Evolution of the Unit.
The dimensions of the light and colour unit here adopted, together
with the scales of division, were in the first instance physiological,
depending entirely on the skill of normal visions for exactitude. The
co-relation of equal values in the different colour scales, was secured
by an elaborate system of cross-checking, rendered necessary because
the establishment of a perfectly colourless neutral tint unit, demanded
an exact balance in values of the different colour scales. These scales
have stood the test of many years’ work by many observers, and in no
case has any alteration been required. The original set is still in use.
The first point which required consideration after the want of standard
colour scales was realized, was the basis and dimensions of the unit.
So far as the writer knew there was no published information bearing on
this which could be used as a guide.
Several arbitrary scales for specific purposes had already been
constructed by selecting a colour depth which could easily be
distinguished, calling it a unit, and scaling it by duplicating and
subdividing. This course was adopted with a coloured glass which
approximately matched Ales and Malt solutions, and another which
matched Nesslerized Ammonia solutions. No insuperable difficulty
occurred in constructing scales available for quantitative work in
these two instances.
The intensity of the colour unit for these arbitrary scales, was that
which appeared to be most convenient for the purpose required, but the
several scales had no common basis. The unit was physiological, and the
exactitude of the scales depended entirely on the skill of the vision
for discriminating small differences.
As the writer’s experimental work progressed, it became evident that
red, yellow, and blue were the only colours suitable for systematic
work. The superimposition of any two, developed a third colour which
apparently had no relation to either. The superimposition of the third
glass modified or destroyed all colour and reduced the amount of light.
This suggested the idea that if the three colours could be so balanced
that the light transmitted was colourless, it would be evidence of
equivalence of intensity in the individual colours.
The real difficulty was in obtaining this equivalence, because a
balance which transmitted a neutral tint by one light developed colour
by another. This necessitated the selection of a standard light. The
light finally selected was that of a so-called sea fog, away from the
contaminating influence of towns. The white fog of Salisbury Plain was
used as being most available. It required two years’ work to establish
equivalence in the unit.
[Illustration: PLATE II NINE CIRCLES ILLUSTRATING THE ANALYSIS OF A BEAM
OF WHITE LIGHT INTO THE SIX COMPOSING COLOURS BY THE ABSORPTIVE METHOD
A. WHITE LIGHT
B. DIVIDED INTO CHROMATIC EQUIVALENTS
C. WHOLLY ABSORBED
COLOURS OF THE FIRST DIVISION DEVELOPED BY THE ABSORPTION OF THE THREE
COMPLIMENTARIES
1
2
3
COLOURS OF THE SECOND DIVISION DEVELOPED BY THE ABSORPTION OF THE FIVE
OTHER RAYS
4
5
6
_To face page 11._] [_Lovibond, Colour Theories._
]