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 As already described arbitrary scales were first used in many colours, but were superseded by these three, which, when graded into scales of equivalent value, were found to cover all daylight colours.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 C_{2}SO_{4}5H_{2}O 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 of _Nickel Sulphate NiSO_{4}7H_{2}O, 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 of _Potassium Bichromate_ K_{2}Cr_{2}O, 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 depth of 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 glasses represent 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.

EXAMPLES.

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 by one 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 then matched 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