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 scale was constructed, ranging in intensity from ·01 to 20·
units.[3]
[3] It was found that the superimposition of two glasses did not
visually disturb equivalence, therefore only two glasses were used for
each observation in constructing the scales.
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.
[Illustration: PLATE IV COMPARISON CURVES OF HEALTHY HUMAN BLOOD WITH
THE BLOOD OF LOWER ANIMALS.
_To face page 31._] [_Lovibond, Colour Theories._
]