultimately occupy, and the technical points of process into which it is useless to enter in a general lecture. After a little time spent in copying these, your own predilections must determine your future course of study; only remember, whatever school you follow, it must be only to learn method, not to imitate result, and to acquaint yourself with the minds of other men, but not to adopt them as your own. Be assured that no good can come of our work but as it arises simply out of our own true natures, and the necessities of the time around us, though in many respects an evil one. We live in an age of base conceit and baser servility--an age whose intellect is chiefly formed by pillage, and occupied in desecration; one day mimicking, the next destroying, the works of all the noble persons who made its intellectual or art life possible to it:--an age without honest confidence enough in itself to carve a cherry-stone with an original fancy, but with insolence enough to abolish the solar system, if it were allowed to meddle with it.[15] In the midst of all this, you have to become lowly and strong; to recognise the powers of others and to fulfil your own. I shall try to bring before you every form of ancient art, that you may read and profit by it, not imitate it. You shall draw Egyptian kings dressed in colours like the rainbow, and Doric gods, and Runic monsters, and Gothic monks--not that you may draw like Egyptians or Norsemen, nor yield yourselves passively to be bound by the devotion, or inspired by the passion of the past, but that you may know truly what other men have felt during their poor span of life; and open your own hearts to what the heavens and earth may have to tell you in yours.

[Footnote 15: Every day these bitter words become more sorrowfully true (September, 1887).]


190. In closing this first course of lectures, I have one word more to say respecting the possible consequence of the introduction of art among the studies of the University. What art may do for scholarship, I have no right to conjecture; but what scholarship may do for art, I may in all modesty tell you. Hitherto, great artists, though always gentlemen, have yet been too exclusively craftsmen. Art has been less thoughtful than we suppose; it has taught much, but erred much, also. Many of the greatest pictures are enigmas; others, beautiful toys; others, harmful and corrupting enchantments. In the loveliest, there is something weak; in the greatest, there is something guilty. And this, gentlemen, if you will, is the new thing that may come to pass,--that the scholars of England may resolve to teach also with the silent power of the arts; and that some among you may so learn and use them, that pictures may be painted which shall not be enigmas any more, but open teachings of what can no otherwise be so well shown;--which shall not be fevered or broken visions any more, but filled with the indwelling light of self-possessed imagination;--which shall not be stained or enfeebled any more by evil passion, but glorious with the strength and chastity of noble human love;--and which shall no more degrade or disguise the work of God in heaven, but testify of Him as here dwelling with men, and walking with them, not angry, in the garden of the earth.

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1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

COLOUR (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering). The visual apparatus of the eye enables us to distinguish not only differences of form, size and brilliancy in the objects looked upon, but also differences in the character of the light received from them. These latter differences, familiar to us as differences in colour, have their physical origin in the variations in wave-length (or frequency) which may exist in light which is capable of exciting the sensation of vision. From the physical point of view, light of a pure colour, or homogeneous light, means light whose undulations are mathematically of a simple character and which cannot be resolved by a prism into component parts. All the visible pure colours, as thus defined, are to be found in the spectrum, and there is an infinite number of them, corresponding to all the possible variations of wave-length within the limits of the visible spectrum (see Spectroscopy). On this view, there is a strict analogy between variations of colour in light and variations of pitch in sound, but the visible spectrum contains a range of frequency extending over about one octave only, whereas the range of audibility embraces about eleven octaves.

Of all the known colours it might naturally be thought that white is the simplest and purest, and, till Sir Isaac Newton's time, this was the prevailing opinion. Newton, however, showed that white light could be decomposed by a prism into the spectral colours red, orange, yellow, green, blue, indigo and violet; the colours appearing in this order and passing gradually into each other without abrupt transitions. White is therefore not a simple colour, but is merely the colour of sunlight, and probably owes its apparently homogeneous character to the fact that it is the average colour of the light which fills the eye when at rest. The colours of the various objects which we see around us are not due (with the exception of self-luminous and fluorescent bodies) to any power possessed by these objects of creating the colours which they exhibit, but merely to the exercise of a selective action on the light of the sun, some of the constituent rays of the white light with which they are illuminated being absorbed, while the rest are reflected or scattered in all directions, or, in the case of transparent bodies, transmitted. White light is thus the basis of all other colours, which are derived from it by the suppression of some one or more of its parts. A red flower, for instance, absorbs the blue and green rays and most of the yellow, while the red rays and usually some yellow are scattered. If a red poppy is illuminated successively by red, yellow, green and blue light it will appear a brilliant red in the red light, yellow in the yellow light, but less brilliant if the red colour is pure; and black in the other colours, the blackness being due to the almost complete absorption of the corresponding colour.

Bodies may be classified as regards colour according to the nature of the action they exert on white light. In the case of ordinary opaque bodies a certain proportion of the incident light is irregularly reflected or scattered from their surfaces. A white object is one which reflects nearly all the light of all colours; a black object absorbs nearly all. A body which reflects only a portion of the light, but which exhibits no predominance in any particular hue, is called grey. A white surface looks grey beside a similar surface more brilliantly illuminated.

The next class is that of most transparent bodies, which owe their colour to the light which is transmitted, either directly through, or reflected back again at the farther surface. A body which transmits all the visible rays equally well is said to be colourless; pure water, for example, is nearly quite colourless, though in large masses it appears bluish-green. A translucent substance is one which partially transmits light. Translucency is due to the light being scattered by minute embedded particles or minute irregularities of structure. Some fibrous specimens of tremolite and gypsum are translucent in the direction of the fibres, and practically opaque in a transverse direction. Coloured transparent objects vary in shade and hue according to their size; thus, a conical glass filled with a red liquid commonly appears yellow at the bottom, varying through orange up to red at the upper part. A coloured powder is usually of a much lighter tint than the substance in bulk, as the light is reflected back after transmission through only a few thin layers. For the same reason the powders of transparent substances are opaque.

Polished bodies, whether opaque or transparent, when illuminated with white light and viewed at the proper angle, reflect the incident light regularly and appear white, without showing much of their distinctive colours.

Some bodies reflect light of one colour and transmit that of another; such bodies nearly always possess the properties of selective or metallic reflection and anomalous dispersion. Most of the coal-tar dyes belong to this category. Solid eosin, for example, reflects a yellowish-green and transmits a red light. Gold appears yellow under ordinary circumstances, but if the light is reflected many times from the surface it appears a ruby colour. On the other hand, a powerful beam of light transmitted through a thin gold-leaf appears green.

Some solutions exhibit the curious phenomenon of dichromatism (from Se-, double, and X pWµa, colour), that is, they appear of one colour when viewed in strata of moderate thickness, but of a different colour in greater thicknesses (see Absorption Of Light).

The blue colour of the sky has been explained by Lord Rayleigh as due to the scattering of light by small suspended particles and air molecules, which is most effective in the case of the shorter waves (blue). J. Tyndall produced similar effects in the laboratory. The green colour of sea-water near the shore is also due to a scattering of light.

The colours of bodies which are gradually heated to white incandescence occur in the order - red, orange, yellow, white. This is because the longer waves of red light are first emitted, then the yellow as well, so that orange results, then so much green that the total effect is yellow, and lastly all the colours, compounding to produce white. Fluorescent bodies have the power of converting light of one colour into that of another (see Fluorescence).

Besides the foregoing kinds of colorization, a body may exhibit, under certain circumstances, a colouring due to some special physical conditions rather than to the specific properties of the material; such as the colour of a white object when illuminated by light of some particular colour; the colours seen in a film of oil on water or in mother-of-pearl, or soapbubbles, due to interference (q.v.); the colours seen through the eyelashes or through a thin handkerchief held up to the light, due to diffraction (q.v.); and the colours caused by ordinary refraction, as in the rainbow, double refraction and polarization (qq.v.).

Composition of Colours

It has been already pointed out that white light is a combination of all the colours in the spectrum. This was shown by Newton, who recombined the spectral colours and produced white. Newton also remarks that if a froth be made on the surface of water thickened a little with soap, and examined closely, it will be seen to be coloured with all the colours of the spectrum, but at a little distance it looks white owing to the combined effect on the eye of all the colours.

The question of the composition of colours is largely a physiological one, since it is possible, by mixing colours, say red and yellow, to produce a new colour, orange, which appears identical with the pure orange of the spectrum, but is physically quite different, since it can be resolved by a prism into red and yellow again. There is no doubt that the sensation of colour-vision is threefold, in the sense that any colour can be produced by the combination, in proper proportions, of three standard colours. The question then arises, what are the three primary colours? Sir David Brewster considered that they were red, yellow and blue; and this view has been commonly held by painters and others, since all the known brilliant hues can be derived from the admixture of red, yellow and blue pigments. For instance, vermilion and chrome yellow will give an orange, chrome yellow and ultramarine a green, and vermilion and ultramarine a purple mixture. But if we superpose the pure spectral colours on a screen, the resulting colours are quite different. This is especially the case with yellow and blue, which on the screen combine to produce white, generally with a pink tint, but cannot be made to give green. The reason of this difference in the two results is that in the former case we do not get a true combination of the colours at all. When the mixed pigments are illuminated by white light, the yellow particles absorb the red and blue rays, but reflect the yellow along with a good deal of the neighbouring green and orange. The blue particles, on the other hand, absorb the red, orange and yellow, but reflect the blue and a good deal of green and violet. As much of the light is affected by several particles, most of the rays are absorbed except green, which is reflected by both pigments. Thus, the colour of the mixture is not a mixture of the colours yellow and blue, but the remainder of white light after the yellow and blue pigments have absorbed all they can. The effect can also be seen in coloured solutions. If two equal beams of white light are transmitted respectively through a yellow solution of potassium bichromate and a blue solution of copper sulphate in proper thicknesses, they can be compounded on a screen to an approximately white colour; but a single beam transmitted through both solutions appears green. Blue and yellow pigments would produce the effect of white only if very sparsely distributed. This fact is made use of in laundries, where cobalt blue is used to correct the yellow colour of linen after washing.

Thomas Young suggested red, green and violet as the primary colours, but the subsequent experiments of J. Clerk Maxwell appear to show that they should be red, green and blue. Sir William Abney, however, assigns somewhat different places in the spectrum to the primary colours, and, like Young, considers that they should be red, green and violet. All other hues can be obtained by combining the three primaries in proper proportions. Yellow is derived from red and green. This can be done by superposition on a screen or by making a solution which will transmit only red and green rays. For this purpose Lord Rayleigh recommends a mixture of solutions of blue litmus and yellow potassium chromate. The litmus stops the yellow and orange light, while the potassium chromate stops the blue and violet. Thus only red and green are transmitted, and the result is a full compound yellow which resembles the simple yellow of the spectrum in appearance, but is resolved into red and green by a prism. The brightest yellow pigments are those which give both the pure and compound yellow. Since red and green produce yellow, and yellow and blue produce white, it follows that red, green and blue can be compounded into white. H. von Helmholtz has shown that the only pair of simple spectral colours capable of compounding to white are a greenish-yellow and blue.

Just as musical sounds differ in pitch, loudness and quality, so may colours differ in three respects, which Maxwell calls hue, shade and tint. All hues can be produced by combining every pair of primaries in every proportion. The addition of white alters the tint without affecting the hue. If the colour be darkened by adding black or by diminishing the illumination, a variation in shade is produced. Thus the hue red includes every variation in tint from red to white, and every variation in shade from red to black, and similarly for other hues. We can represent every hue and tint on a diagram in a manner proposed by Young, following a very similar suggestion B of Newton's. Let RGB (fig. 1) be an FIG. I. equilateral triangle, and let the angular points be coloured red, green and blue of such intensities as to produce white if equally combined; and let the colour of every point of the triangle be determined by combining such proportions of the three primaries, that three weights in the same proportion would have their centre of gravity at the point. Then the centre of the triangle will be a neutral tint, white or grey; and the middle points of the sides Y, S, P will be yellow, greenish-blue and purple. The hue varies all round the perimeter. The tint varies along any straight line through W. To vary the shade, the whole triangle must be uniformly darkened.

The simplest way of compounding colours is by means of Maxwell's colour top, which is a broad spinning-top over the spindle of which coloured disks can be slipped (fig. 2). The disks are slit radially so that they can be slipped partially over each other and the surfaces exposed in any desired ratio. Three disks are used together, and a match is obtained between these and a pair of smaller ones mounted on the same spindle. If any five colours are taken, two of which may be black and white, a match can be got between them by suitable adjustment. This shows that a relation exists between any four colours (the black being only needed to obtain the proper intensity) and that consequently the number of independent colours is three. A still better instrument for combining colours is Maxwell's colour box, in which the colours of the spectrum are combined by means of prisms. Sir W. Abney has also invented an apparatus for the same purpose, which is much the same in principle as Maxwell's colour box. Several methods of colour photography depend on the fact that all varieties of colour can be compounded from red, green and blue in proper proportions.

Any two colours which together give white are called complementary colours. Greenish-yellow and blue are a pair of complementaries, as already men tioned. Any number of pairs may be obtained by a simple device due to Helmholtz and represented in fig. 3. A beam of white light, decomposed by the prism P, is recompounded into white light by the lens 1 and focussed on a screen at f. If the thin prism p is inserted near the lens, any set of colours may be deflected to another point n, thus producing two coloured and complementary images of the source of light.

Nature of White Light

The question as to whether white light actually consists of trains of waves of regular frequency has been discussed in recent years by A. Schuster, Lord Rayleigh and others, and it has been shown that even if it consisted of a succession of somewhat irregular impulses, it would still be resolved, by the dispersive property of a prism or grating, into trains of regular frequency. We may still, however, speak of white light as compounded of the rays of the spectrum, provided we mean only that the two systems are mathematically equivalent, and not that the homogeneous trains exist as such in the original light.

See also Newton's Opticks, bk. i. pt. ii.; Maxwell's Scientific Papers; Helmholtz's papers in Poggendorf's Annalen; Sir G. G. Stokes, Burnett Lectures for 1884-5-6; Abney's Colour Vision (1895). (J. R. C.)


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Bible wiki

Up to date as of January 23, 2010

From BibleWiki

The subject of colours holds an important place in the Scriptures.

White occurs as the translation of various Hebrew words. It is applied to milk (Gen 49:12), manna (Ex 16:31), snow (Isa 1:18), horses (Zech 1:8), raiment (Eccl 9:8). Another Hebrew word so rendered is applied to marble (Est 1:6), and a cognate word to the lily (Song 2:16). A different term, meaning "dazzling," is applied to the countenance (Song 5:10).

This colour was an emblem of purity and innocence (Mk 16:5; Jn 20:12; Rev 19:8, 14), of joy (Eccl 9:8), and also of victory (Zech 6:3; Rev 6:2). The hangings of the tabernacle court (Ex 27:9; 38:9), the coats, mitres, bonnets, and breeches of the priests (Ex 39:27,28), and the dress of the high priest on the day of Atonement (Lev 16:4,32), were white.

Black, applied to the hair (Lev 13:31; Song 5:11), the complexion (Song 1:5), and to horses (Zech 6:2,6). The word rendered "brown" in Gen 30:32 (R.V., "black") means properly "scorched", i.e., the colour produced by the influence of the sun's rays. "Black" in Job 30:30 means dirty, blackened by sorrow and disease. The word is applied to a mourner's robes (Jer 8:21; 14:2), to a clouded sky (1 Kg 18:45), to night (Mic 3:6; Jer 4:28), and to a brook rendered turbid by melted snow (Job 6:16). It is used as symbolical of evil in Zech 6:2, 6 and Rev 6:5. It was the emblem of mourning, affliction, calamity (Jer 14:2; Lam 4:8; 5:10).

Red, applied to blood (2 Kings 3;22), a heifer (Num 19:2), pottage of lentils (Gen 25:30), a horse (Zech 1:8), wine (Prov 23:31), the complexion (Gen 25:25; Song 5:10). This colour is symbolical of bloodshed (Zech 6:2; Rev 6:4; 12:3).

Purple, a colour obtained from the secretion of a species of shell-fish (the Murex trunculus) which was found in the Mediterranean, and particularly on the coasts of Phoenicia and Asia Minor. The colouring matter in each separate shell-fish amounted to only a single drop, and hence the great value of this dye. Robes of this colour were worn by kings (Jdg 8:26) and high officers (Est 8:15). They were also worn by the wealthy and luxurious (Jer 10:9; Ezek 27:7; Lk 16:19; Rev 17:4). With this colour was associated the idea of royalty and majesty (Jdg 8:26; Song 3:10; 7:5; Dan 5:7, 16,29).

Blue. This colour was also procured from a species of shell-fish, the chelzon of the Hebrews, and the Helix ianthina of modern naturalists. The tint was emblematic of the sky, the deep dark hue of the Eastern sky. This colour was used in the same way as purple. The ribbon and fringe of the Hebrew dress were of this colour (Num 15:38). The loops of the curtains (Ex 26:4), the lace of the high priest's breastplate, the robe of the ephod, and the lace on his mitre, were blue (Ex 28:28, 31, 37).

Scarlet, or Crimson. In Isa 1:18 a Hebrew word is used which denotes the worm or grub whence this dye was procured. In Gen 38:28,30, the word so rendered means "to shine," and expresses the brilliancy of the colour. The small parasitic insects from which this dye was obtained somewhat resembled the cochineal which is found in Eastern countries. It is called by naturalists Coccus ilics. The dye was procured from the female grub alone. The only natural object to which this colour is applied in Scripture is the lips, which are likened to a scarlet thread (Song 4:3). Scarlet robes were worn by the rich and luxurious (2 Sam 1:24; Prov 31:21; Jer 4:30. Rev 17:4). It was also the hue of the warrior's dress (Nah 2:3; Isa 9:5). The Phoenicians excelled in the art of dyeing this colour (2Chr 2:7).

These four colours--white, purple, blue, and scarlet--were used in the textures of the tabernacle curtains (Ex 26:1, 31, 36), and also in the high priest's ephod, girdle, and breastplate (Ex 28:5, 6, 8, 15). Scarlet thread is mentioned in connection with the rites of cleansing the leper (Lev 14:4, 6, 51) and of burning the red heifer (Num 19:6). It was a crimson thread that Rahab was to bind on her window as a sign that she was to be saved alive (Josh 2:18; 6:25) when the city of Jericho was taken.

Vermilion, the red sulphuret of mercury, or cinnabar; a colour used for drawing the figures of idols on the walls of temples (Ezek 23:14), or for decorating the walls and beams of houses (Jer 22:14).

This entry includes text from Easton's Bible Dictionary, 1897.

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