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 of | Complex Colour
Standard | of Standard
Brightness. | Brightness.
|
3· Red. | 6· Blue, 10· Violet.
Simple Colour | Complex Colour
Brighter than | Duller than
Standards. | Standards.
|
7· Yellow, | Red 6, Orange 5,
Light 2· | Black 2.
The number of complex colour charts is limited to the six represented
in Fig. 1 as 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.
[Illustration: 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 substance is 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.”
[Illustration: PLATE V ABSORPTION 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.