Evolution of the Method.
The writer was formerly a brewer, and this work had its origin in an
observation that the finest flavour in beer was always associated
with a colour technically called “golden amber,” and that, as the
flavour deteriorated, so the colour assumed a reddish hue. It was these
variations in tint that suggested the idea of colour standards as a
reliable means of reference.
The first experiments were made with coloured liquids in test tubes
of equal diameter, and by these means some useful information was
obtained; but as the liquids soon changed colour, frequent renewals
were necessary, and there was always a difficulty and uncertainty in
their exact reproduction.
To obviate this, glass in different colours was tried, and long
rectangular wedges with regularly graded tapers were ground and
polished for standards, whilst correspondingly tapered glass vessels
were made for the beers. These were arranged to work side by side,
and perpendicularly, before two apertures of an optical instrument,
which gave a simultaneous view of both. The apertures were provided
with a fixed centre line, to facilitate the reading off of comparisons
of thickness. There was no difficulty in obtaining glass which
approximated to the required colour when used in one thickness only.
But as thickness varied, the test no longer held good for both
standards, their rates of colour change being different, making the
method unreliable.[1]
[1] It was afterwards found that these colour changes through
variations of intensities were due to a natural law to be described
under the heading of “Specific colour.” (_See_ page 32.)
The system about to be described is one of analytical absorption, and
has been published from time to time in the form of papers, read before
Societies interested in the question of colour standardization; as also
in two descriptive works by the present writer. The earlier works were
necessarily fragmentary, but gathered system as the subject progressed.
At an early stage in the investigations it was realized that the
handbooks of the period dealt largely with theoretical differences
which were of little service to the technical worker. Under these
circumstances the writer applied for advice to the late Mr. Browning
of the Strand, who gave it as his opinion that no work existed which
could be of service to the writer. All that could be done was to go on
until something should be arrived at. On this, all theoretical reading
was put aside, and the work proceeded on the simple lines of observing,
recording, and classifying experimental facts.
In working with glass of different colours it was found that some
combinations developed colour, whilst other combinations destroyed
it. This suggested the probability of a governing natural law; and
experimental work was undertaken in the hope of discovering it. The
result was the construction of a mechanical scale of colour standards,
which are now in use in over one thousand laboratories, and no
question of their practical accuracy arises. The principal conditions
for ensuring accuracy and constancy of results are embodied in the
following code of nine precautions, which have been published for
nearly twenty years without being disputed. They may therefore be
considered as governing laws, at least for the present. The colour
theory adopted for these Governing Laws has grown out of a series of
experimental facts capable of demonstration, and is summed up in the
following code of nine Laws.
Laws 1, 2, and 3 relate to White and Coloured Light, and are as
follows:--
1. Normal white light is made up of the six colour rays, Red, Orange,
Yellow, Green, Blue and Violet in equal proportions. When these rays
are in unequal proportions the light is abnormal and coloured.
2. The particular colour of an abnormal beam is that of the
one preponderating ray, if the colour be simple, or of the two
preponderating rays if the colour be complex. The depth of colour is in
proportion to the preponderance.
3. The rays of a direct light are in a different condition to the
same rays after diffusion, and give rise to a different set of colour
phenomena.
Laws 4, 5, 6, and 7 deal with The Limitations of the Vision to
appreciate Colour.
4. The vision is not simultaneously sensitive to more than two colours
in the same beam of light. The colour of any other abnormal ray is
merged in the luminosity of the beam.
5. The two colours to which the vision is simultaneously sensitive
are always adjacent in their spectrum order, Red and Violet being
considered adjacent for this purpose.
6. The vision is unable to appreciate colour in an abnormal beam
outside certain limits, from two causes:
(_a_) The colour of an abnormal beam may be masked to the vision from
excess of luminosity.
(_b_) The luminous intensity of the abnormal beam may be too low to
excite definite colour sensations.
7. The vision has a varying rate of appreciation for different colours
by time, the lowest being for red. The rate increases in rapidity
through the spectrum, until the maximum rate is reached with violet.
And since this varying rate necessitates a time limit for critical
observations, five seconds has been adopted as the limit, no variations
being perceptible in that time.
Laws 8 and 9 relate to Colour Constants.
8. The colour of a given substance of a given thickness is constant so
long as the substance itself, and the conditions of observation, remain
unaltered.
9. Every definite substance has its own specific rate of colour
development for regularly increasing thicknesses.