CHAPTER VII.Quantitative Colour Nomenclature.

CHAPTER VII.Quantitative Colour Nomenclature.THE GLASS STANDARD SCALES.At an early stage of the investigation, it was found that coloured glass gave better results than coloured solutions, and that Red, Yellow, and Blue, were the only colours suitable for systematic work; it was also found that any colour could be produced by their combination. As already described arbitrary scales were first used in many colours, but were superseded by these three, which, when graded into scales of equivalent value, were found to cover all daylight colours.Upon this evidence, scales of red, yellow and blue were constructed of glass slips, each scale being all of one colour, with a regular variation of intensity from 0·01 to 20·0 units, equal units of the three scales being in equivalence with each other. The dimensions of the unit are necessarily arbitrary, but the scales comply with the essentials of a scientific standard, in that the divisions are equal, and the unit recoverable. The equality of the unit divisions in the scales, is demonstrated by a system of cross-checking. The test of colour equivalence has already been described on pages 10 and 28.The power of recovering the unit, is by co-relation to well-known physical colour constants, such as is easily obtained by definite intensities of percentage solutions, of selected pure chemical compounds in distilled water, at standard temperatures. For example, a one per cent. solution of pure crystallized copper sulphate C2SO45H2O at 60° F. when viewed in the optical instrument in a 1-inch stratum, must be matched by a combination of Yellow 1·58 and Blue 1·55.The inch of distilled water itself constitutes very little of this colour; the colour of distilled water is remarkably uniform, and might almost be taken as a colour constant, thus: A 2-foot stratum is matched by Yellow 0·1 and Blue 0·34, a 4-foot stratum by Yellow 1·0 and Blue 1·45.A one per cent. solution ofNickel Sulphate NiSO47H2O, tem.60° F. in a 2-inch stratum must be matched by 2·2 Blue and 2·0 Yellow units.A one per cent. solution ofPotassium BichromateK2Cr2O, Tem. 60° and in a 2-inch stratum after being dulled by 0·5 neutral tint units must be matched by 34·0 yellow and 9·6 red units.METHOD OF DEVELOPING, MEASURING AND NAMING COLOUR.The single sensation colours, Red, Yellow and Blue, are matchable by a single glass from the corresponding colour scale; the depth of colour is directly indicated by the value of the glass used.The single sensation colours, Orange, Green and Violet, are matchable by a combination of equal units, from two of the standard scales, the depthof colour is directly indicated by the unit value of either of the glasses, thus: 2·0 Blue + 2·0 Red develop 2·0 units Violet.A given neutral grey is matchable by a combination of equal units from the three standard scales, the depth of grey, is directly indicated by the unit value on either of the glasses used, thus:—3·0 Red + 3·0 yellow + 3·0 blue develop 3·0 units neutral tint.The complex colour sensations, red and yellow oranges, yellow and blue greens, blue and red violets are matchable by unequal glasses from two of the standard scales; the colour developed is not directly indicated by the unit value of the glasses, but is recorded by means of an equation, the first half of which contains the separate values of the glasses used, and the second half the names and the depth of the colours they transmit. For instance—The equation of a colour matched by 17·0 red and 2·6 blue units, is as follows:—Standard Glasses. Colour Developed.Red. Blue. Violet. Red.17·0 + 2·6 = 2·6 + 14·4The colour developed is a red violet in these proportions.A colour matched byStandard Glasses. Colour Developed.Red. Yellow. Orange. Red.10·0 + 3·0 = 3·0 + 7·0The colour developed is a red orange in these proportions.A colour matched byStandard Glasses. Colour Developed.Yellow. Blue. Green. Yellow.3·0 + 1·5 = 1·5 + 1·5The colour developed is a yellow green in these proportions.A colour matched byStandard Glasses. Colour Developed.Blue. Red. Blue. Violet.6·0 + 1·8 = 4·2 + 1·8The colour developed is a blue violet in these proportions.The standard glass colours are necessarily of a given brightness, and colours for measurement may be either brighter, or sadder than the standards.A given complex colour of less than glass standard brightness, is matchable by unequal numbers from the three standard scales; the smallest unit value always represents the “black,” or neutral unit factor. The equation is as follows:—A colour matched byStandard Glasses. Colour Developed.Red. Yellow. Blue. Neutral Tint. Green. Blue.1·0 + 3·0 + 9·0 = 1·0 + 2·0 + 6·0The colour is a blue green, in the proportion of six to two, saddened by one of neutral tint.A given complex colour of greater brightness than the glass standards, is first dulled by the interception of neutral tint units, until measurable in the manner described above; the intercepting glassesrepresent the unit value of excess of brightness, and is shown in the equation as light units, for instance—Standard Glasses. Colour Developed.Neutral Tint. Yellow. Blue. Light. Green. Yellow.1·5 + 7·5 + 0·5 = 1·5 + 0·5 + 7·0The colour is a yellow green in the proportions of 7·0 of yellow, to 0·5 of green, and 1·5 brighter than the standards.Every daylight colour being thus measurable by a suitable combination of standard glasses, with or without the addition of a Light, or a Neutral Tint factor, it follows that any colour can be described both qualitatively, and quantitatively, in terms of the colour sensations yielded by the standard glasses and their combination. The distinct colour sensations are those, which, by common consent are known as Red, Yellow, Blue, Orange, Green and Violet, and they are yielded by single glasses, or by pairs as already described; all colours therefore fall into the following categories:—A.—Single colour sensations:—1. Transmitted by single glass standards:Red.Yellow.Blue.2. Transmitted by equivalent pairs of standard glasses:Orange.Green.Violet.B.—Double colour sensations transmitted by unequal pairs of standard glasses.Red orange, transmitted by unequal units of red and yellow, red preponderating.Yellow orange, transmitted by unequal units of red and yellow, yellow preponderating.Yellow green, transmitted by unequal units of yellow and blue, yellow preponderating.Blue green, transmitted by unequal units of yellow and blue, blue preponderating.Blue violet, transmitted by unequal units of blue and red, blue preponderating.Red violet, transmitted by unequal units of blue and red, red preponderating.C.—Any of the above colours with the addition or subtraction of neutral tint.Neutral tint itself, is transmitted by a combination of equal units of the standard glasses, thus three units red, yellow and blue, when superposed, transmit three units neutral tint.Examples.Three units red, of standard brightness, completely describes a colour matched by a red glass of three units, and is denotedR. 3·0Three units red saddened by one neutral tint, completely describes a colour matched by a red glass standard of four units red, combined with a blue and yellow of one unit each, and is denotedR. 3·0 + N.T. 1·0A given red of three units, which is one unit brighter than standards, after having been saddened byone unit each of red, yellow and blue, is matched by three units of red and is correctly described byRed 3·0 + Light 1·0Three units of violet, of standard brightness, is matched by a red and a blue glass of three units, and is correctly described byV. 3·0Three units of orange, of standard brightness, is matched by a red and a yellow glass of three units, and is correctly described byO. 3·0A binary red violet of standard brightness, in which red preponderates by one unit, is matched by four units red, and a blue of three units, and is correctly described byR. 1·0 + V. 3·0A binary red orange, of standard brightness, in which orange preponderates by three units, is matched by red four and yellow three units, and is correctly described byR. 1·0 + O. 3·0A red orange, of less than standard brightness by one unit, in which orange preponderates by three units, is matched by a red five, yellow four, blue one, and is correctly described byR. 1·9 + O. 3·0 + N.T. 1·0A red violet, in which red preponderates by one unit, and is one unit brighter than standard, is first dulled by one unit red, yellow and blue, and thenmatched by four red and three blue, and is correctly described byR. 1·0 + V. 3·0 + Light 1·0A red orange, in which red preponderates by one unit, and is one unit brighter than standard, is first dulled by one red, yellow and blue, and then matched by four red, and three yellow, and is correctly described byR. 1·0 + O. 3·0 + Light 1·0

At an early stage of the investigation, it was found that coloured glass gave better results than coloured solutions, and that Red, Yellow, and Blue, were the only colours suitable for systematic work; it was also found that any colour could be produced by their combination. As already described arbitrary scales were first used in many colours, but were superseded by these three, which, when graded into scales of equivalent value, were found to cover all daylight colours.

Upon this evidence, scales of red, yellow and blue were constructed of glass slips, each scale being all of one colour, with a regular variation of intensity from 0·01 to 20·0 units, equal units of the three scales being in equivalence with each other. The dimensions of the unit are necessarily arbitrary, but the scales comply with the essentials of a scientific standard, in that the divisions are equal, and the unit recoverable. The equality of the unit divisions in the scales, is demonstrated by a system of cross-checking. The test of colour equivalence has already been described on pages 10 and 28.

The power of recovering the unit, is by co-relation to well-known physical colour constants, such as is easily obtained by definite intensities of percentage solutions, of selected pure chemical compounds in distilled water, at standard temperatures. For example, a one per cent. solution of pure crystallized copper sulphate C2SO45H2O at 60° F. when viewed in the optical instrument in a 1-inch stratum, must be matched by a combination of Yellow 1·58 and Blue 1·55.

The inch of distilled water itself constitutes very little of this colour; the colour of distilled water is remarkably uniform, and might almost be taken as a colour constant, thus: A 2-foot stratum is matched by Yellow 0·1 and Blue 0·34, a 4-foot stratum by Yellow 1·0 and Blue 1·45.

A one per cent. solution ofNickel Sulphate NiSO47H2O, tem.60° F. in a 2-inch stratum must be matched by 2·2 Blue and 2·0 Yellow units.

A one per cent. solution ofPotassium BichromateK2Cr2O, Tem. 60° and in a 2-inch stratum after being dulled by 0·5 neutral tint units must be matched by 34·0 yellow and 9·6 red units.

The single sensation colours, Red, Yellow and Blue, are matchable by a single glass from the corresponding colour scale; the depth of colour is directly indicated by the value of the glass used.

The single sensation colours, Orange, Green and Violet, are matchable by a combination of equal units, from two of the standard scales, the depthof colour is directly indicated by the unit value of either of the glasses, thus: 2·0 Blue + 2·0 Red develop 2·0 units Violet.

A given neutral grey is matchable by a combination of equal units from the three standard scales, the depth of grey, is directly indicated by the unit value on either of the glasses used, thus:—

3·0 Red + 3·0 yellow + 3·0 blue develop 3·0 units neutral tint.

The complex colour sensations, red and yellow oranges, yellow and blue greens, blue and red violets are matchable by unequal glasses from two of the standard scales; the colour developed is not directly indicated by the unit value of the glasses, but is recorded by means of an equation, the first half of which contains the separate values of the glasses used, and the second half the names and the depth of the colours they transmit. For instance—

The equation of a colour matched by 17·0 red and 2·6 blue units, is as follows:—

Standard Glasses. Colour Developed.Red. Blue. Violet. Red.17·0 + 2·6 = 2·6 + 14·4

The colour developed is a red violet in these proportions.

A colour matched by

Standard Glasses. Colour Developed.Red. Yellow. Orange. Red.10·0 + 3·0 = 3·0 + 7·0

The colour developed is a red orange in these proportions.

A colour matched by

Standard Glasses. Colour Developed.Yellow. Blue. Green. Yellow.3·0 + 1·5 = 1·5 + 1·5

The colour developed is a yellow green in these proportions.

A colour matched by

Standard Glasses. Colour Developed.Blue. Red. Blue. Violet.6·0 + 1·8 = 4·2 + 1·8

The colour developed is a blue violet in these proportions.

The standard glass colours are necessarily of a given brightness, and colours for measurement may be either brighter, or sadder than the standards.

A given complex colour of less than glass standard brightness, is matchable by unequal numbers from the three standard scales; the smallest unit value always represents the “black,” or neutral unit factor. The equation is as follows:—

A colour matched by

Standard Glasses. Colour Developed.Red. Yellow. Blue. Neutral Tint. Green. Blue.1·0 + 3·0 + 9·0 = 1·0 + 2·0 + 6·0

The colour is a blue green, in the proportion of six to two, saddened by one of neutral tint.

A given complex colour of greater brightness than the glass standards, is first dulled by the interception of neutral tint units, until measurable in the manner described above; the intercepting glassesrepresent the unit value of excess of brightness, and is shown in the equation as light units, for instance—

Standard Glasses. Colour Developed.Neutral Tint. Yellow. Blue. Light. Green. Yellow.1·5 + 7·5 + 0·5 = 1·5 + 0·5 + 7·0

The colour is a yellow green in the proportions of 7·0 of yellow, to 0·5 of green, and 1·5 brighter than the standards.

Every daylight colour being thus measurable by a suitable combination of standard glasses, with or without the addition of a Light, or a Neutral Tint factor, it follows that any colour can be described both qualitatively, and quantitatively, in terms of the colour sensations yielded by the standard glasses and their combination. The distinct colour sensations are those, which, by common consent are known as Red, Yellow, Blue, Orange, Green and Violet, and they are yielded by single glasses, or by pairs as already described; all colours therefore fall into the following categories:—

A.—Single colour sensations:—

1. Transmitted by single glass standards:

Red.Yellow.Blue.

2. Transmitted by equivalent pairs of standard glasses:

Orange.Green.Violet.

B.—Double colour sensations transmitted by unequal pairs of standard glasses.

Red orange, transmitted by unequal units of red and yellow, red preponderating.

Yellow orange, transmitted by unequal units of red and yellow, yellow preponderating.

Yellow green, transmitted by unequal units of yellow and blue, yellow preponderating.

Blue green, transmitted by unequal units of yellow and blue, blue preponderating.

Blue violet, transmitted by unequal units of blue and red, blue preponderating.

Red violet, transmitted by unequal units of blue and red, red preponderating.

C.—Any of the above colours with the addition or subtraction of neutral tint.

Neutral tint itself, is transmitted by a combination of equal units of the standard glasses, thus three units red, yellow and blue, when superposed, transmit three units neutral tint.

Three units red, of standard brightness, completely describes a colour matched by a red glass of three units, and is denoted

R. 3·0

Three units red saddened by one neutral tint, completely describes a colour matched by a red glass standard of four units red, combined with a blue and yellow of one unit each, and is denoted

R. 3·0 + N.T. 1·0

A given red of three units, which is one unit brighter than standards, after having been saddened byone unit each of red, yellow and blue, is matched by three units of red and is correctly described by

Red 3·0 + Light 1·0

Three units of violet, of standard brightness, is matched by a red and a blue glass of three units, and is correctly described by

V. 3·0

Three units of orange, of standard brightness, is matched by a red and a yellow glass of three units, and is correctly described by

O. 3·0

A binary red violet of standard brightness, in which red preponderates by one unit, is matched by four units red, and a blue of three units, and is correctly described by

R. 1·0 + V. 3·0

A binary red orange, of standard brightness, in which orange preponderates by three units, is matched by red four and yellow three units, and is correctly described by

R. 1·0 + O. 3·0

A red orange, of less than standard brightness by one unit, in which orange preponderates by three units, is matched by a red five, yellow four, blue one, and is correctly described by

R. 1·9 + O. 3·0 + N.T. 1·0

A red violet, in which red preponderates by one unit, and is one unit brighter than standard, is first dulled by one unit red, yellow and blue, and thenmatched by four red and three blue, and is correctly described by

R. 1·0 + V. 3·0 + Light 1·0

A red orange, in which red preponderates by one unit, and is one unit brighter than standard, is first dulled by one red, yellow and blue, and then matched by four red, and three yellow, and is correctly described by

R. 1·0 + O. 3·0 + Light 1·0

CHAPTER VIII.The Colour Scales.A normal vision under ordinary conditions, has no hesitation in correctly naming the sensations produced by the triad groups red, yellow and blue, or by the single rays orange, green and violet. It can also correctly describe a complex colour sensation, by naming the two associated colours, such as red orange, yellow orange, blue green, blue violet, etc.; but when called upon to decide differences of colour depth, it can only do so by using arbitrary terms of no precise scientific value, such as light, medium, dark, etc.This deficiency is because the vision has in itself no arrangement for the quantitative definition of colour depth. This want can only be met by co-relating colour sensations, to some physical colour constants.This co-relation has now been effected by a series of glass standard colour scales, which are numerically graded for colour depth, the scales themselves being colour constants by co-relation to percentage solutions, of such coloured chemicals, as copper sulphate, nickel sulphate, potassium permanganate,etc. These substances as well as many others, are always available for checking the constancy of the scales, or for recovering the unit if lost.As already mentioned, the system of taper scales proved to be useless for the purpose, not only because the rate of colour increase was never in proportion to the rate of thickness increase, but also because no two substances are equal in this respect, each having a rate specific to itself.The prismatic spectrum colours were not available for several reasons, first as being unsuitable for critical comparisons under daylight conditions, as being too weak except “in camera”; also they were found to be too crowded for the separation of a working area of monochromatic colour, and some corrections would have been necessary for variation in the refractions of different colour rays. This is more fully dealt with under the heading of The Spectrum in relation to Colour Standardization, page 36.THE EQUIVALENCE OF THE COLOUR SCALES.The method employed for obtaining equality of the unit divisions, and colour equivalence between the different scales was as follows:—Two slips of red glass in a light shade were made exactly equal in colour, and considered as initial units; these were then superimposed and matched by a single glass, which was then considered as of two units, this and one of the initial units were superimposed, and matched by a single glass of three colour units, and so on, until a progressive red scalewas constructed, ranging in intensity from ·01 to 20· units.[3]The yellow and blue scales, were similarly constructed, taking care that their similar unit values were in colour equivalence with the red units, the test of equivalence being, that when equal units of the three scales were superimposed against a normal white light, a neutral grey was transmitted, in which no trace of colour could be perceived by the common consent of the whole staff of trained observers.The scales were then considered as in colour equivalence with each other. The system of cross-checking was so elaborate, that after the equivalence of the first unit was established, nearly four years was occupied in the work before the scales were passed as satisfactory.It may be urged that the unit is arbitrary, but this applies also to the unit of any other standard scale; it is sufficient that the essentials of a philosophic scale are complied with, in that the divisions are equal, and the unit recoverable.PLATE IVCOMPARISON CURVES OF HEALTHY HUMAN BLOOD WITH THE BLOOD OF LOWER ANIMALS.1 Healthy Human Blood. 2 Pike's Blood. 3 Bullock's Blood. 4 Lamb's Blood. 5 Pig's Blood. 6 Frog's Blood.To face page 31.[Lovibond, Colour Theories.

A normal vision under ordinary conditions, has no hesitation in correctly naming the sensations produced by the triad groups red, yellow and blue, or by the single rays orange, green and violet. It can also correctly describe a complex colour sensation, by naming the two associated colours, such as red orange, yellow orange, blue green, blue violet, etc.; but when called upon to decide differences of colour depth, it can only do so by using arbitrary terms of no precise scientific value, such as light, medium, dark, etc.

This deficiency is because the vision has in itself no arrangement for the quantitative definition of colour depth. This want can only be met by co-relating colour sensations, to some physical colour constants.

This co-relation has now been effected by a series of glass standard colour scales, which are numerically graded for colour depth, the scales themselves being colour constants by co-relation to percentage solutions, of such coloured chemicals, as copper sulphate, nickel sulphate, potassium permanganate,etc. These substances as well as many others, are always available for checking the constancy of the scales, or for recovering the unit if lost.

As already mentioned, the system of taper scales proved to be useless for the purpose, not only because the rate of colour increase was never in proportion to the rate of thickness increase, but also because no two substances are equal in this respect, each having a rate specific to itself.

The prismatic spectrum colours were not available for several reasons, first as being unsuitable for critical comparisons under daylight conditions, as being too weak except “in camera”; also they were found to be too crowded for the separation of a working area of monochromatic colour, and some corrections would have been necessary for variation in the refractions of different colour rays. This is more fully dealt with under the heading of The Spectrum in relation to Colour Standardization, page 36.

The method employed for obtaining equality of the unit divisions, and colour equivalence between the different scales was as follows:—

Two slips of red glass in a light shade were made exactly equal in colour, and considered as initial units; these were then superimposed and matched by a single glass, which was then considered as of two units, this and one of the initial units were superimposed, and matched by a single glass of three colour units, and so on, until a progressive red scalewas constructed, ranging in intensity from ·01 to 20· units.[3]

The yellow and blue scales, were similarly constructed, taking care that their similar unit values were in colour equivalence with the red units, the test of equivalence being, that when equal units of the three scales were superimposed against a normal white light, a neutral grey was transmitted, in which no trace of colour could be perceived by the common consent of the whole staff of trained observers.

The scales were then considered as in colour equivalence with each other. The system of cross-checking was so elaborate, that after the equivalence of the first unit was established, nearly four years was occupied in the work before the scales were passed as satisfactory.

It may be urged that the unit is arbitrary, but this applies also to the unit of any other standard scale; it is sufficient that the essentials of a philosophic scale are complied with, in that the divisions are equal, and the unit recoverable.

PLATE IVCOMPARISON CURVES OF HEALTHY HUMAN BLOOD WITH THE BLOOD OF LOWER ANIMALS.1 Healthy Human Blood. 2 Pike's Blood. 3 Bullock's Blood. 4 Lamb's Blood. 5 Pig's Blood. 6 Frog's Blood.

PLATE IVCOMPARISON CURVES OF HEALTHY HUMAN BLOOD WITH THE BLOOD OF LOWER ANIMALS.

To face page 31.[Lovibond, Colour Theories.

CHAPTER IX.Colour Charts.A colour chart is constructed by placing two colour scales at right angles to each other, with their zeros at the angle.A measured simple colour, finds its position directly on its corresponding colour scale at the point of its measured value.A measured complex colour, finds its position within the angle, at that point where perpendiculars drawn through the two colour values meet.The above statements are complete only for colours of standard brightness, should the colour be brighter or duller than standards, a light factor is necessary, the value of which is furnished by the measurement itself, and must be written in numerals near the colour point.By this method the chart position of even the most complicated colour is indicated by a single point which is determined by the analytical value of the composing factors.Examples.Simple Colour ofStandardBrightness.Complex Colourof StandardBrightness.Simple ColourBrighter thanStandards.Complex ColourDuller thanStandards.3· Red.6· Blue, 10· Violet.7· Yellow, Light 2·Red 6, Orange 5, Black 2.The number of complex colour charts is limited to the six represented inFig. 1as lying in their order on a continuous spectrum. The red and violet mixtures having no visible spectrum position are represented in the ultra violet. The ordinates of the charts are made by erecting the overlying red, yellow and blue scales as perpendiculars.Fig. 1.The information to be obtained by charting measured colour is more extensive than appears at first sight, as by varying the character of the co-ordinates, and charting suitable series of measurements, new fields of investigation are opened, thus throwing light on some hitherto obscure questions, of which the following are some instances.SPECIFIC COLOUR.It has sometimes been assumed that colour increase was in direct ratio to intensity increase, but this is never the case, each substance has its own rate, specific to itself. It is conceivable that the colours of two substances may coincide at one point, but as their densities increase, or decrease, their rates of change vary.The term “Specific Colour” is based on the experimental fact, that the colour of a given substanceis constant, so long as the substance itself and the conditions of observation, remain unaltered. During experimental work a sufficient number of instances have accumulated to warrant the writer in advancing and using the term “Specific Colour” as describing a new natural law, as rigid in its application as that of “Specific Gravity” or “Specific Heat.”PLATE VABSORPTION CURVES OF FIVE COLOUR CONSTANTS.1 Potassium Bichromate. 2 Copper Sulphate. 3 Nickel Sulphate. 4. Alkaline Litmus. 5 Acid LitmusTo face page 33.[Lovibond, Colour Theories.When this principle is applied to the measurement of regularly increasing thicknesses, curves of colour changes can be established, which are specific for the substance in question, and afford a certain means of identifying similar substances in future. This is effected by varying the nature of the co-ordinates, making the ordinates to represent the tintometrical scale of colour units irrespective of colour, whilst the abscissae represent the scale of increasing thicknesses. Then by plotting the separate factors of each measurement according to their unit values, a series of curves is established, specific to the substance in question, and applicable to none other.We have now two systems of charting colour, in the first, the complete sensation is represented by a single point, as in Plate IV. In the second, each factor is represented by a separate point, and by connecting points of similar colours, a series of curves is established which represents a quantitative analysis of the progressive colour development, as in Plate V.

A colour chart is constructed by placing two colour scales at right angles to each other, with their zeros at the angle.

A measured simple colour, finds its position directly on its corresponding colour scale at the point of its measured value.

A measured complex colour, finds its position within the angle, at that point where perpendiculars drawn through the two colour values meet.

The above statements are complete only for colours of standard brightness, should the colour be brighter or duller than standards, a light factor is necessary, the value of which is furnished by the measurement itself, and must be written in numerals near the colour point.

By this method the chart position of even the most complicated colour is indicated by a single point which is determined by the analytical value of the composing factors.

Examples.

Simple Colour ofStandardBrightness.Complex Colourof StandardBrightness.Simple ColourBrighter thanStandards.Complex ColourDuller thanStandards.3· Red.6· Blue, 10· Violet.7· Yellow, Light 2·Red 6, Orange 5, Black 2.

The number of complex colour charts is limited to the six represented inFig. 1as lying in their order on a continuous spectrum. The red and violet mixtures having no visible spectrum position are represented in the ultra violet. The ordinates of the charts are made by erecting the overlying red, yellow and blue scales as perpendiculars.

Fig. 1.

The information to be obtained by charting measured colour is more extensive than appears at first sight, as by varying the character of the co-ordinates, and charting suitable series of measurements, new fields of investigation are opened, thus throwing light on some hitherto obscure questions, of which the following are some instances.

It has sometimes been assumed that colour increase was in direct ratio to intensity increase, but this is never the case, each substance has its own rate, specific to itself. It is conceivable that the colours of two substances may coincide at one point, but as their densities increase, or decrease, their rates of change vary.

The term “Specific Colour” is based on the experimental fact, that the colour of a given substanceis constant, so long as the substance itself and the conditions of observation, remain unaltered. During experimental work a sufficient number of instances have accumulated to warrant the writer in advancing and using the term “Specific Colour” as describing a new natural law, as rigid in its application as that of “Specific Gravity” or “Specific Heat.”

PLATE VABSORPTION CURVES OF FIVE COLOUR CONSTANTS.1 Potassium Bichromate. 2 Copper Sulphate. 3 Nickel Sulphate. 4. Alkaline Litmus. 5 Acid Litmus

PLATE VABSORPTION CURVES OF FIVE COLOUR CONSTANTS.

To face page 33.[Lovibond, Colour Theories.

When this principle is applied to the measurement of regularly increasing thicknesses, curves of colour changes can be established, which are specific for the substance in question, and afford a certain means of identifying similar substances in future. This is effected by varying the nature of the co-ordinates, making the ordinates to represent the tintometrical scale of colour units irrespective of colour, whilst the abscissae represent the scale of increasing thicknesses. Then by plotting the separate factors of each measurement according to their unit values, a series of curves is established, specific to the substance in question, and applicable to none other.

We have now two systems of charting colour, in the first, the complete sensation is represented by a single point, as in Plate IV. In the second, each factor is represented by a separate point, and by connecting points of similar colours, a series of curves is established which represents a quantitative analysis of the progressive colour development, as in Plate V.

CHAPTER X.Representations of Colour in Space of Three Dimensions.The relations of the different colours to one another, and to neutral tint are, perhaps, best represented to the mind by a solid model, or by reference to three co-ordinate axes, as employed in solid geometry (seeFig. 2).Fig. 2.Let the three adjacent edges OR, OB, OY, of the above cube be three axes, along which are measured degrees of Red, Yellow and Blue respectively, starting from the origin O. Every point in space onthe positive side of this origin will then represent a conceivable colour, the constituents of which in degrees of red, yellow and blue are measured by the three co-ordinates of the points. Pure reds lie all along the axis OR, pure yellows on the axis OY, and pure blues on the axis OB.All normal oranges, normal greens, and normal violets lie on the diagonals of the faces of the cubes OO1, OG, OV respectively.Pure neutral tints lie on the diagonal ON of the cube, equally inclined to the three principal axes.Red violets will be found on the plane ROB, between OV and OR.Blue violets on the same plane between OV and OB.“Saddened” red violets all within the wedge or open space enclosed by the three planes, whose boundaries are OB, OV, ON.The other colours, red and yellow oranges, blue and yellow greens, pure and saddened, are found in corresponding positions in relation to the other cases.[4]

The relations of the different colours to one another, and to neutral tint are, perhaps, best represented to the mind by a solid model, or by reference to three co-ordinate axes, as employed in solid geometry (seeFig. 2).

Fig. 2.

Let the three adjacent edges OR, OB, OY, of the above cube be three axes, along which are measured degrees of Red, Yellow and Blue respectively, starting from the origin O. Every point in space onthe positive side of this origin will then represent a conceivable colour, the constituents of which in degrees of red, yellow and blue are measured by the three co-ordinates of the points. Pure reds lie all along the axis OR, pure yellows on the axis OY, and pure blues on the axis OB.

All normal oranges, normal greens, and normal violets lie on the diagonals of the faces of the cubes OO1, OG, OV respectively.

Pure neutral tints lie on the diagonal ON of the cube, equally inclined to the three principal axes.

Red violets will be found on the plane ROB, between OV and OR.

Blue violets on the same plane between OV and OB.

“Saddened” red violets all within the wedge or open space enclosed by the three planes, whose boundaries are OB, OV, ON.

The other colours, red and yellow oranges, blue and yellow greens, pure and saddened, are found in corresponding positions in relation to the other cases.[4]

CHAPTER XI.The Spectrum in relation to Colour Standardization.The spectrum has naturally been considered as a suitable source for colour standards, but the power of analysing has disclosed some difficulties, which have yet to be overcome.Concerning the prismatic spectrum, there has always been a difficulty in apportioning the different colours to specific areas, and further, before this spectrum is available for colour standardization, some method of correction for the unequal distribution of colours must be devised.Neither of these difficulties occur in the use of the diffraction spectrum, where the pure colours are apportioned by Professor Rood from A to H in the manner shown in table on next page.Professor Rood further divides the spectrum from A to H into 100 equal divisions, allotting 20 unit divisions of 72,716 wave lengths to the space between each two colour lines. This allots a space of 3,635 W.L. to each unit division, as shown in Table III.TABLE III.Wave Length Position.No. ofWave Lengthsfrom Colourbetween each.DivisionW.L.K. perDivision760,400 A.Red760,400——72,717== 203,635Orange687,683——72,716== 203,635Yellow614,967——72,716== 203,635Green542,251——72,716== 203,635Blue469,535——72,716== 203,635396,819 H.Violet396,819363,581Total W.L. between A. & H.363,581100Having provided equal wave length positions for the six pure colours, the intermediate colours are necessarily binaries in definite proportions, accounted for by a regular overlapping of two bounding colours in opposite directions from zero to 20, as shown in the following table from Red to Orange, representing the space between these two pure colours.RedW.L760,40020191817161514131211109876543210W.L687,683Orange.01234567891011121314151617181920202020202020202020202020202020202020202020It follows, that apart from the six monochromes, all spectrum complex colours in a single wave length must be binaries, whose united values equal 20.On comparing Professor Rood’s scales of divisions with those of the tintometrical scales already described,they appear to coincide in several particulars, for instance:—The monochromes correspond both in number and in name.Their positions in the scales correspond.Their unit divisions are equal in number, and in dimensions.Their colour positions correspond, when an artificial tintometrical spectrum is made by regularly overlapping monochromes.It follows that when the two scales are superimposed as in Plate V., showing similar monochromes as lying in the same perpendicular, the same wave length numbers apply to both; concerning the dimensions between the monochromes, the spaces occupied by 72,716 wave lengths between the spectrum monochromes, also represent similar spaces in the tintometrical scales, and one-twentieth of this 3,635 represents the space of a single unit in each case.In connexion with these co-related dimensions, some information is obtainable bearing on the limitation of a monochromatic vision for discriminating small colour differences. Under ordinary daylight conditions, the unit in the lighter shades of the tintometrical scale is divided into 100 fractional parts, each fraction therefore represents a space occupied by thirty-six wave lengths in the spectrum scale. This may be near the limit of dimension for monochromatic vision in such a gradually changing colour scale, as that of the spectrum, and may be some guide as to suitable slit areas in the synthetical building up of complex coloured light.PLATE VISIX COLOUR CHARTS IN ONE OR ANOTHER OF WHICH ANY SIMPLE OR COMPLEX COLOUR FINDS A DEFINITE POSITION.1 Red Violet. 2 Red Orange. 3 Yellow Orange. 4 Yellow Green. 5 Blue Green. 6 Blue Violet.To face page 39.[Lovibond, Colour Theories.In Plate VI. are shown the six tintometrical colour charts, as lying in their order on the tintometrical spectrum, illustrating that any measured colour factor lies in a perpendicular drawn through both spectra, and occupying the same wave length position, and may therefore be designated by that wave length number.This explanation is not intended to convey that the colour energies do not really overlap beyond the boundaries of the dual combinations, but only that the vision is unable to distinguish as colour, such overlapping if it exists.POINTS OF DIFFERENCE.On further comparisons of the two scales there are some points of difference which have a bearing on their values as colour standards.There is a variation in the length of the two scales, the spectrum terminating at H, whilst the tintometrical scale is extended to a sixth division in the region of the ultra violet, showing overlapping combinations of Red and Violet, strictly analogous to the overlapping binaries in the other five sections.These red and violet combinations constitute one-sixth of the cycle of distinguishable colours, and cannot be omitted in any system of colour standardization, therefore their absence in the continuous spectrum is a drawback.A second drawback, is the limited number of spectrum complex colours, in consequence of each colour being blended only with overlapping colour value, which lies in its own wave length, whereas innature each colour may be blended with any value of the overlapping colour. In the tintometrical standards, similar effects are obtained by changing the value of the graded slips.It is true, that complex colours other than those in the same wave length, may be developed by blending two colours from different parts of the spectrum, but the ray proportions of colours so produced, are necessarily more complex than those developed by specific absorption; the first being a method of synthesis towards complexity, and the second a method of analysis towards simplicity, and although two colours so produced may be similar in name, red for instance, they must differ in character. This view may tend to reconcile some of the theoretical differences between Scientists and Artists.THE ULTRA VIOLET DIVISION.The complete range of daylight colours not being fully comprised in a continuous spectrum, may be considered as a cycle of radiant energies, sensitive to the vision as colour, which can be represented as a circle as in Plate VII. The outer and broken circle represents a bent spectrum, the unoccupied division corresponding in position with that of the red and violet mixtures in the complete cycle.This arrangement does not alter the relative positions of the Fraunhoper lines A, B, C, D, E, F, G and H in reference to either scale, but, it theoretically breaks that sequence of the successive wave lengths in the Red Violet which holds good in the other five divisions from A to H.PLATE VIITo face page 40.[Lovibond, Colour Theories.In order to theoretically avoid this juxtaposition of wave length contrast, it is only necessary to imagine that the violet energy beyond H in the ultra violet, is overlapped by the infra red energy of a succeeding spectrum, filling this section with a series of overlapping binaries analogous in wave length sequence to that of the other sections.A RESIDUAL RED RAY.Apart from the colours of everyday life there is, in sunlight and most direct artificial lights, an additional red energy which differs materially from the red energy in diffused daylight.It was first noticed whilst establishing the colour equivalence of the tintometrical light unit, by developing a red sensation which disturbed constancy of reading under certain conditions of light.So far as the writer knows, this energy has never been investigated as separate from the other spectrum red. The following observations must be considered as tentative only.SOME PROPERTIES.It does not obey the laws of absorption which govern the red of diffused daylight. When the six transparent pigmentary colours are illuminated by direct sunlight, and viewed through a sufficient number of Neutral Tint units, the colours all disappear, all appearing red alike, with only differences in luminosity.The spectrum position of this red energy is in the A. B. region, and further interception byNeutral Tint whilst narrowing the band, intensifies the colour, until obstructed by the large number of intercepting glass surfaces.It has no photographic action on the six sensitised papers dealt with in the photographic section.LIGHT INTENSITIES.The apparatus for determining the unit values of light intensities in the following series of measurements, consisted of a conical rectangular hopper tapering from 2 feet to 2 inches square. This was adapted so that the light from the small end, commanded the stage of the optical instrument sufficiently close to cut off outside light. The wide end facing a north sky was adapted with sliding shutters, to regulate the area of incident light; of the six water-colour pigments which nearest corresponded to the standard colours, washed to their full depth on Whatman’s paper, six measurements were made. These measurements are shown in Table IV, and classified in Table V.It will be noted that the readings are constant for all the colours between 16 and 26 units, except a variation of light ·15 in the 24-inch opening, which is in effect as if the cone was not present, and ·2 in the 8-inch area of orange.Note.—Experiments in this branch give some information relating to the perception of colour under daylight conditions, by limiting the range of intensities within which colour can be distinguished and differentiated, whilst their separate photographic action (page 48) suggests the impression that colour phenomena, outside these limits, may be a physiological expression of widely varying underlying energies.TABLE IV.Pigment.SquareInchesAperture.LightIntensity.Black.Red.Orange.Carmine210·520·2·3—"411·519·1·4—"614·4618·95·59—"1020—16·91·1—"1222—16·91·1—"Open26—16·91·1—YellowLemon Yellow210—6·9·1—"411—6·9·1—"614—6·9·1—"816—7·0——"1020—7·0——"1222—7·0——"Open26—7·0——Blue.Violet.Cobalt Blue210—10·5——"411—10·5——Green."614—10·5·2—Violet."816—10·5·5—"1020—10·5·5—"1222—10·5·5—"Open26—10·5·5—Red.Orange.Light.Chrome Orange210—3·46·0—"411—3·46·0—"614—3·06·2·05"816—3·06·0·05"1020—3·26·0·05"1222—3·26·0—"Open26—3·26·0—Yellow.Green.Emerald Green210—·26·4·05"411——6·4·05"614——6·4·05"816——6·62·0"1020——6·62·0"1222——6·62·0"Open26——6·6·05Red.Violet.Mauve210—3·07·4—"411—3·07·4—"614—3·07·4—"816—2·87·2—"1020—2·87·2—"1222—2·87·2—"Open26—2·87·2—TABLE V.COLOURED SURFACESTable of Varying Luminous IntensitiesInchesSquare.LightUnits.Red.Yellow.Blue.“Carmine.”“Lemon.”“Cobalt.”R.Or.Blk.Y.Or.B.Vi.Blk.21020·2·3·56·9·111·5——41419·1·4·56·9·111·5——61418·95·59·466·9·110·7Gr.·2·181616·91·1—7·0—10·5Vi.·5—102016·91·1—7·0—10·5·5—122216·91·1—7·0—10·5·5—Open242616·91·1—7·0—10·5·5—InchesSquare.LightUnits.Orange.Green.Violet.“Chrome.”“Emerald.”“Fr. Mauve.”Or.R.Light.Gr.Y.Light.Vi.R.2106·3·4·056·4·2·057·43·4146·3·4·056·4·2·057·43·6146·23··056·4—·057·23·8166·3·—6·6—·27·22·810206·3·2—6·6—·27·22·812226·3·2—6·6—·27·22·8Open24266·3·2—6·6—·057·22·8

The spectrum has naturally been considered as a suitable source for colour standards, but the power of analysing has disclosed some difficulties, which have yet to be overcome.

Concerning the prismatic spectrum, there has always been a difficulty in apportioning the different colours to specific areas, and further, before this spectrum is available for colour standardization, some method of correction for the unequal distribution of colours must be devised.

Neither of these difficulties occur in the use of the diffraction spectrum, where the pure colours are apportioned by Professor Rood from A to H in the manner shown in table on next page.

Professor Rood further divides the spectrum from A to H into 100 equal divisions, allotting 20 unit divisions of 72,716 wave lengths to the space between each two colour lines. This allots a space of 3,635 W.L. to each unit division, as shown in Table III.

TABLE III.

Wave Length Position.No. ofWave Lengthsfrom Colourbetween each.DivisionW.L.K. perDivision760,400 A.Red760,400——72,717== 203,635Orange687,683——72,716== 203,635Yellow614,967——72,716== 203,635Green542,251——72,716== 203,635Blue469,535——72,716== 203,635396,819 H.Violet396,819363,581Total W.L. between A. & H.363,581100

Having provided equal wave length positions for the six pure colours, the intermediate colours are necessarily binaries in definite proportions, accounted for by a regular overlapping of two bounding colours in opposite directions from zero to 20, as shown in the following table from Red to Orange, representing the space between these two pure colours.

RedW.L760,40020191817161514131211109876543210W.L687,683Orange.01234567891011121314151617181920202020202020202020202020202020202020202020

It follows, that apart from the six monochromes, all spectrum complex colours in a single wave length must be binaries, whose united values equal 20.

On comparing Professor Rood’s scales of divisions with those of the tintometrical scales already described,they appear to coincide in several particulars, for instance:—

The monochromes correspond both in number and in name.

Their positions in the scales correspond.

Their unit divisions are equal in number, and in dimensions.

Their colour positions correspond, when an artificial tintometrical spectrum is made by regularly overlapping monochromes.

It follows that when the two scales are superimposed as in Plate V., showing similar monochromes as lying in the same perpendicular, the same wave length numbers apply to both; concerning the dimensions between the monochromes, the spaces occupied by 72,716 wave lengths between the spectrum monochromes, also represent similar spaces in the tintometrical scales, and one-twentieth of this 3,635 represents the space of a single unit in each case.

In connexion with these co-related dimensions, some information is obtainable bearing on the limitation of a monochromatic vision for discriminating small colour differences. Under ordinary daylight conditions, the unit in the lighter shades of the tintometrical scale is divided into 100 fractional parts, each fraction therefore represents a space occupied by thirty-six wave lengths in the spectrum scale. This may be near the limit of dimension for monochromatic vision in such a gradually changing colour scale, as that of the spectrum, and may be some guide as to suitable slit areas in the synthetical building up of complex coloured light.

PLATE VISIX COLOUR CHARTS IN ONE OR ANOTHER OF WHICH ANY SIMPLE OR COMPLEX COLOUR FINDS A DEFINITE POSITION.1 Red Violet. 2 Red Orange. 3 Yellow Orange. 4 Yellow Green. 5 Blue Green. 6 Blue Violet.

PLATE VISIX COLOUR CHARTS IN ONE OR ANOTHER OF WHICH ANY SIMPLE OR COMPLEX COLOUR FINDS A DEFINITE POSITION.

To face page 39.[Lovibond, Colour Theories.

In Plate VI. are shown the six tintometrical colour charts, as lying in their order on the tintometrical spectrum, illustrating that any measured colour factor lies in a perpendicular drawn through both spectra, and occupying the same wave length position, and may therefore be designated by that wave length number.

This explanation is not intended to convey that the colour energies do not really overlap beyond the boundaries of the dual combinations, but only that the vision is unable to distinguish as colour, such overlapping if it exists.

On further comparisons of the two scales there are some points of difference which have a bearing on their values as colour standards.

There is a variation in the length of the two scales, the spectrum terminating at H, whilst the tintometrical scale is extended to a sixth division in the region of the ultra violet, showing overlapping combinations of Red and Violet, strictly analogous to the overlapping binaries in the other five sections.

These red and violet combinations constitute one-sixth of the cycle of distinguishable colours, and cannot be omitted in any system of colour standardization, therefore their absence in the continuous spectrum is a drawback.

A second drawback, is the limited number of spectrum complex colours, in consequence of each colour being blended only with overlapping colour value, which lies in its own wave length, whereas innature each colour may be blended with any value of the overlapping colour. In the tintometrical standards, similar effects are obtained by changing the value of the graded slips.

It is true, that complex colours other than those in the same wave length, may be developed by blending two colours from different parts of the spectrum, but the ray proportions of colours so produced, are necessarily more complex than those developed by specific absorption; the first being a method of synthesis towards complexity, and the second a method of analysis towards simplicity, and although two colours so produced may be similar in name, red for instance, they must differ in character. This view may tend to reconcile some of the theoretical differences between Scientists and Artists.

The complete range of daylight colours not being fully comprised in a continuous spectrum, may be considered as a cycle of radiant energies, sensitive to the vision as colour, which can be represented as a circle as in Plate VII. The outer and broken circle represents a bent spectrum, the unoccupied division corresponding in position with that of the red and violet mixtures in the complete cycle.

This arrangement does not alter the relative positions of the Fraunhoper lines A, B, C, D, E, F, G and H in reference to either scale, but, it theoretically breaks that sequence of the successive wave lengths in the Red Violet which holds good in the other five divisions from A to H.

PLATE VII

To face page 40.[Lovibond, Colour Theories.

In order to theoretically avoid this juxtaposition of wave length contrast, it is only necessary to imagine that the violet energy beyond H in the ultra violet, is overlapped by the infra red energy of a succeeding spectrum, filling this section with a series of overlapping binaries analogous in wave length sequence to that of the other sections.

Apart from the colours of everyday life there is, in sunlight and most direct artificial lights, an additional red energy which differs materially from the red energy in diffused daylight.

It was first noticed whilst establishing the colour equivalence of the tintometrical light unit, by developing a red sensation which disturbed constancy of reading under certain conditions of light.

So far as the writer knows, this energy has never been investigated as separate from the other spectrum red. The following observations must be considered as tentative only.

It does not obey the laws of absorption which govern the red of diffused daylight. When the six transparent pigmentary colours are illuminated by direct sunlight, and viewed through a sufficient number of Neutral Tint units, the colours all disappear, all appearing red alike, with only differences in luminosity.

The spectrum position of this red energy is in the A. B. region, and further interception byNeutral Tint whilst narrowing the band, intensifies the colour, until obstructed by the large number of intercepting glass surfaces.

It has no photographic action on the six sensitised papers dealt with in the photographic section.

The apparatus for determining the unit values of light intensities in the following series of measurements, consisted of a conical rectangular hopper tapering from 2 feet to 2 inches square. This was adapted so that the light from the small end, commanded the stage of the optical instrument sufficiently close to cut off outside light. The wide end facing a north sky was adapted with sliding shutters, to regulate the area of incident light; of the six water-colour pigments which nearest corresponded to the standard colours, washed to their full depth on Whatman’s paper, six measurements were made. These measurements are shown in Table IV, and classified in Table V.

It will be noted that the readings are constant for all the colours between 16 and 26 units, except a variation of light ·15 in the 24-inch opening, which is in effect as if the cone was not present, and ·2 in the 8-inch area of orange.

Note.—Experiments in this branch give some information relating to the perception of colour under daylight conditions, by limiting the range of intensities within which colour can be distinguished and differentiated, whilst their separate photographic action (page 48) suggests the impression that colour phenomena, outside these limits, may be a physiological expression of widely varying underlying energies.

TABLE IV.

Pigment.SquareInchesAperture.LightIntensity.Black.Red.Orange.Carmine210·520·2·3—"411·519·1·4—"614·4618·95·59—"1020—16·91·1—"1222—16·91·1—"Open26—16·91·1—YellowLemon Yellow210—6·9·1—"411—6·9·1—"614—6·9·1—"816—7·0——"1020—7·0——"1222—7·0——"Open26—7·0——Blue.Violet.Cobalt Blue210—10·5——"411—10·5——Green."614—10·5·2—Violet."816—10·5·5—"1020—10·5·5—"1222—10·5·5—"Open26—10·5·5—Red.Orange.Light.Chrome Orange210—3·46·0—"411—3·46·0—"614—3·06·2·05"816—3·06·0·05"1020—3·26·0·05"1222—3·26·0—"Open26—3·26·0—Yellow.Green.Emerald Green210—·26·4·05"411——6·4·05"614——6·4·05"816——6·62·0"1020——6·62·0"1222——6·62·0"Open26——6·6·05Red.Violet.Mauve210—3·07·4—"411—3·07·4—"614—3·07·4—"816—2·87·2—"1020—2·87·2—"1222—2·87·2—"Open26—2·87·2—

TABLE V.COLOURED SURFACESTable of Varying Luminous Intensities

InchesSquare.LightUnits.Red.Yellow.Blue.“Carmine.”“Lemon.”“Cobalt.”R.Or.Blk.Y.Or.B.Vi.Blk.21020·2·3·56·9·111·5——41419·1·4·56·9·111·5——61418·95·59·466·9·110·7Gr.·2·181616·91·1—7·0—10·5Vi.·5—102016·91·1—7·0—10·5·5—122216·91·1—7·0—10·5·5—Open242616·91·1—7·0—10·5·5—

InchesSquare.LightUnits.Orange.Green.Violet.“Chrome.”“Emerald.”“Fr. Mauve.”Or.R.Light.Gr.Y.Light.Vi.R.2106·3·4·056·4·2·057·43·4146·3·4·056·4·2·057·43·6146·23··056·4—·057·23·8166·3·—6·6—·27·22·810206·3·2—6·6—·27·22·812226·3·2—6·6—·27·22·8Open24266·3·2—6·6—·057·22·8

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