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                       LIGHT AND COLOUR THEORIES

[Illustration: TINTOMETER. Form of Instrument for Opaque Observation.]




[Illustration: Reproductions of some Medals awarded to JOSEPH W. LOVIBOND’S
Method of Colour Analysis FOR Scientific and Commercial Purposes.]




                               LIGHT AND
                            COLOUR THEORIES

                    and their Relation to Light and
                        Colour Standardization

                                  By

                          JOSEPH W. LOVIBOND

               ILLUSTRATED BY 11 PLATES COLOURED BY HAND

[Illustration: Logo]

                                London

            E. & F. N. SPON, Limited, 57 HAYMARKET

                               New York

                SPON & CHAMBERLAIN, 123 LIBERTY STREET

                                 1915




CONTENTS


                                                             PAGE

  List of Plates                                              vii

  Purpose                                                      ix


                          CHAPTER I.
  Introduction                                                  1


                          CHAPTER II.
  Evolution of the Method                                       5


                          CHAPTER III.
  Evolution of the Unit                                         9


                          CHAPTER IV.
  Derivation of Colour from White Light                        11


                          CHAPTER V.
  Standard White Light                                         14


                          CHAPTER VI.
  Qualitative Colour Nomenclature                              17


                          CHAPTER VII.
  Quantitative Colour Nomenclature

                          CHAPTER VIII.
  The Colour Scales                                            28


                          CHAPTER IX.
  Colour Charts                                                31


                          CHAPTER X.
  Representations of Colour in Space of Three Dimensions       34


                          CHAPTER XI.
  The Spectrum in relation to Colour Standardization           36


                          CHAPTER XII.
  The Physiological Light Unit                                 45


                          APPENDIX I.
  Colour Education                                             59


                          APPENDIX II.
  The Possibilities of a Standard Light and Colour Unit        69

                          APPENDIX III.
  Dr. Dudley Corbett’s Radiometer                              83


  Index                                                        89




ERRATA.


    Plate I. Newton’s Theory. The Indigo line is erroneously placed
    between the Violet and the Red; it should be between the Blue
    and the Violet.

    Page 40.--_Fifth line from the bottom, for_ Fraunhoper _read_
    Fraunhofer.


_To face p. vi., Lovibond, Light and Colour Theories._] [P.R. 1317




LIST OF PLATES


                                                           TO FACE
                                                             PAGE

  Plate  I. Six Colour Theories                                 4

    "   II. Circles Illustrating Absorption of White Light     11

    "  III. Diagram Illustrating Analysis of White Light       13

    "   IV. First System of Charting Colour                    31

    "    V. Second System of Charting Colour                   33

    "   VI. Six Tintometrical Colour Charts                    39

    "  VII. Two Circles                                        40

    " VIII. Absorption Curves of Dyes                          76

    "   IX. Fading Curves of Dyes                              78

    "    X. Comparison Curves of Healthy and Diseased Blood    80

    "   XI. Specific Colour Curves of Healthy and Diseased
            Human Blood                                        82




PURPOSE


The purpose of this work is to demonstrate that colour is a
determinable property of matter, and to make generally known methods
of colour analysis and synthesis which have proved of great practical
value in establishing standards of purity in some industries.

The purpose is also to show that the methods are thoroughly scientific
in theory and practice, and that the results are not likely to be
changed by further discoveries. Also that out of the work done a new
law has been developed, which the writer calls the Law of Specific
Colour Development, meaning that every substance has its own rate of
colour development for regularly increasing thicknesses.


THE THEORY.

Of the six colours in white light--red, orange, yellow, green, blue
and violet; Red, Yellow and Blue are regarded as dominants, because
they visually hold the associated colours orange, green and violet in
subjection.

An equivalent unit of pure red, pure yellow and pure blue is adopted,
and incorporated into glass. The unit is multiplied to obtain greater
intensities, and divided to obtain lesser intensities.

The coloured glasses are called absorbents. The red absorbent transmits
violet, red and orange, but the red ray alone is visible as colour,
until the other absorbents are superimposed, and the character of the
group of rays changed. In the same way yellow transmits orange and
green, and blue transmits green and violet, whilst the yellow and blue
alone are visible as colour. Orange, green and violet are here called
subordinates, which may be developed as follows:--

    Or. = R. + Y.   Gr. = Y. + B.   Vi. = B. + R.

Twenty-five years’ experience in the application of the theory and the
method to the requirements of practical work, have given no reason for
change. Following will be found a list of awards from International
Juries and Scientific Societies, also a list of industries in which the
writer’s method is giving entire satisfaction.


Awards by International Juries.

  St. Louis                1 Silver Medal.
                           2 Bronze Medals.

  Brussels                 1 Gold Medal.
                           3 Silver Medals.

  Turin                    Diploma of Honour.
                           1 Gold Medal.
                           1 Silver Medal.


Awards by Scientific Societies.

  Sanitary Institute of Great Britain--
      _Bronze_ Medal for Colourometrical Water Analysis.

  Royal Sanitary Institute--
      _Bronze_ Medal for Measuring Smoke Densities.

  International Congress on School Hygiene--
      _Bronze_ Medal for Colour Educator.

  Royal Sanitary Institute--
      _Silver_ Medal for Colour Educator.

  Smoke Abatement Society, Sheffield--
      _Diploma_ for System of Colour Measurement.

  Royal Sanitary Institute--
      _Bronze_ Medal for Quantitative Estimation of Colour Blindness.

  Franco-British Exhibition--
      _Gold_ Medal for Colour Educator.

  International Medical Congress--
      _Bronze_ Medal for Tintometer as Medical requisite.

  Royal Sanitary Institute--
      _Silver_ Medal for recent developments.

  Royal Sanitary Institute--
      _Silver_ Medal Corbett’s Radiometer.

Formal Adoption of Tintometer Standards by--

  The Petroleum Industry.
  The Massachusetts Board of Health.
  The International Association of Leather Trades Chemists.
  The Inter-states Cotton Seed Oil Association.
  The Bureau of Engraving and Printing, China.

In general use by the following Industries--

  Brewing and Malting.
  Tanning.
  Wine and Spirit Merchants.
  Dyeing and Printing.
  Paint, Oil and Varnish Merchants.
  Millers.
  Water Works Chemists.
  Ceramic Works.
  For estimating per cent. of Carbon in Steel.
  For estimating per cent. of Ammonia.
  For estimating Colours for Anthropological Classifications.
  For estimating Smoke Densities.
  For estimating Haemoglobin in the Blood.
  For estimating Colour of Whale Oil, etc., etc.


THE METHOD.

The colour composition of any object may be measured by superimposing
units of different colours until the colour of the object is matched. A
convenient apparatus is furnished for this purpose. The composition of
the colour is learned by merely reading the markings on the glasses.

It is of course necessary that in the isolation of colour rays,
some unit for measuring the intensity of both light and colour be
established. As will be explained later, all such units are necessarily
arbitrary. In this method the unit has been established by taking the
smallest amount of colour easily perceptible to the ordinary vision.
This unit or “one” is divided into tenths in the darker shades, and
hundredths in the lighter scale. One one-hundredth is the smallest
amount of colour measurable by a normal trained vision.

When equivalent units in the three colours are superimposed, their
equivalent value (not their aggregate value) represents so much white
light absorbed. For instance, 2 R. + 2 Y. + 2 B. absorbs two units of
white light.

When the absorptive power of the colour standards is less than the
intensity of the light, associated white light remains.

  JOSEPH W. LOVIBOND.

  The Colour Laboratories,
  Salisbury.

  _December, 1914._




Light and Colour Theories




CHAPTER I.

Introduction.


It may at first appear strange that colour, one of the most important
indices of value in the Arts, Manufactures, and Natural Products,
should have no common nomenclature or reliable standard for reference,
the reproduction of a given colour depending for exactitude on the
memory of a sensation; whereas this branch of science requires a
physical means of recording a colour, with a power of recovery. It
remains to be shown that this power of record and recovery is possible,
and depends only on the observance of a few simple natural laws easy of
application.

The study of colour is carried on by two principal methods: the
spectroscopic, where the colours are partially separated as a
continuous band by a regular variation in their indices of refraction,
the colours gradually merging into each other by overlapping in
opposite directions; or by absorption, where a colour is developed by
absorbing its complementary, and is isolated as a single or complex
colour. This latter is nature’s own method.

It is necessary to touch on some theoretical differences which exist
between Scientists and Artists, as to which are Primary colours, as
confusion of this character retards investigation. Scientists adopt
Red, Green, and Violet as Primaries, regarding all other colours as
mixtures of these; whilst Artists and Colourists adopt Red, Yellow and
Blue as the Primaries, and all other colours as made from them.

The theory of the Scientists is based on the phenomena developed by
mixing coloured lights taken from different parts of the spectrum.
This is a method of synthesis, each added colour being a progressive
stage towards the complexity of white light. In this case the colour
developed is that of the preponderating ray of a complex beam. The
theory of the Artists is based on the phenomena developed by mixed
pigments. This is a method of analysis, tending towards ray simplicity,
each added pigment reducing the complexity of the colour developed by
its power of selective absorption.

The theoretical differences between the two schools appear to have
arisen from supposing that a given colour developed by the two
methods should correspond; but considering the differences in their
ray composition, this would be impossible, for although both may be
describable by one general colour term, as for instance a Red, they
would be of two varieties. It remains to be shown that one theory may
cover both sets of phenomena.

The Red, Green, and Violet theory appears to be based on two principal
assumptions: first that there are only three fundamental colours;
and second that the rays taken from different colour areas are pure
colours. Both assumptions are open to question. In regard to the first,
there is no difficulty in isolating six colours; and as to the second,
it can be demonstrated that the colours do overlap in every part, with
a double overlapping in the middle colours, and are therefore not
simple but complex.


PAST THEORIES.

In a work of this nature it is unnecessary to deal minutely with the
theories which have been adopted from time to time since Newton’s
discovery of the continuous spectrum. It will, however, be useful to
touch on the principal points where theorists are agreed, and also on
some of their points of difference, the latter in order to find, if
possible, the causes of their difference.


TABLE I.

  ----------------+-----+-------------------------------------------------
                  | No. |
                  | of  |               Primary Colours.
                  |Rays.|
  ----------------+-----+-------------------------------------------------
  Newton (later)  |  7  | Red, Orange, Yellow, Green, Blue, Indigo, Violet
                  |     |
  Werner          |  6  | Red, Orange, Yellow, Green, Blue,         Violet
                  |     |
  Newton and      |     |
  Helmholtz       |  5  | Red,         Yellow, Green, Blue,         Violet
   (early)        |     |
                  |     |
  Hering          |  4  | Red,         Yellow, Green, Blue
                  |     |
  Chevieul,       |     |
  Brewster, Hay,  |     |
  Redgrave, Field |  3  | Red,         Yellow,        Blue
                  |     |
  Young, Helmholtz|     |
    (later)       |  3  | Red,                 Green,               Violet
  ----------------+-----+-------------------------------------------------

                       Note to Plate I.

    The respective positions of the primaries of each theory in
    regard to the whole cycle of distinguishable colours are
    illustrated above, and the primaries of each theory are shown
    in their several spectrum positions, the spectrum being shown
    as bent in circular form.

The six principal theories of primary colours are given in Table I,
and illustrated on Plate I, with the names of the primaries of each
theory opposite the names of some of their principal advocates. It
should not be forgotten when comparing these wide divergences, that
each theory has been the result of experimental evidence, in what was,
at the time, and remains up to the present, a new and progressive
branch of science.

They agree that the spectrum colours are purer than the pigmentary
colours, and that by reason of their being referable to wave length
positions, they are most adaptable as standards of colour. There has
also been common agreement that certain colours are primaries, and
that all other colours are mixtures of these, but there has been wide
divergence as to their number and even the colours themselves.

[Illustration: PLATE I ILLUSTRATING THE POSITION OF THE PRIMARIES IN
RELATION TO THE COLOURS REQUIRED TO BE PRODUCED BY THE THEORETICAL
MIXTURES AND ALSO IN RELATION TO THE CHROMATIC CIRCLE.

  A SEVEN RAY THEORY. NEWTON.

  A SIX RAY THEORY. WERNER.

  A FIVE RAY THEORY. NEWTON (_Early_). HELMHOLTZ (_Early_).

  A FOUR RAY THEORY. HERING.

  A THREE RAY THEORY. CHEVREUL. BREWSTER. HAY. REDGRAVE. FIELD.

  A THREE RAY THEORY. YOUNG-HELMHOLTZ.

  _To face page 4._]      [_Lovibond, Colour Theories._
]




CHAPTER II.

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.




CHAPTER III.

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._
]




CHAPTER IV.

Derivation of Colour from White Light.


The method of analysing white light into its colour constituents by
means of coloured glass absorbents of known intensity and purity, is
illustrated by the set of nine circles in Plate II, which demonstrate
that colour is developed by the absorption of the complementary colour
rays. The ratios of transmission are equal.

In this set of illustrations the circles represent light of 20 units
luminous intensity, and the absorptive value of the three glass colours
is each of 20 units, therefore the whole of the light and colour
energies are presumed to be dealt with.

In the first set of three circles, _A_ represents a beam of normal
white light. _B_ a similar beam as divided into the six colour rays,
Red, Orange, Yellow, Green, Blue and Violet in equal proportions, _C_
as wholly absorbed by Red, Yellow, and Blue glasses, each of 20 units
colour intensity.

Figures 1, 2 and 3 represent the specific action of Red, Yellow, and
Blue glass on the white light.

_Red_ absorbs Yellow, Green and Blue, transmitting Violet, Red and
Orange, developing Red only.

_Yellow_ absorbs Blue, Violet, and Red, transmitting Orange, Yellow and
Green, developing Yellow only.

_Blue_ absorbs Red, Orange and Yellow, transmitting Green, Blue, and
Violet, developing Blue only.

By this method of development, Red, Yellow or Blue, when seen alone
are visually monochromatic, although composite in structure, each
containing a group of three rays, the middle ray alone exciting the
colour sensation.

Circles 4, 5, and 6 illustrate the development of Orange, Green and
Violet from the triad groups, by intercepting the light with two glass
colours.

Circle 4, Red on Yellow, develops Orange by absorbing Yellow, Green,
Blue, Violet and Red.

Circle 5, Yellow on Blue, develops Green by absorbing Blue, Violet,
Red, Orange and Yellow.

Circle 6, Blue on Red, develops Violet by absorbing Red, Orange,
Yellow, Green and Blue.

By this method of demonstration the six colours fall naturally into
two groups. The first group includes Red, Yellow, and Blue, whilst the
second group includes Orange, Green, and Violet. The colours of the
second group, Orange, Green, and Violet, are true monochromes, each
being isolated from the light, by the absorption of the five other rays.

These illustrations deal with light and colour of 20 units
intensity;[2] as the intensity of the light here is exactly equal to
the absorptive power of the standards, no free light remains; where
the absorptive power of the colour standards is less than the light,
associated white light remains; for instance, if only one unit of
colour was developed, 19 units associated white light would remain.

[2] For description of the light and colour units, refer to chap. III,
page 9.

[Illustration: PLATE III
  _To face page 13._]      [_Lovibond, Colour Theories._
]

This method of colour development by analytical absorption is further
illustrated by Plate III, showing the effect of superimposition of the
three colours in their several combinations as intercepting a beam of
white light.

Not all lights which appear white to the vision are truly normal
white; colour may be masked by excess of luminosity, and only become
evident when the luminosity has been reduced, by placing neutral tint
standards between the light and the observer. Direct sunlight, and some
artificial lights, are instances. (Law 6 (_a_) page 8.)

On the other hand, an abnormal light may be too low for the vision to
discriminate colour. This may be observed in nature by the gradual loss
of colour in flowers, etc., in the waning intensities of evening light.
The order of their disappearance is shown in Chart I.




CHAPTER V.

Standard White Light.


The colour of a substance is determined by the ray composition of
the light it reflects, or transmits to the vision, the colour would
therefore vary with every change in the ray proportions of the incident
light; it follows that constancy in colour measurement can only be
obtained by a colourless light. Up to the present diffused daylight is
the only light which complies with the condition of ray equality.

The absolute equality of the six spectrum colours may be difficult
to establish in any light, and their constancy in equivalence under
varying light intensities may be open to argument. But, as everyday
work is carried on mainly under daylight conditions, and as the vision
is the final arbiter for colour work, theoretical questions outside the
discriminating power of the vision, need be no bar to the establishment
of a working standard white light; and in saying that diffused daylight
is normal white, it is only intended to mean: In so far as a normal
vision can determine.

Apart from any theoretical explanation it is an experimental fact, that
the abnormal rays of direct sunlight, and some artificial lights, may
be so modified by diffusion as to be available for a limited range of
colour work. In the case of diffused north sunlight, when taken from
opposite the sun’s meridian, the modification is sufficient to make it
available as a standard white light. In the case of artificial lights,
their use is, as yet, limited to visual matching (not recording) and
arbitrary comparisons.


THE BLACK UNIT.

Ideal black is total absence of light, and can only be realized as a
sensation, in the presence of light, which may however be in contrast
or in association.

The nearest approach to ideal black by contrast, is to view a hole in
a box with a blackened interior, so arranged that no light entering
the hole, can be reflected back to the vision: in this way associated
light, if not entirely absent, is reduced to a minimum and total
darkness is practically realized by the vision in contrast with the
surrounding light.

Pigmentary black viewed under diffused daylight conditions is always
associated with white light, as no substance, however black it may
be, absorbs all the impinging light; as examples, the following
measurements of three white and three black pigments were made at an
angle of 45 degrees with a light intensity of 25 units.

This is a true quantitative analysis of the 25 units of white light
after reflection from the black pigments. The black units represent
the proportion of white light absorbed, whilst the beams reflected
from the pigments consist of the colour values developed which are
associated with the unabsorbed white light.


TABLE II.

  ----------------------+--------+------+------+------+-------+------
                        |  Lime  |Blue  |Lamp  |Ivory |Zinc   | White
                        |Sulphate|Black |Black |Black |White  | Lead
  ----------------------+--------+------+------+------+-------+------
  Standard light units  |  25·0  |      |      |      |       |
  ----------------------+        |      |      |      |       |
  Black units           |        |      |      |      |       |
    (light absorbed)    |        |  9·0 |  9·2 |  9·2 |   --  |   ·08
  Violet units          |        |      |      |      |       |
    (colour developed)  |        |  2·2 |  1·4 |  1·4 |   --  |   --
  Blue  units           |        |      |      |      |       |
    (colour developed)  |        |  1·0 |  1·9 |   ·4 |   ·01 |   ·05
  Green units           |        |      |      |      |       |
    (colour developed)  |        |  --  |  --  |  --  |   --  |   ·07
  Associated white light|        | 12·8 | 12·5 | 14·0 | 24·99 | 24·80
  ----------------------+--------+------+------+------+-------+------
      Totals            |  25·0  | 25·0 | 25·0 | 25·0 | 25·0  | 25·0
  ----------------------+--------+------+------+------+-------+------

The analyses demonstrate that black is not itself an active energy
analogous to colour, but is a minus quantity distinguishable by
contrast with the original light. The reflected beam consists of the
colour developed, associated with the residue of unaltered light.

_Note._--Suitable proportions of Violet and Blue give character and
value to black, whilst Orange and Yellow are less pleasing as tending
to rustiness.




CHAPTER VI.

Qualitative Colour Nomenclature.


SIMPLE COLOURS.

The vision can separate six monochromatic colours from a beam of white
light, therefore in practical work six must be dealt with, no matter
how they may be theoretically accounted for. They naturally take the
accepted spectrum names and symbols already in use. To these are added
two other terms, Bk. to signify black, and L. for light; these terms
deal with the brightness, or dinginess, of a colour.


Simple Terms. Symbols.

    Red               R
    Orange            O
    Yellow            Y
    Green             G
    Blue              B
    Violet            V
    Black            Bk
    White             L


COMPLEX COLOURS.

The order of the association of simple colours to form complex, is
governed by two factors. The first is a physiological limitation of
the vision, which is unable to simultaneously distinguish more than two
colours, in the same beam of light, this limits the most complex colour
to two colour names. The second limitation is one of association, based
on the experimental fact, that the particular two must be adjacent in
their spectrum order, spectrum red and violet being considered adjacent
for this purpose. Under these conditions, any given colour must be
either a monochrome, or a bichrome, and all complex colours must be
bichromes. Therefore the only possible combinations are as follows:--

  Red    and Orange
  Orange  "  Yellow
  Yellow  "  Blue
  Blue    "  Green
  Green   "  Violet
  Violet  "  Red

The classified order of associating symbols for describing the
components of the whole range of distinguishable colours is set out in
the following tables:--

  -------------+-------------+------------
   Monochromes | Monochromes |Monochromes
  of a Standard|Brighter than|Duller than
   Brightness. | Standards.  |Standards.
  -------------+-------------+------------
        R.     |    R. L.    |  R. Bk.
        O.     |    O. L.    |  O. Bk.
        Y.     |    Y. L.    |  Y. Bk.
        G.     |    G. L.    |  G. Bk.
        B.     |    B. L.    |  B. Bk.
        V.     |    V. L.    |  V. Bk.
  -------------+-------------+------------

[Illustration]

The separation of the six monochromatic sensations from a point of
white light, and the formation of binary sensations by the combination
of adjacent colours, is graphically illustrated in the above diagram.

In order to make the qualitative symbols quantitative it is only
necessary to add the numerical unit value to each factor as found by
direct experiment.




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




CHAPTER VIII.

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._
]




CHAPTER IX.

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.




CHAPTER X.

Representations of Colour in Space of Three Dimensions.


The relations of the different colours to one another, and to neutral
tint are, perhaps, best represented to the mind by a solid model, or
by reference to three co-ordinate axes, as employed in solid geometry
(_see_ Fig. 2).

[Illustration: Fig. 2.]

Let the three adjacent edges OR, OB, OY, of the above cube be three
axes, along which are measured degrees of Red, Yellow and Blue
respectively, starting from the origin O. Every point in space on
the positive side of this origin will then represent a conceivable
colour, the constituents of which in degrees of red, yellow and blue
are measured by the three co-ordinates of the points. Pure reds lie all
along the axis OR, pure yellows on the axis OY, and pure blues on the
axis OB.

All normal oranges, normal greens, and normal violets lie on the
diagonals of the faces of the cubes OO^1, OG, OV respectively.

Pure neutral tints lie on the diagonal ON of the cube, equally inclined
to the three principal axes.

Red violets will be found on the plane ROB, between OV and OR.

Blue violets on the same plane between OV and OB.

“Saddened” red violets all within the wedge or open space enclosed by
the three planes, whose boundaries are OB, OV, ON.

The other colours, red and yellow oranges, blue and yellow greens, pure
and saddened, are found in corresponding positions in relation to the
other cases.[4]

[4] This method of illustration was suggested by Dr. Herbert Munro.




CHAPTER XI.

The Spectrum in relation to Colour Standardization.


The spectrum has naturally been considered as a suitable source for
colour standards, but the power of analysing has disclosed some
difficulties, which have yet to be overcome.

Concerning the prismatic spectrum, there has always been a difficulty
in apportioning the different colours to specific areas, and further,
before this spectrum is available for colour standardization, some
method of correction for the unequal distribution of colours must be
devised.

Neither of these difficulties occur in the use of the diffraction
spectrum, where the pure colours are apportioned by Professor Rood from
A to H in the manner shown in table on next page.

Professor Rood further divides the spectrum from A to H into 100 equal
divisions, allotting 20 unit divisions of 72,716 wave lengths to the
space between each two colour lines. This allots a space of 3,635 W.L.
to each unit division, as shown in Table III.

TABLE III.

  -----------+-----------------------+-----------+--------+--------
             |                       |   No. of  |        |
             |                       |    Wave   |        |
             |                       |  Lengths  |        | W.L.K.
             | Wave Length Position. |    from   |Division|   per
             |                       |   Colour  |        |Division
             |                       |  between  |        |
             |                       |   each.   |        |
  -----------+-----------------------+-----------+--------+--------
  760,400 A. |Red            760,400 |           |        |
             |-----------------------+   72,717  | == 20  |  3,635
             |Orange         687,683 |           |        |
             |-----------------------+   72,716  | == 20  |  3,635
             |Yellow         614,967 |           |        |
             |-----------------------+   72,716  | == 20  |  3,635
             |Green          542,251 |           |        |
             |-----------------------+   72,716  | == 20  |  3,635
             |Blue           469,535 |           |        |
             |-----------------------+   72,716  | == 20  |  3,635
  396,819 H. |Violet         396,819 |           |        |
  -----------+-----------------------+-----------+--------+--------
  363,581    |Total W.L. between A. & H. 363,581 |   100  |
  -----------+-----------------------------------+--------+--------

Having provided equal wave length positions for the six pure colours,
the intermediate colours are necessarily binaries in definite
proportions, accounted for by a regular overlapping of two bounding
colours in opposite directions from zero to 20, as shown in the
following table from Red to Orange, representing the space between
these two pure colours.

  Red  20 19 18 17 16 15 14 13 12 11 10  9  8  7  6  5  4  3  2  1  0  W.L
  W.L                                                                  687,
  760,  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20   683
   400 -------------------------------------------------------------- Orange.
       20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
       ==============================================================

It follows, that apart from the six monochromes, all spectrum complex
colours in a single wave length must be binaries, whose united values
equal 20.

On comparing Professor Rood’s scales of divisions with those of the
tintometrical scales already described, they appear to coincide in
several particulars, for instance:--

The monochromes correspond both in number and in name.

Their positions in the scales correspond.

Their unit divisions are equal in number, and in dimensions.

Their colour positions correspond, when an artificial tintometrical
spectrum is made by regularly overlapping monochromes.

It follows that when the two scales are superimposed as in Plate V.,
showing similar monochromes as lying in the same perpendicular, the
same wave length numbers apply to both; concerning the dimensions
between the monochromes, the spaces occupied by 72,716 wave lengths
between the spectrum monochromes, also represent similar spaces in the
tintometrical scales, and one-twentieth of this 3,635 represents the
space of a single unit in each case.

In connexion with these co-related dimensions, some information is
obtainable bearing on the limitation of a monochromatic vision for
discriminating small colour differences. Under ordinary daylight
conditions, the unit in the lighter shades of the tintometrical
scale is divided into 100 fractional parts, each fraction therefore
represents a space occupied by thirty-six wave lengths in the spectrum
scale. This may be near the limit of dimension for monochromatic vision
in such a gradually changing colour scale, as that of the spectrum, and
may be some guide as to suitable slit areas in the synthetical building
up of complex coloured light.

[Illustration: PLATE VI SIX COLOUR CHARTS IN ONE OR ANOTHER OF WHICH
ANY SIMPLE OR COMPLEX COLOUR FINDS A DEFINITE POSITION.
  _To face page 39._]      [_Lovibond, Colour Theories._
]

In Plate VI. are shown the six tintometrical colour charts, as lying
in their order on the tintometrical spectrum, illustrating that any
measured colour factor lies in a perpendicular drawn through both
spectra, and occupying the same wave length position, and may therefore
be designated by that wave length number.

This explanation is not intended to convey that the colour energies
do not really overlap beyond the boundaries of the dual combinations,
but only that the vision is unable to distinguish as colour, such
overlapping if it exists.


POINTS OF DIFFERENCE.

On further comparisons of the two scales there are some points of
difference which have a bearing on their values as colour standards.

There is a variation in the length of the two scales, the spectrum
terminating at H, whilst the tintometrical scale is extended to a
sixth division in the region of the ultra violet, showing overlapping
combinations of Red and Violet, strictly analogous to the overlapping
binaries in the other five sections.

These red and violet combinations constitute one-sixth of the cycle of
distinguishable colours, and cannot be omitted in any system of colour
standardization, therefore their absence in the continuous spectrum is
a drawback.

A second drawback, is the limited number of spectrum complex colours,
in consequence of each colour being blended only with overlapping
colour value, which lies in its own wave length, whereas in nature
each colour may be blended with any value of the overlapping colour. In
the tintometrical standards, similar effects are obtained by changing
the value of the graded slips.

It is true, that complex colours other than those in the same wave
length, may be developed by blending two colours from different parts
of the spectrum, but the ray proportions of colours so produced, are
necessarily more complex than those developed by specific absorption;
the first being a method of synthesis towards complexity, and the
second a method of analysis towards simplicity, and although two
colours so produced may be similar in name, red for instance, they
must differ in character. This view may tend to reconcile some of the
theoretical differences between Scientists and Artists.


THE ULTRA VIOLET DIVISION.

The complete range of daylight colours not being fully comprised in a
continuous spectrum, may be considered as a cycle of radiant energies,
sensitive to the vision as colour, which can be represented as a
circle as in Plate VII. The outer and broken circle represents a bent
spectrum, the unoccupied division corresponding in position with that
of the red and violet mixtures in the complete cycle.

This arrangement does not alter the relative positions of the
Fraunhoper lines A, B, C, D, E, F, G and H in reference to either
scale, but, it theoretically breaks that sequence of the successive
wave lengths in the Red Violet which holds good in the other five
divisions from A to H.

[Illustration: PLATE VII
  _To face page 40._]      [_Lovibond, Colour Theories._
]

In order to theoretically avoid this juxtaposition of wave length
contrast, it is only necessary to imagine that the violet energy beyond
H in the ultra violet, is overlapped by the infra red energy of a
succeeding spectrum, filling this section with a series of overlapping
binaries analogous in wave length sequence to that of the other
sections.


A RESIDUAL RED RAY.

Apart from the colours of everyday life there is, in sunlight and
most direct artificial lights, an additional red energy which differs
materially from the red energy in diffused daylight.

It was first noticed whilst establishing the colour equivalence of the
tintometrical light unit, by developing a red sensation which disturbed
constancy of reading under certain conditions of light.

So far as the writer knows, this energy has never been investigated as
separate from the other spectrum red. The following observations must
be considered as tentative only.


SOME PROPERTIES.

It does not obey the laws of absorption which govern the red of
diffused daylight. When the six transparent pigmentary colours are
illuminated by direct sunlight, and viewed through a sufficient number
of Neutral Tint units, the colours all disappear, all appearing red
alike, with only differences in luminosity.

The spectrum position of this red energy is in the A. B. region, and
further interception by Neutral Tint whilst narrowing the band,
intensifies the colour, until obstructed by the large number of
intercepting glass surfaces.

It has no photographic action on the six sensitised papers dealt with
in the photographic section.


LIGHT INTENSITIES.

The apparatus for determining the unit values of light intensities
in the following series of measurements, consisted of a conical
rectangular hopper tapering from 2 feet to 2 inches square. This was
adapted so that the light from the small end, commanded the stage of
the optical instrument sufficiently close to cut off outside light.
The wide end facing a north sky was adapted with sliding shutters, to
regulate the area of incident light; of the six water-colour pigments
which nearest corresponded to the standard colours, washed to their
full depth on Whatman’s paper, six measurements were made. These
measurements are shown in Table IV, and classified in Table V.

It will be noted that the readings are constant for all the colours
between 16 and 26 units, except a variation of light ·15 in the 24-inch
opening, which is in effect as if the cone was not present, and ·2 in
the 8-inch area of orange.

    Note.--Experiments in this branch give some information
    relating to the perception of colour under daylight conditions,
    by limiting the range of intensities within which colour can
    be distinguished and differentiated, whilst their separate
    photographic action (page 48) suggests the impression that
    colour phenomena, outside these limits, may be a physiological
    expression of widely varying underlying energies.


TABLE IV.

  -------------+---------+----------+--------+--------+---------+-------
               | Square  |          |        |        |         |
  Pigment.     | Inches  | Light    | Black. |  Red.  | Orange. |
               |Aperture.|Intensity.|        |        |         |
  -------------+---------+----------+--------+--------+---------+-------
  Carmine      |      2  |     10   |   ·5   |   20·2 |    ·3   | ----
    "          |      4  |     11   |   ·5   |   19·1 |    ·4   | ----
    "          |      6  |     14   |   ·46  |   18·95|    ·59  | ----
    "          |      8  |     16   |  ----  |   16·9 |   1·1   | ----
    "          |     10  |     20   |  ----  |   16·9 |   1·1   | ----
    "          |     12  |     22   |  ----  |   16·9 |   1·1   | ----
    "          |    Open |     26   |  ----  |   16·9 |   1·1   | ----
  -------------+---------+----------+--------+--------+---------+-------
               |         |          |        | Yellow |         |
  Lemon Yellow |      2  |     10   |  ----  |    6·9 |    ·1   | ----
    "     "    |      4  |     11   |  ----  |    6·9 |    ·1   | ----
    "     "    |      6  |     14   |  ----  |    6·9 |    ·1   | ----
    "     "    |      8  |     16   |  ----  |    7·0 |   ----  | ----
    "     "    |     10  |     20   |  ----  |    7·0 |   ----  | ----
    "     "    |     12  |     22   |  ----  |    7·0 |   ----  | ----
    "     "    |    Open |     26   |  ----  |    7·0 |   ----  | ----
  -------------+---------+----------+--------+--------+---------+-------
               |         |          |        |  Blue. | Violet. |
  Cobalt Blue  |      2  |     10   |  ----  |   11·5 |   ----  | ----
    "     "    |      4  |     11   |  ----  |   11·5 |   ----  | ----
               |         |          |        |        |  Green. |
    "     "    |      6  |     14   |  ----  |   10·7 |    ·2   | ----
               |         |          |        |        |  Violet.|
    "     "    |      8  |     16   |  ----  |   10·5 |    ·5   | ----
    "     "    |     10  |     20   |  ----  |   10·5 |    ·5   | ----
    "     "    |     12  |     22   |  ----  |   10·5 |    ·5   | ----
    "     "    |    Open |     26   |  ----  |   10·5 |    ·5   | ----
  -------------+---------+----------+--------+--------+---------+-------
               |         |          |        |   Red. | Orange. | Light.
  Chrome Orange|      2  |     10   |  ----  |    3·4 |   6·0   | ----
    "      "   |      4  |     11   |  ----  |    3·4 |   6·0   | ----
    "      "   |      6  |     14   |  ----  |    3·0 |   6·2   | ----
    "      "   |      8  |     16   |  ----  |    3·0 |   6·0   |  ·05
    "      "   |     10  |     20   |  ----  |    3·2 |   6·0   |  ·05
    "      "   |     12  |     22   |  ----  |    3·2 |   6·0   |  ·05
    "      "   |    Open |     26   |  ----  |    3·2 |   6·0   | ----
  -------------+---------+----------+--------+--------+---------+-------
               |         |          |        |Yellow. |  Green. |
  Emerald Green|      2  |     10   |  ----  |   ·2   |    6·4  |  ·05
    "       "  |      4  |     11   |  ----  |  ----  |    6·4  |  ·05
    "       "  |      6  |     14   |  ----  |  ----  |    6·4  |  ·05
    "       "  |      8  |     16   |  ----  |  ----  |    6·6  | 2·0
    "       "  |     10  |     20   |  ----  |  ----  |    6·6  | 2·0
    "       "  |     12  |     22   |  ----  |  ----  |    6·6  | 2·0
    "       "  |    Open |     26   |  ----  |  ----  |    6·6  |  ·05
  -------------+---------+----------+--------+--------+---------+-------
               |         |          |        |  Red.  | Violet. |
  Mauve        |      2  |     10   |  ----  |  3·0   |    7·4  | ----
    "          |      4  |     11   |  ----  |  3·0   |    7·4  | ----
    "          |      6  |     14   |  ----  |  3·0   |    7·2  | ----
    "          |      8  |     16   |  ----  |  2·8   |    7·2  | ----
    "          |     10  |     20   |  ----  |  2·8   |    7·2  | ----
    "          |     12  |     22   |  ----  |  2·8   |    7·2  | ----
    "          |    Open |     26   |  ----  |  2·8   |    7·2  | ----
  -------------+---------+----------+--------+--------+---------+-------


TABLE V.

COLOURED SURFACES

Table of Varying Luminous Intensities

  -------+------+----------------+----------+------------------
         |      |      Red.      |  Yellow. |     Blue.
         |      +----------------+----------+------------------
  Inches |Light |   “Carmine.”   | “Lemon.” |   “Cobalt.”
  Square.|Units.|                |          |
         |      +-----+-----+----+----+-----+-----+-----+------
         |      |  R. | Or. |Blk.| Y. | Or. | B.  | Vi. | Blk.
  -------+------+-----+-----+----+----+-----+-----+-----+------
       2 |  10  |20·2 |  ·3 | ·5 | 6·9|  ·1 |11·5 | ----| ----
       4 |  14  |19·1 |  ·4 | ·5 | 6·9|  ·1 |11·5 | ----| ----
       6 |  14  |18·95|  ·59| ·46| 6·9|  ·1 |10·7 |  ·2 |  ·1
       8 |  16  |16·9 | 1·1 |----| 7·0| ----|10·5 |  ·5 | ----
      10 |  20  |16·9 | 1·1 |----| 7·0| ----|10·5 |  ·5 | ----
      12 |  22  |16·9 | 1·1 |----| 7·0| ----|10·5 |  ·5 | ----
    Open |      |     |     |    |    |     |     |     |
      24 |  26  |16·9 | 1·1 |----| 7·0| ----|10·5 |  ·5 | ----
  -------+------+----------------+----------+------------------
  -------+------+----------------+----------------+------------
         |      |     Orange.    |     Green.     |  Violet.
         |      +----------------+----------------+------------
  Inches |Light |    “Chrome.”   |   “Emerald.”   |“Fr. Mauve.”
  Square.|Units.|                |                |
         |      +----+----+------+----+----+------+-----+------
         |      |Or. | R. |Light.| Gr.| Y. |Light.| Vi. | R.
  -------+------+----+----+------+----+----+------+-----+------
       2 |  10  | 6· | 3·4|  ·05 | 6·4| ·2 |  ·05 | 7·4 | 3·
       4 |  14  | 6· | 3·4|  ·05 | 6·4| ·2 |  ·05 | 7·4 | 3·
       6 |  14  | 6·2| 3· |  ·05 | 6·4|----|  ·05 | 7·2 | 3·
       8 |  16  | 6· | 3· | ---- | 6·6|----|  ·2  | 7·2 | 2·8
      10 |  20  | 6· | 3·2| ---- | 6·6|----|  ·2  | 7·2 | 2·8
      12 |  22  | 6· | 3·2| ---- | 6·6|----|  ·2  | 7·2 | 2·8
    Open |      |    |    |      |    |    |      |     |
      24 |  26  | 6· | 3·2| ---- | 6·6|----|  ·05 | 7·2 | 2·8
  -------+------+----------------+----------------+------------




CHAPTER XII.

The Physiological Light Unit.


DIFFUSED LIGHT.

The physiological values of light intensities determined by the
absorptive method, differ in some respects, from the intensities based
on the inverse ratio of the squares of distance between the shadows of
two lights.

However valuable this method of calculating light intensities may
be from a mathematical point of view, it does not express the
physiological appreciation of light differences.

The dimensions of the light unit used in the following experiments have
already been described.

This method cannot deal exhaustively with intense direct lights, on
account of the presence (activity) of the disturbing red ray which
prevents total absorption. Such lights must first be modified by
intercepting media of known diffusive value, or by reflection from a
white surface.

One object of these experiments was to obtain more insight into the
physiological conditions of light, as bearing on the question of
standardization.

The first two experiments are records of intensity changes by time,
in Morning and Evening light, and are of interest, as bearing on the
lowest luminosities for reading, for viewing objects at different
distances, and for defining the limits at which colours visually
disappear. The measurements are marked in neutral tint units and
plotted in curves in Charts 1 and 2.

The numbers on the single curve in the Morning chart represent total
absorption of the direct light 20 degrees above the horizon. The upper
curve in the Evening chart also represents the direct light, whilst
the under curve represents the light values as reflected from a lime
sulphate surface; except in the case of the reading notes when it
represents the printed paper surface. The difference between the two
curves is the loss of light incident on reflection, but this must not
be rigidly interpreted for all cases, as there is reason for supposing
it varies with different lights.


DIRECT LIGHTS.

In measuring the physiological intensity of direct lights, the presence
(activity) of the unabsorbable red ray, prevents their being dealt
with directly by the absorptive method. Such lights can, however, be
made measurable by a sufficient diffusion, as already explained in the
case of direct sunlight, the proportion of diffusion required, being
more or less according to the intensity of the light; in the following
examples, one reflection from a white surface or from the ordinary
grease spot arrangement, was found to be sufficient.

    Note.--Light reflected from grease is not above suspicion, it
    being governed by the law of specific absorption already dealt
    with.

[Illustration: Chart 1.

Evening Light March 18^{th}. 1906.

  _Upper Curve 20° above Horizon
  Lower Curve as Reflected from a
  1 Foot Square Lime Sulphate Screen_
]

[Illustration: Chart 2.

Morning Light Dec^r. 17^{th}. 1907.

_20° above Horizon_]

[Illustration: Chart 3.

Luminous Intensities of _1, 2 & 3 Standard Candles_.]

[Illustration: Chart 4.

Luminosity of Gas from Burner

_Reduced to Equal 1, 2 & 3 Candles at 1 Foot._]

[Illustration: Chart 5.

Luminosities of Paraffin Lamp

_Reduced to 1, 2 & 3 Candles at 1 Foot._]

The experiments were carried out on a home-made twelve-foot photometer,
with the usual protected lantern at each end, one being removed for
present purposes. The grease spot carrier was replaced by a one-foot
square diaphragm, with a standard white surface; this travelled the
whole length of the photometer at right angles to the light, and the
readings made at one-foot intervals at an angle of 45 degrees, 10
inches distance from the white surface.

Charts 3, 4 and 5 deal with experiments, with one, two and three
standard candles of the London Gas Referees. The curves represent
the physiological rate of declining luminosities by distance. Some
characteristic differences from other lights are brought together in
Charts 4 and 5. The slight irregularities in the curves are probably
due to the readings being made at half unit intervals. These acting
sometimes in opposite directions, fully account for want of symmetry in
the curves.

A noticeable feature in these experiments is the small amount of
physiological luminosity added by each successive candle to the
first. Theoretically, if one candle equals 21 units intensity, two
should equal 42, and three 63, whereas the physiological additions of
luminosity are not only much less, but vary with different luminous
intensities, as will be seen by the following comparisons:--

               1        2        3
             Candle  Candles  Candles  Total.

  Theoretical   21  +  21    +  21    =  63

  Physiological 21  +   3    +   6    =  30

Chart 4 represents the Luminosity of gas from a batswing burner
reduced to one, two and three candle power at one foot distance.

               1        2        3
             Candle  Candles  Candles  Total.

  Theoretical   21  +  21    +  21    =  63

  Physiological 21  +   3    +   7    =  31

Chart 5 represents the Luminosity of a hand Paraffin Lamp similarly
reduced.


               1        2        3
             Candle  Candles  Candles  Total.

  Theoretical   21  +  21    +  21    =  63

  Physiological 21  +   3    +   6    =  30


PHOTOGRAPHIC ENERGIES OF DIFFERENT RAYS

The following experiments are preliminary only, and were undertaken to
determine the relationship of the several colours of the tintometrical
scales, to their associated photographic values, under different
conditions of light and times of exposure, the end in view being the
hope of standardizing screens, papers and impressions.

The standardized colours act as selective light absorbents on the same
principle as screens for trichromatic colour work, but differ from
these in their having a definite standard of colour depth and colour
purity.

Work of this character requires paper of a known degree of
sensitiveness, but on inquiry it was found that no reliable standard
had as yet been established. On this, six makes of “white” paper were
purchased in the open market and submitted to exposures under the
following conditions:--

Six slips of the sensitized papers were covered by a thin metal plate,
pierced by six rows of apertures, a complete row lying over each
sample of paper. The rows contained seven apertures each, one being
left uncovered to receive the full energy of the impinging light. The
remaining six were covered respectively by a Red, Orange, Yellow,
Green, Blue or Violet standardized screen of 15 units colour intensity;
the whole was fitted into a suitable exposure frame. Many exposures
were made, from which the four following were selected:--

  No. 1. 20 min. exposure to a dull sky.
   "  2. 10  "      "     " bright sunlight.
   "  3. 20  "      "     "          "
   "  4. 30  "      "     "          "

The results are arranged in Tables VI and VII.

Table VII contains results of different exposures under sunlight
conditions.


TABLE VI.

DULL SOUTH LIGHT.

20 Minutes’ Exposure on White Sensitized Papers.

  -----+-----------------+-----------------+------------------
   Pap-|                 |                 |
   ers |   OPEN SCREEN.  |   RED SCREEN.   |  ORANGE SCREEN.
       |                 |                 |
  -----+-----------------+-----------------+------------------
       | Blk.  Or.  Red. | Blk.  Or.  Yel. | Blk.  Or.  Red.
    1  | 7·8   4·2  6·0  | 2·0   4·8  3·0  | ----   ·3   ·2
       |                 |            Red. |
    2  | 6·0   5·0  5·5  | 1·9   3·1   ·4  | ---- ----  ----
       |                 |            Yel. |
    3  | 7·8   3·2  7·0  | 2·7   4·7  3·0  | ---- ----  ----
       |                 |            Red. |
    4  | 7·2   4·3  6·0  | 2·2   3·2   ·2  | ---- ----  ----
    5  | 5·8   7·2  4·0  | 1·9   2·3   ·8  | ---- ----  ----
    6  | 5·6   7·4  4·0  | 1·8   2·6    6  | ---- ----  ----
  -----+-----------------+-----------------+------------------

  -----+-----------------+-----------------+-----------------+------------------
   Pap-|                 |                 |                 |
   ers | YELLOW SCREEN.  |  GREEN SCREEN.  |  BLUE SCREEN.   |   VIOLET SCREEN.
       |                 |                 |                 |
  -----+-----------------+-----------------+-----------------+------------------
       | Blk.  Or.  Red. | Blk.  Or.  Red. | Blk.  Or.  Red. | Blk.  Or.  Red.
    1  | 2·0   3·0  1·6  | ----  ---- ---- |  3·9  7·6   3·5 |  ·2    ·4   1·0
       |                 |                 |            Yel. |
    2  |  ·4    ·4  1·0  | ----  ---- ---- |  4·3  9·2   4·5 |  ·2    ·8    ·8
       |                 |                 |            Red. |
    3  | 1·2    ·5   ·8  | ----  ---- ---- |  3·7  9·3   2·0 |  ·7    ·6    ·9
       |                 |                 |            Yel. |
    4  | 1·2    ·1  1·2  | ---- ----  ---- |  3·4  1·1   1·0 |  ·7    ·7    ·9
    5  | 1·0    ·3   ·3  | ---- ----  ---- |  3·3  8·7   2·0 |  ·3    ·3   1·2
       |                 |                 |            Red. |
    6  |  ·2    ·3   ·4  | ---- ----  ---- |  3·6  8·9   1·5 |  ·4    ·2    ·9
  -----+-----------------+-----------------+-----------------+------------------


TABLE VII.

BRIGHT SUNLIGHT (South), 18--9--12.

10 Minutes’ Exposure on White Sensitized Papers.

  ---+-------------+---- --------+--------------+---------------
  NO.| OPEN SCREEN.| RED SCREEN. |ORANGE SCREEN.|YELLOW SCREEN.
  ---+-------------+-------------+--------------+---------------
     |Blk. Or. Red.|Blk. Or. Yel.|Blk.  Or. Red.|Blk.  Or. Red.
   1 | 8·4 1·6  6·5|2·8  6·8  1·9| ·34 1·41  ·55| 2·3  7·1 --
   2 | 8·2 2·0  6·3|2·7  4·7  3·6|--   --   --  |  ·8  1·9  ·70
   3 | 7·8 2·2  7·5|2·4  7·0  4·1|--   --   --  | 2·4  4·2  ·80
   4 | 7·8 1·6  8·6|2·4  5·4  3·2|--   --   --  | 1·5  2·3  ·70
     |             |             |              |
   5 | 6·4 4·4  7·7|2·4  5·2  3·0|--   --   --  |  ·9  1·8  ·80
   6 | 5·2 6·3  5·5|2·3  5·1  3·0|--   --   --  | 1·1  2·0  ·80

BRIGHT SUNLIGHT (South), 18--9--12.

20 Minutes.

     |    Vio. Red.|     Or. Red.|      Or. Yel.|          Red.
  1  |10·6 1·4  5·0|7·0  2·2  8·3|1·1  3·6   ·9 |10·0  --   7·0
     |             |             |          Red.|      Or.
  2  |10·0 --    ·8|7·0  2·6  7·4|--   1·0   ·3 | 7·4  2·0  3·6
     |     Or.     |             |              |
  3  | 9·2 1·6  2·2|4·5  5·5  8·0|1·8  7·4  --  | 5·2  4·2  8·1
     |     Or.     |             |              |
  4  |10·6  ·4  3·5|6·0  4·2  8·3| ·44 1·36  ·8 | 9·0  2·5  5·0
  5  |10·0  ·8  5·7|6·2  4·0  8·3|--    ·7   ·45| 7·0  5·0  4·0
     |             |             |              |
  6  | 6·2 4·6  7·7|5·0  7·5  4·0| ·2   ·4   ·5 | 5·4  7·1  4·5

BRIGHT SUNLIGHT (South), 18--9--12.

30 Minutes.

     |     Or. Red.|     Or. Red.|      Or. Red.|      Or. Red.
   1 |10·6  ·9  1·5|6·4  4·4  5·7|2·6  7·8  1·1 | 5·2  3·4 10·4
   2 | 9·2 1·4  2·9|6·2  5·3  5·5| ·1   ·7   ·4 | 7·8  3·0  5·2
   3 |10·8 1·2  1·5|7·2  3·4  8·4| ·7  2·0   ·2 | 9·2  1·6  5·2
   4 |10·0 2·0  1·5|6·2  4·4  6·9| ·5  1·0   ·7 | 8·2  2·0  7·3
     |             |             |              |
   5 | 9·6 4·4  4·5|5·8  5·0  7·2| ·2  1·9   ·7 | 6·2  3·8  6·5
   6 | 6·6 7·9  5·0|4·8  7·2  5·0| ·2   ·5   ·55| 5·0  7·5  2·0
  ---+-------------+-------------+--------------+---------------

BRIGHT SUNLIGHT (South), 18--9--12.

10 Minutes’ Exposure on White Sensitized Papers.

  ---+-------------+-------------+---------------
  NO.|GREEN SCREEN.| BLUE SCREEN.|VIOLET SCREEN.
  ---+-------------+-------------+---------------
     |Blk. Or. Red.|Blk. Or. Red.|Blk.  Or.  Yel.
   1 | ·26 ·69  ·10|5·2  4·4 8·4 |1·75  3·65  ·40
   2 |--   ·15  ·03|6·0  3·8 6·7 |1·7   3·9   ·40
   3 | ·22 ·02  ·04|5·6  5·9 6·5 |1·5   3·5  1·2
   4 | ·19 ·05  ·12|4·8  5·6 6·6 |1·3   3·2  --
     |             |             |           Red.
   5 |--   --   -- |4·0 10·5 2·0 | ·6   1·7  1·0
   6 |--   --   -- |4·0 11·5 1·0 | ·5   1·9  1·1

BRIGHT SUNLIGHT (South), 18--9--12.

20 Minutes.

    |     Or. Yel.|     Or. Red.|      Or.  Red.
  1 |3·1  6·1 1·8 |8·0  2·0 8·5 |4·5   7·5  4·5
    |             |     Or.     |
  2 |1·9  3·7  ·8 |8·8   ·6 3·6 |5·0   6·5  4·5
    |             |             |
  3 |1·0  7·0 1·6 |8·6  1·4 3·5 |4·0   8·5  4·5
    |             |             |
  4 |1·8  4·8 1·2 |8·0  2·2 6·8 |4·5   7·5  4·5
  5 |1·7  4·9 1·4 |7·6  2·4 8·0 |4·0   8·5  4·0
    |         Red.|             |
  6 |1·8  3·4  ·4 |6·4  4·4 8·2 |3·5   8·5  1·0

BRIGHT SUNLIGHT (South), 18--9--12.

30 Minutes.

     |    Or.  Yel.|     Or. Red.|      Or. Red.
   1 |1·0  6·8  ·2 |6·4  2·6 7·5 |4·2   6·8  6·0
   2 |1·8  3·4  ·2 |8·4   ·8 3·8 |5·4   8·1  1·0
   3 |2·2  5·2 2·2 |8·0  2·0 7·5 |4·5   8·5  4·0
   4 |2·5  4·7 2·0 |8·0  1·6 7·4 |3·7  10·3  1·0
     |             |             |          Yel.
   5 |1·6  4·2 2·8 |7·6  2·6 6·8 |3·1   9·4  1·5
   6 |1·5  3·9  ·2 |6·4  7·6 5·5 |3·4   9·7  1·5
  ---+-------------+-------------+---------------

It would be unsafe to draw definite conclusions from a few experiments,
but so far as permissible, the results show considerable differences,
both in depth and in colour, of the energy of the different rays, for
instance--

Compare Nos. 1 and 6 under 20 min. sunlight.

          Black.  Orange.   Red.

  No. 1   10·6  +  1·4   +  5·0
  No. 6    6·2  +  4·6   +  7·7

or again 3 and 6, the maximum and minimum, under 30 min. exposure.

         Black.  Orange.   Red.

  No. 3   10·8  +  1·2  +  1·5
  No. 6    6·6  +  7·9  +  5·0

The sensitiveness of Nos. 2, 4 and 5 appears to have been exhausted by
20 min. exposure to sunlight, further exposure showing no reaction;
whilst the sensitiveness of Nos. 1, 3 and 6 do not appear to have been
exhausted by 30 min. sunlight exposure.

Other noticeable points are the small action under the Orange, Green
and Violet screens, and the greater, although variable proportion of
colour to black under all the colour screens.


TRICHROMATIC COLOUR SCREENS.

Table VIII contains the colour measurements of five sets of screens for
trichromatic colour work which have come from time to time under the
writer’s notice.

The measurements are the tintometrical colour units required to match
the screens under daylight conditions, and are classified under the
theoretically accepted terms of Red, Green and Violet.


TABLE VIII.

  ----+-------------------------+-------------------------+--------------------
  Set.|       RED SCREENS.      |      GREEN SCREENS.     | VIOLET SCREENS.
  ----+-------------------------+-------------------------+--------------------
   No.| Red.  Or.   Vi.  Light. | Yel. Green. Or.  Light. | Blue. Green. Black.
    1 |119·0  ----  ----   1·6  | 27·0  36·0  ----   9·0  |  32·0   1·4   6·6
    2 | 24·2  +·8   ----  ----  |  8·9  ----    ·1  ----  |  13·0  ----   3·1
    3 | 26·4  ----   ·8   ----  |  7·5  ----   1·4  ----  |  16·6  ----   3·2
    4 | 22·6  ----  ----  ----  |  7·6  ----   1·4  ----  |  15·3  ----   3·1
    5 | 21·0  ----  9·0   ----  | 28·0  14·0  ----  ----  |  28·0  ----   2·8
  ----+-------------------------+---------------------------+--------------------

The Red screens are all practically pure except No. 5, which transmits
also 30 per cent. Violet. No. 1 is distinguished by its greater colour
depth and purity, the degree of which latter is recorded at 1·6 light
units brighter than standard.

In the Green division, only Nos. 1 and 5 transmit any green, No. 1
transmitting also 42·8 per cent. yellow and being 9·0 units brighter
than standards; No. 4 transmits 66·6 per cent. yellow to 33·3 per cent.
green; Nos. 2, 3 and 4 are all yellows tinged with orange.

In the Violet division, Nos. 2, 3, 4 and 5 are all pure blues; No. 1 is
tinged with green.




Appendix I

COLOUR EDUCATION


The time has passed when it might have been considered necessary to
preface a handbook on the teaching of colour by arguments to prove that
it is a legitimate subject of instruction in schools, but it has not
hitherto been sufficiently recognized that the early stages of such
instruction must be on sound lines and that nothing must be taught
which will afterwards have to be unlearned.

Apart from the pleasure its sensations give to all properly constituted
persons, the study of colour has an intellectual value in common with
other branches of science. It strengthens the judgment by constantly
requiring thought and precision in definition, it also develops the
faculty of colour perception even to the point of curing some forms of
colour blindness. In addition to this, it forms a necessary part of
the instruction in all schools in which drawing is properly taught by
methods which demand from the pupils faithful representations of the
appearances of actual objects in colour as they are seen.

In the past the systematic study of colour has been more or less
neglected from two principal causes: first, the want of a comprehensive
scheme of colour nomenclature capable of describing all colours in
terms precise enough for general understanding and record; and, second,
the absence of any reliable means of reproducing any specific colour
if lost or faded. Both these conditions may now be secured by the use
of the standardized coloured glasses supplied with the Colour Educator,
and this work is intended to bring the subject before teachers in such
a way as to make each point perfectly easy of demonstration to a class
in a systematic manner.

To-day the value and importance of a keen perception of colour and of
an apparatus furnishing definite colour standards, though perhaps not
much appreciated by the general public, are widely recognized in the
industrial and scientific world; and it is evident that in these days
of keen commercial competition between nations we cannot afford to
neglect any means which will enable us to maintain present industries
and to develop new ones.

=General Remarks.=--It is not advisable to introduce colour theories to
pupils before they know the names of the different colours, but, as the
glasses used in the apparatus are graded for colour-depth according to
a set of scales now generally accepted as of standard value, a short
description of the derivation of the colour names will be of service
when the pupils are sufficiently advanced.

The names of the six spectrum colours, Red, Orange, Yellow, Green,
Blue, and Violet, are accepted by common consent as describing the
principal hues into which a beam of white light can be resolved by
a diffraction grating or by prismatic refraction. They are also the
colours distinguishable in objects of everyday life, and the following
Educational Method is based on the fact that they can be separated at
will from ordinary daylight. Therefore the first educational step is to
associate these six colour rays with their respective names, the pupils
being made to understand that there are many degrees of depth in each
colour.

=The Applications of Colour to the Work of Everyday Life= are so
universal that a complete list is almost impossible, though some of
the most important are mentioned below. In a general way the visual
characteristics of every visible object are determined by contrasts of
light and colour, outline itself being governed by differences of light
intensity.

In Nature, colour is practically universal. There are few objects
perfectly white. Most of them have colour of greater or less
complexity; even snow under a cloudless sky has a blue tint which
is measurable against such white objects as pure lime sulphate,
zinc white, etc., the blue tint being manifestly reflected from the
cloudless sky, as it disappears under a cloudy overcast.

=Some of the Scientific Applications of Colour.=--It associates colour
sensations with definite names and values; discovers and classifies
cases of colour-blindness, and is a preparation for the physical
study of light. It is also essential for studying the physiological
structure of the organs of vision, for disclosing abnormal conditions
of the blood, and for measuring the colour of the hair and skin for
the anthropological classification of races. It is used in general
laboratory work for analytical and original investigation; and
it furnishes standards of value for the petroleum industry, the
International Tanning Association, the inter-States Cotton Seed Oil
Association, etc. It is also one of the principal factors in all
artistic industries; for, besides having an important educational
value in questions of harmony, contrast, and taste, it is of direct
commercial value in such industries as dyeing, calico printing,
all woollen industries, wall-paper printing, paint making, house
decoration, etc.

=General Instructions for Using the Apparatus.=--The apparatus consists
of a frame having six little windows, fitted with sliding shutters, and
a tray containing eighteen standardized glasses.

The frame must be placed on a table or stand facing the children, with
a window or some other source of diffused white light behind it. Care
must be taken not to have a coloured background of any kind.

The glasses are in three colours, Red, Yellow, and Blue, of different
depths. The depth of colour is marked in figures on each glass, and
corresponding numbers in the three colours are of equal intensity. It
is of importance that the six spectrum colour terms should be the only
ones used in the preliminary stages. The first step in colour teaching
must be to develop and train the perceptive faculties of the children
so as to enable them to express in words the colour sensations which
are excited. For this purpose it is necessary to begin by demonstrating
that the six spectrum colours Red, Orange, Yellow, Green, Blue, and
Violet, are derived from white light.

Red, Yellow, and Blue should first be dealt with, and _for practical
work_ each pupil should be supplied with three water-colour pigments
closely corresponding in hue to the standardized glasses, viz., Crimson
Lake, Lemon Yellow, and Cobalt Blue; also with a piece of white paper
ruled into six small rectangular spaces corresponding to the little
windows of the apparatus.

As each colour is demonstrated by means of the glass in the apparatus,
each pupil should paint the corresponding pigment in its proper place
on the paper.

The little shutters being placed in all the six little windows, remove
the top left-hand shutter and insert a deep _Red_ glass, thus showing

=Red.=--The pupils will name this and then paint the corresponding
colour on their papers.

Next, remove the shutter below the first one, and insert a _Yellow_
glass of the same numerical value as the Red one, thus showing

=Yellow.=--The pupils will name this and then paint the corresponding
colour on their papers.

Now remove the shutter next below and insert a _Blue_ glass of the same
numerical value as the Red and Yellow ones, thus showing

=Blue.=--The pupils will name this and then paint the corresponding
colour on their papers.

            +--------+--------+
            | 1      |        |
            |        |        |
            |  RED   |        |
            |        |        |
            +--------+--------+
            | 2      |        |
            |        |        |
            | YELLOW |        |
            |        |        |
            +--------+--------+
            |3       |        |
            |        |        |
            | BLUE   |        |
            |        |        |
            +--------+--------+

There are now exposed to view the three colours which are by artists
commonly called primaries, but it will be found convenient to term Red,
Yellow, and Blue the _Dominant_ colours of this system.

The second step is to show how the three secondary or subordinate
colours are derived or developed.

Remove the top right-hand shutter and insert a deep Red and a deep
Yellow glass of equal depth, showing the pupils that these two colours
combined in equal proportions develop

=Orange.=--The pupils will name this, and will then mix their Red and
Yellow pigments to obtain a similar Orange which will be painted in its
proper place on their papers.

Now remove the shutter next below and insert a deep Yellow and a deep
Blue glass of equal depth, showing the pupils that these two colours
combined in equal proportions develop

=Green.=--The pupils will name this, and will then mix their Blue and
Yellow pigments to obtain a similar Green which will be painted in its
proper place on their papers.

            +----------+----------+
            | 1        | 4        |
            |          |          |
            |   RED    |   ORANGE |
            |          |          |
            +----------+----------+
            | 2        | 5        |
            |          |          |
            |  YELLOW  |  GREEN   |
            |          |          |
            +----------+----------+
            |3         | 6        |
            |          |          |
            |  BLUE    |  VIOLET  |
            |          |          |
            +----------+----------+

Remove the last right-hand shutter and insert a deep Red and a deep
Blue glass of equal depth, showing that these two colours combined in
equal proportions develop

=Violet.=--The pupils will name this, and mix their Blue and Red
pigments to obtain a similar Violet, which will also be painted in its
proper place on their papers.

Now are exposed to view on the left-hand side the three primary or
dominant colours, and on the right-hand side the three secondary
or subordinate colours, and the whole frame is filled with the six
spectrum colours in equal colour depth. Corresponding to the colours
in the frame, each pupil’s paper should show a similar arrangement of
colours, and the pupils can be taught their spectrum order by reading
them in rows horizontally--Red, Orange, Yellow, Green, Blue, and Violet.

The teacher should now take out the coloured glasses and replace the
shutters, except the two top windows, one of which is left open to
show white light, and the other filled with three equally deep glasses
in Red, Yellow, and Blue, showing either black or neutral grey, and
demonstrating the total or partial absorption of light according
to their higher or lower numerical unit value. It is of the utmost
importance to bear in mind that the glasses are graded for diffused
daylight, and that all artificial lights are more or less coloured and
would give a different effect. The same remark applies to light taken
direct from coloured objects.

In this set the six windows are in one horizontal line, and should be
uncovered in the following order:

                 { No. 1 Window for Red.
    Dominants    {  "  3   "        Yellow.
                 {  "  5   "        Blue.
                 {  "  2   "        Orange.
    Subordinates {  "  4   "        Green.
                 {  "  6   "        Violet.

One advantage in this method is that when all the colours are in they
are arranged in their spectrum order.

=Complex Colours.=--We have demonstrated that single standard glasses
develop the three Dominant colours, Red, Yellow, and Blue, and that
pairs of equal standard glasses develop the three Subordinate colours,
Orange, Green, and Violet. In order to produce complex colours
two standard glasses of unequal value must be used. The degree of
inequality does not alter the spectroscopic names of complex colours,
variation in proportions being only a statement of degree.

              Complex Name.        High value.  Lower.

    To develop a Red-Orange insert a Red and Yellow.
          "      Yellow-Orange   "   Yellow and Red.
          "      Yellow-Green    "   Yellow and Blue.
          "      Blue-Green      "   Blue and Yellow.
          "      Blue-Violet     "   Blue and Red.
          "      Red-Violet      "   Red and Blue.

A reference to the circles, 7, 8, and 9, on the cards supplied with
the apparatus will show that all complex colours are members of the
same triad group, and experiments have shown that the six combinations
above are the only ones distinguishable in Nature, subject, however, to
unlimited variations in brightness or dullness. It remains to be shown
how these variations are effected.

Variations in brightness are produced by inserting with the two glasses
forming a complex colour the third spectrum colour, always bearing
in mind that it must be less in value than either of the other two.
The addition of the third colour has a dulling or saddening effect,
the degree of which is determined by the numerical value of the third
colour. The colour produced by the addition of the third colour may
be termed a saddened or dingy colour, the appearance being that of a
brighter colour seen in shadow.

Reviewing the foregoing, it is demonstrated that primary or dominant
colours are transmitted by a single coloured standard glass; the
secondary or subordinate colours are transmitted by two equal standard
glasses of different colours; the complex colours by two unequal
standard glasses of different colours. Saddened pure colours are
developed either by one coloured standard glass combined with two equal
standard glasses of different colours and lesser value or by two equal
standard glasses of different colours and a third of lesser value.
Saddened complex colours by three unequal standard glasses of different
colours. Greys, which are steps towards blackness, are produced by
three equal standard glasses of different colours.

It is well known that Colour Blindness is a defect in the vision often
involving the confusion of such utterly distinct colour sensations as
Red and Green, Orange and Violet, and many others as widely different.
In the cases of Red and Green the confusions are specially disastrous
should the subject be a railwayman or a sailor. It is not, however, so
well known that many cases of so-called Colour Blindness are in reality
cases of Colour Ignorance, and the capacity for distinguishing between
colours and shades is often latent, and only waiting to be developed by
Education.

When a child persistently misnames colours after having received an
amount of instruction sufficient to remove colour ignorance in a normal
case, the errors are probably due to some form of colour blindness.

Such cases should be registered for further examination, taking note of
the miscalled colours. It would be an additional precaution to change
the position of the colours in the windows of the apparatus, in order
to prevent the association of colours with their positions as first
given in the instructions.

_Note._--The paints which most nearly correspond to the colour
standards in the Colour Educator are tabulated below, the third column
containing their measured colour proportions when they are washed
thickly on white paper (_Whatman’s_).

  --------------+---------------+-----------------------------------------------
                |               |     Colour Composition in Standard Units.
   Spectrum     | Artists’      +-------+-------+-------+-------+-------+-------
     Names      | Names.        |       |       |       |       |       |
                |               | Red.  |Orange.|Yellow.| Green.| Blue. |Violet.
  --------------+---------------+-------+-------+-------+-------+-------+-------
                |               |       |       |       |       |       |
   Dominants:   |               |       |       |       |       |       |
     Red        | Crimson Lake  |  18·0 |   0·4 |  ---- |  ---- |  ---- | ----
     Yellow     | Lemon Yellow  |  ---- |  ---- |   6·6 |  ---- |  ---- | ----
     Blue       | Cobalt        |  ---- |  ---- |  ---- |  ---- |   9·0 | ----
                |               |       |       |       |       |       |
   Subordinates:|               |       |       |       |       |       |
     Orange     | Chrome Orange |  ---- |   7·2 |   1·6 |  ---- |  ---- | ----
     Green      | Emerald Green |  ---- |  ---- |   0·40|   5·4 |  ---- | ----
     Violet     | Mauve         |   3·3 |  ---- |  ---- |  ---- |  ---- |  8·2
  --------------|---------------+-------+-------+-------+-------+-------+-------

In mixing two dominant colours (artists’ primaries) to develop
subordinates (artists’ secondaries), their relative colour depth must
be taken into account; for instance, Crimson Lake has a natural colour
depth nearly three times that of Lemon Yellow; therefore, in order to
develop a normal Orange nearly three times the quantity of Yellow must
be used, presuming that they were originally ground to an equal degree
of fineness.

It is desirable that a record of the children’s own painting should be
preserved, with a view to discriminating between errors arising from
Colour Blindness and Colour Ignorance, the former perpetuating itself,
and the latter naturally remedying itself. For this purpose painting
books containing sets of diagrams corresponding to the figures in the
foregoing pamphlet can be supplied.




Appendix II

THE POSSIBILITIES OF A STANDARD LIGHT AND COLOUR UNIT.[5]

[5] Reprinted from the _Journal of the Society of Dyers and
Colourists_, March, 1913. No. 3, vol. xxix.

The past attempts to standardize light and colour are mainly limited
to those radiant energies which excite light and colour sensations
under diffused daylight conditions, because in direct sunlight, and in
most artificial lights, there are other colour energies, which, unless
sufficiently modified by diffusion, disturb the colour readings. There
are also latent colour energies, which only become distinguishable
by special means. They do not, however, appear to influence diffused
daylight colour work.

The definition of a normal vision is one which agrees with a majority
of others. This definition has proved satisfactory up to the present,
as the normals are many and the colour blind few.

_Light Intensities._--There are two methods of determining light
intensities by means of a graded scale of light absorbents.

First. By total absorption of the light, when the intensity is directly
represented by the unit value of the absorbents required. This method
is applicable for low lights, internal surfaces, such as a desk, etc.,
where a standard light is not available for comparison.

Second. By the reduction of a standard light by absorption until it
equals the light of the object. In this case the standard must be
originally brighter than the object.

_Constants._--The first requirement in establishing a scale of light
and colour units is a means of co-relating visual sensations to a scale
of physical colour constants, in order to secure a power of record and
recovery. There is no natural scale available for quantitative colour
work, but artificial scales can be constructed, and made constants by
co-relation at different points with physical colour constants, and by
cross-checking the intervals between these.

The scales used in the “tintometer” consist of red, yellow and blue
glass, so graded in equivalents that combinations of equal units
transmit colourless light. Full details of these have already been
placed before the Society (see this _Jour._, 1887, p. 186, and 1908, p.
36).

=Scale of Luminous Intensities.= _The Light Unit._--The natural
terminals in a scale of luminous intensities are black and white, and
the first question which arises is what is black, and what is white?
as when used in a popular sense each term covers a wide range of
differences.

In the author’s sense the term black means total absence of light,
and the term white means a diffused daylight of given intensity, as
reflected from a lime sulphate surface. In this sense black and white
are the terminals of a scale of light intensities; the scale is divided
into units and fractions of units. The unit itself is physiological,
and is not in progressive accord with the mathematical light unit based
on the inverse squares of distance.

_The Black Unit._--Ideal black is practically obtained under daylight
conditions by viewing a hole in a box with blackened interior, so
arranged that no entering light can be reflected back to the vision.

The box used for this purpose is illustrated in Fig. 3, and has one
surface covered with standard white for the purpose of easy comparison
with the pigments. The standard black aperture (1) is in the middle.
The pigmentary blacks (2 to 10) are arranged over this, and the
pigmentary whites and greys (11 to 20) underneath, each being numbered
in accord with its intensity as tabulated.

[Illustration: Fig. 3.]

The degrees of blackness are the number of absorptive units required to
reduce the standard white to equal the pigments in each case.


Light Absorbed by Various Pigments.

  ---+---------------------+--------+----------+-------
  No.|                     |Absorbed|Unabsorbed|Initial
     |                     | Light. |  Light.  | Light.
  ---+---------------------+--------+----------+-------
   1 |Black Hole in Box    |   36   |   --     |  36
   2 |Optical Black        |   20   |   16     |  36
   3 |Lamp Black           |   17   |   19     |  36
   4 |Vegetable Black A    |   17   |   19     |  36
   5 |    "       "   B    |   14   |   22     |  36
   6 |    "       "   C    |   15   |   21     |  36
   7 |Indian Ink on Paper  |   14   |   22     |  36
   8 |     "    Solid      |   12   |   24     |  36
   9 |Boot Black           |   11   |   25     |  36
  10 |Black Lead           |    9   |   27     |  36
  ---+---------------------+--------+----------+-------

This gives a working scale of colourless light intensities, the
terminals being black and white, with a range of 36 units.

_The Standard White._--White is the natural terminal of the luminous
end of the scale, and it is necessary to select a physical objective
white as a constant. Pure precipitated lime sulphate has been adopted,
and departures from the light intensity of this are recorded in units
of lessened light intensity throughout the scale, comprising all
degrees of colourless whites, greys, and blacks.

Strictly speaking, white is a qualitative term only, until the degree
of variation from the zero of the scale has been established. The
measured variation then takes its position in the scale of luminous
intensities according to its numerical unit value.


Light Absorbed by Various White and Grey Pigments.

  ---+--------------+--------+---------+-------
  No.|  Pigments.   |Absorbed|Reflected|Initial
     |              | Light. |  Light. | Light.
  ---+--------------|--------+---------+-------
   11| Grey Paint E |  6·0   |   30·0  |   36
   12|     "      D |  5·0   |   31·0  |   36
   13|     "      C |  4·0   |   32·0  |   36
   14|     "      B |  2·0   |   34·0  |   36
   15| White Paint A|  0·7   |   35·3  |   36
   16| White Paper D|  0·3   |   35·7  |   36
   17|      "      C|  0·2   |   35·8  |   36
   18|      "      B|  0·25  |   35·75 |   36
   19|      "      A|  0·15  |   35·85 |   36
   20| Chinese White|  0·006 |   35·994|   36
  ---+--------------+--------+---------+-------

As the scale is differentiated into hundredths of a unit, there can be
100 variations of white pigments in a single unit, each quite easily
distinguishable from the others.

Any definite mixture of black and white finds a position on the
diagonal of a chart whose co-ordinates are the black and white scales;
for example, the 20 measured pigments are charted on Figs. 4 and 5, the
latter being on an enlarged scale, as the whites would be too crowded
to be noted on Fig. 4.

[Illustration: Fig. 4.]

The merging of white into grey, and of grey into black is gradual,
having no strict lines of demarcation.

An example of this method of determining light intensities is
illustrated in Fig. 6 by the light intensities at which different
objects are discernible. The points of most interest are, that colour
is indistinguishable as such in lights below 15 units intensity; and
that ordinary work, such as reading a newspaper, requires for comfort a
minimum of 28 units.

_The Colour Unit._--The colour unit is physiological, and its
dimensions are determined by the dimensions of the colourless light
from which it is derived. This deduction is based on the experimental
fact that colourless light is a mixture of the six colour rays--red,
orange, yellow, green, blue, and violet--in equal proportion, as
illustrated in Fig. 7, showing that a white light of 20 units light
intensity is made up to the six colour rays, each of 20 units colour
intensity. This is demonstrated by the fact that any proportion of
any colour can be developed at will by means of the glass standard
scales already mentioned; it follows that the smallest disturbance of
equivalence between the composing rays results in the development of
colour.

[Illustration: Fig. 5.]

The above remarks apply to both simple and complex colours, and
the complex colours are always dichromes, being governed by
another physiological fact, which is: That the vision is unable to
simultaneously distinguish more than two colours in the same beam of
light. The order of their association is definite, and may be described
by saying that the combined two are always adjacent in their spectrum
order, red and violet being considered adjacent for this purpose. It
follows that all complex colours are binaries, and the only possible
combinations are as follows:--

            Red    with Orange.
            Orange   "  Yellow.
            Yellow   "  Green.
            Green    "  Blue.
            Blue     "  Violet.
            Violet   "  Red.

[Illustration: Fig. 6.]

In the author’s colour nomenclature, a monochrome is qualitatively
described by a single term, and a complex colour by a combination of
two single terms. For a quantitative description, it is only necessary
to add the measured unit value to each term. When there is excess of
brightness, or a saddening factor, these also must be quantitatively
estimated.

The colours developed by means of these scales are governed by the same
law of selective absorption which governs the development of natural
colours, any of which can be matched and reproduced by means of their
established ray proportions.

[Illustration: Fig. 7.]

The governing law is simple, and may be stated by saying that the
colour developed is always complementary to the colour absorbed, not in
the generally accepted sense that their mixture necessarily makes white
light, but in the sense that they are opposite in the cycle of daylight
colours.

The dimensions of the unit are necessarily arbitrary, it was originally
selected as being a convenient depth for distinguishing differences,
the scale was then constructed by equal additions and sub-divisions;
the two essentials of a scientific scale being complied with, in
that the divisions were equal and the unit recoverable. The power
of recovery lies in the fact that different parts of the scale are
co-related to physical colour constants, which can be prepared in any
laboratory.

[Illustration: PLATE VIII ABSORPTION CURVES OF SIX DYES.
  _To face page 76._]      [_Lovibond, Colour Theories._
]

_Specific Colour._--The relationship of colour increase to intensity
increase in substances has hitherto been somewhat obscure. It has been
sometimes considered that they were in direct proportion, but in the
absence of a means of recording colour sensations, no definite results
were obtainable.

[Illustration: Fig. 8.]

Sufficient information is now available to warrant the formulation of
the following law: “That every substance has its own rate of colour
development for regularly increasing intensities, which, when once
established, becomes a constant for identifying similar substances
in future.” This is the meaning of specific colour, and when a
series of measurements at regularly increasing densities of a given
substance have been made, the specific colour rate of that substance
is established. This can be charted in curves and used as a basis for
estimating quantities, properties, changes of condition, differences in
value, detecting adulteration, etc.

_Applications._--The author has permission to use the names of several
gentlemen who have used the tintometrical scales for various purposes.

Sir Arthur H. Church, F.R.S., has employed the tintometrical standards
for the purpose of registering the colours of certain wild flowers.

Sir Boverton Redwood has used the scales and system for petroleum
investigations. At his instance the specific colour rate of petroleum
was established, and the several composing colours plotted in
curves, as in Fig. 8, where the ordinates represent the scale of
units irrespective of colour, and the abscissæ the scale of strata
thicknesses.

The measurements were made at two-inch intervals, and the four
perpendicular lines are at the colour points selected for valuing
the four distinguishing marks, technically known as “Water White,”
“Superfine,” “Prime,” and “Standard.” Intermediate qualities find their
position in the scale of curves according to their measured colour
values.

This method of standardizing commercial values has also been adopted by
the International Tanners’ Association, the Inter-States Cotton Seed
Oil Association, and other oil industries. Also for scale, solid fats,
and such substances as can be easily melted and measured by transmitted
light.

=Varying Effects of Different Lights.= _Pathological
Applications._--The law of specific colour development was made use of
by Dr. George Oliver in determining the degrees of hæmoglobin in the
blood. The method is fully explained in his Croonian Lecture before the
Royal College of Physicians of London, July 11, 1896.

_Detection of Forgeries._--The system and apparatus is used by
Professor A. S. Osborne, Examiner of Questioned Documents, New York
City, for determining the variety of ink, the age of the writing,
and the detection of forgeries. A full description of the process will
be found in his work entitled _Questioned Documents_, published by the
Lawyers’ Co-operative Society, Rochester, N.Y.

[Illustration: PLATE IX SIX ANILINE DYES.
CURVES ILLUSTRATING THE RATE OF FADING BY EXPOSURE TO LIGHT.
  _To face page 78._]       [_Lovibond, Colour Theories._
]

The application to chemical analysis is too well known to require
enlargement here.

[Illustration: Fig. 9.]

_Dyes._--As an example of the use of the system in the valuation of
dyes, Fig. 9 illustrates the specific colour curves of four samples
of Methylene Blue. No. 1 was priced at 5_s._ 9_d._ and No. 2 at 5_s._
per lb., Nos. 3 and 4 were not priced, the solutions were measured in
percentages from 0·001 to 0·048 in distilled water. To find the cost
per unit of colour in the priced samples is only a question of simple
arithmetic, which furnishes data for the valuation of the unpriced
samples.

The yield in the dye vat may not be in direct relation to the solutions
in water, the establishment of this is a question for the expert, and
presents no apparent difficulty.

The use of the scheme in recording the degree of fading of dyes has
been previously dealt with in the _Journal_ (_q.v._, 1908, p. 36).

_Limitations and Precautions._--It has been shown that we have
analytical control, within certain limits, of light and colour under
daylight conditions.

The general limits for colourless light range from total darkness to 28
units, when the unabsorbable red ray comes into evidence.

For colour, the general limits range from 28 to 18 units, between 18
and 15 all colours become indistinct, but at varying rates, below 15,
colour is not distinguishable.

The principal disturbing conditions in making observations are _want
of colour education_ and _insufficient diffusion_. In the case of the
latter, the first evidence is the disturbance of constancy by the
penetrating red ray. A partial remedy is to interpose a white diffusion
screen, such as tissue paper.

_Time of Observation._--This should not exceed five consecutive
seconds, as the keenness of perception decreases by time, but varies
for different colours.

_Angle of Incidence._--Sixty degrees is safe for most solids, but for
bright or polished surfaces, such as varnishes, polished metals, etc.,
the angle must be lessened as the degree of smoothness increases. For
very rough surfaces, such as loosely woven stuffs, etc., care must be
taken that the lay of the fibre is uniform.

_Distance from the Object._--Ten inches has been adopted for general
work, but certain visions require more or less as their focus varies
from normal.

[Illustration: PLATE X COMPARISON CURVES OF HEALTHY AND DISEASED BLOOD.
  _To face page 80._]  [_Lovibond, Colour Theories._
]

_Unabsorbable Colours._--In addition to the daylight colours already
dealt with, there are, in direct lights, colours which do not obey the
laws of absorption governing those of diffused daylight.

The work already done on these unabsorbable rays has only been
incidental, where they happened to interfere with the standardization
of diffused daylight colours. The sensations excited are red and
violet. They blend, producing red-violet mixtures, but in unequal
proportions, the red being dominant.

The red is developed in intense lights by constant interception of
neutral tint absorbents. In the case of a 4-volt incandescent light,
the first absorption simply reduces the light intensity without
developing colour; the light is colourless up to 14 units. At 16
units the light begins to assume a reddish hue, which rapidly becomes
a brilliant intense red by further interceptions of neutral tint
absorbents.

Violet is developed by constant absorption by blue standards, which
grows in intensity by successive additions up to about 120 units.
Beyond this point the brilliancy decreases.

Preliminary experiments point to this ray as fatal to vegetation, and
presumably also to lower forms of organic life.

There remains a factor of considerable importance which has not yet
received the attention it deserves, the physiological changes resulting
from environment.

This aspect of the question has come under the notice of the author
by measuring the vision of experts who excelled in given hues. It was
generally found that their vision was sensitive to a small increment of
their particular colour in harmonies where it was silent to a normal
vision.

In seeking an explanation of this phenomenon there are at least three
possible lines for working:

First. Is the vision naturally more sensitive to that particular energy?

Second. Is it by careful education?

Third. Is it an unconscious adaptation to surroundings, such as other
organs undergo under changes of environment?

[Illustration: PLATE XI SPECIFIC COLOUR CURVES OF HEALTHY AND DISEASED
HUMAN BLOOD
  _To face page 82._]      [_Lovibond, Colour Theories._
]




Appendix III

    THE APPLICATION OF THE NATURAL LAW OF SPECIFIC COLOUR RATE BY
    DR. DUDLEY CORBETT TO THE EXACT MEASUREMENT OF X-RAY DOSAGE.


Dr. Dudley Corbett.

The gradations in the tint given by the Sabouraud-Noiré pastille
when exposed to X-rays are so fine, especially in that region of the
colour scale where lies the erythema dose, that many have felt the
want of a more accurate means of reading these tints, as well as a
series of reliable standards for comparison. Hitherto the only methods
at all generally used have been Hampson’s radiometer and Bordier’s
radio-chronometer, the former in this country, and the latter on the
Continent. Hampson’s instrument has two disadvantages: it can only be
used with electric light, and the standards are made of tinted paper
liable to get soiled, and to vary slightly with the changes in the
pigment employed. Its advantage is that it may be used as a sliding
scale, thus economising the pastilles. Some persons, however, have
considerable difficulty in reading the tints, the scale rising only by
gradations of 1/4 B.

In the construction of any such instrument, the really important
point is to obtain a reliable standard for Tint B--_i.e._, the normal
epilation dose. The tints on the Sabouraud card itself are not always
identical, some representing a dose which will only just epilate,
others an almost dangerous dose for unfiltered rays. The Tint B, which
is the standard, allows a margin of error of 20 per cent. on either
side. In other words, 4/5 B will almost always epilate, while 1-1/5
B is nearly the limit of safety. This observation is in accordance
with the experience of other workers on this subject. In my instrument
Tint B has been obtained by measuring the pastille with Lovibond’s
tintometer immediately after exposure to the X-rays. The pastille was
turned to a tint corresponding to an epilation dose which was known to
be safe, as proved by clinical results. This tint was measured directly
both by daylight and by artificial light from an 8-candle power carbon
filament lamp with frosted glass shade. I am indebted to Mr. Dean for
suggesting the use of Lovibond’s instrument for this purpose.

The methods employed in the experimental work have been described in
the _British Journal of Dermatology_ for August, 1913. By using a
very constant focus tube and averaging a large number of readings and
correlating the results with those obtained in clinical practice, we
were able to construct the curves indicating the colour developed by
the pastille. In these curves, shown in Fig. 10, the ordinates are the
Lovibond colour units, the abscissa the time during which the current
was actually passing. When using an interrupter working at a constant
speed, the actual time was taken, otherwise the number of current
interruptions as measured by a dipper tachymeter was used.

As was to be expected, the daylight and electric light curves were
quite different. In each case the standard yellow glasses employed were
kept constant throughout the curve, that for daylight being 15 units,
that for electric light 13 units. When these were combined with blue
and red glasses in varying units and fractions of a unit, they gave
a colour range which matched the pastille exactly in the changes it
undergoes from the unexposed condition to the 2 B Tint.

The curves are plotted in accordance with Mr. Lovibond’s practice--that
is, not as a direct representation of the standard glasses used, but as
showing the colour sensation received by the eye.

[Illustration: Fig. 10.]

First, as to the daylight curve: In order to match the unexposed
pastille we interpose between the pastille and the observer’s eye
a yellow and a blue glass, plus a certain amount of neutral tint
(composed of three equal colour units). Thus the colour sensation
received is a yellow green, together with a certain amount of white
light. As the pastille darkens under irradiation, both the green and
the white light disappear, until at a point just below the 1/2 B dose,
there is no other colour present but yellow. After this red glasses
are required--_i.e._, the colour sensation is a yellow orange, which
gradually deepens owing to an increase in the proportion of red. The
yellow curve thus rises till just below the 1/2 B point, and then falls
as the orange increases.

Next, as to the electric light curve. The unexposed pastille has but
a trace of green, which is soon lost. The orange begins much earlier
than in daylight, and thus at Tint B has reached a higher point than in
daylight. From this point the orange and yellow parts of both curves
run practically parallel with one another up to 2 B. Beyond 2 B the
readings become more difficult. I have not determined the point when
no more colour develops, as it has no great practical value, though it
might well be of interest from a physico-chemical standpoint.

In my radiometer the standards are composed of the Lovibond standard
glasses in combination. The apparatus itself consists of an optical
instrument or viewing box. This is divided by a central partition,
so that on looking through the eyepiece one sees a white background
through two small circular apertures. On one side, level with the
background, is a fitting to take the pastille in its holder. On the
other side is a groove in the instrument itself for the insertion of
the standard glasses. A similar groove is fitted on the pastille side
of the instrument to take neutral tints if required. The colour of the
pastille as seen by reflected light can thus be compared with that
obtained by transmitted light through the standard glasses seen against
the white background. A difference of 1/5 B or 1 H is quite easily
perceivable.

It is usually of no great importance to obtain extremely accurate
measurement of the smaller fractions below 1/3 B. Where this is
necessary, neutral tints must be used when working with daylight. With
electric light these are unnecessary. When required the neutral tints
are interposed between the pastille and the eye to absorb the white
light reflected from the pastille. The neutral glasses required are 1·5
for the unexposed pastille, 0·6 for 1/4 B, and 0·2 for 1/3 B. These
values are subject to slight variations due to changes in the varnish
of the pastille emulsion. The difficulty can always be avoided by using
electric light, where a trace of neutral tint is needed only when
matching the unexposed pastille--an unimportant point.

_Method of Use._--The choice of daylight or artificial light is a
personal matter, but one should practise reading the scale with both.
The use of the instrument shows that the pastille fades very nearly as
quickly under electric light as it does under daylight. The following
precautions should be observed: In daylight work in a good white
light, avoid shadows and yellow light of any kind. With electric light
use an 8-candle power carbon-filament lamp with frosted glass and a
suitable white shade so arranged that the pastille is 8 inches from the
lamp. No other light should be allowed to reach the pastille during
examination. A low power metal-filament lamp may be used, but greater
accuracy will be obtained with a carbon-filament lamp which was used
for the experimental work. The lamp should be discarded as soon as
the light becomes yellow from prolonged use. Whether in daylight or
electric light, the examination must be rapid to avoid the fading of
the pastille. When it is desired to give an accurate 1 B dose, it is
better to put up the 4/5 B standard first. It is then easy to calculate
how much more exposure is required for the extra 1/5 B. It is important
to adjust the pastille carefully so that none of the unirradiated green
portion is visible through the small aperture, as this will upset the
reading. In very accurate dosage new pastilles should be used, as a
bleached pastille never returns exactly to its original tint. When such
a bleached pastille is irradiated the colour changes start a little
farther down the curve, and thus the tint for a given dose must be
taken a little above the normal tint. This increase is very slight,
but is nevertheless quite appreciable, and may amount to as much as
5 and 10 per cent. Even then the margin for error is ample in the
neighbourhood of the B tint, and if a pastille is not used more than
three times, and is well bleached in daylight after each exposure, no
serious error is likely to occur. A standard white background should
always be used, and discarded for a new one when it gets dirty. The
colour standards usually provided are those in common use--namely,
1/4, 1/3, 1/2, 4/5, 1, 1', and 2 B, but it is quite easy to make up
standards for any point on the curve. The symbol “B,” as the erythema
or epilation dose, has been retained, as it was thought inadvisable to
add to the number of such symbols already existing.

To sum up:

1. The experimental work has determined the exact colour changes
occurring in the Sabouraud pastille when exposed to X-rays.

2. These experiments have established a permanent standard for Tint
B, which matches the pastille exactly, does not fade, is easily kept
clean. These coloured glasses can be readily and accurately reproduced,
as they are standardized spectroscopically by a firm who specialise in
such work. The standard will therefore remain constant so long as the
Sabouraud emulsion remains unaltered.

3. Glasses may be prepared of the correct tint for any fraction or
multiple of this dose up to 2 B or 10 H.

4. Either daylight or electric light can be used.

5. The optical instrument itself, by cutting off extraneous light,
greatly assists the colour comparisons, so that the practical error
need never exceed 10 per cent.




INDEX


  Abnormal light, 13

  Analysis of white light, 11, 13

  Arbitrary scales, 9

  Artists and scientists, 1, 2


  Beam of white light, 7, 11

  Black units, 16, 23

  Black, ideal, 15, 11

  Blood curves, 33


  Code of laws, 7

  Colour charts, 31, 39
    education, 59
    equivalence, 10
    nomenclature, 17, 21
    qualitative, 17, 19, 24
    quantitative, 24
    scales, 20, 29
    standardization, 20, 69
    theories, 2, 3

  Coloured surfaces, 44

  Colours, complex, 18
    sadder than standards, 25
    simple, 17

  Corbett, Dr. Dudley’s, radiometer, 83


  Daylight colours, 24, 84
    measurement, 14

  Diffused light, 10, 43

  Direct lights, 46

  Dulled colour, 10, 18-26, 36


  Equivalent colour units, 7, 9, 29


  Fog, whiteness of, 10


  Glass standards, 20


  Human blood curves, 82


  Laws of colour, 7

  Light, 11, 13, 42, 45-46, 81-84
    abnormal, 8
    brighter than standards, 14, 27
    direct, 46
    for colour work, 42
    intensities, 42, 43
    white, 14

  Lovibond’s new colour theory, 5


  Matching colours brighter than standards, 18
    complex colours, 17

  Measuring and naming colours, 21

  Monochromes, 17, 19

  Munro, Dr., 34


  Neutral tint, 18, 24, 26

  North light, 15


  Past theories, 3

  Photographic energies, 53

  Physical colour constants, 8

  Pigmentary black, 15, 71

  Primary colours, 1

  Prismatic spectrum colours, 29


  Qualitative analysis of colour, 17, 19, 24

  Quantitative analysis of colour, 24


  Radiometer, 83

  Rate of colour absorption, 8

  Rays, six colour, 7

  Red ray, 41


  Scales, arbitrary colour, 9
    coloured, 10, 20
    cross checking of, 9

  Scientists and artists, 1, 2

  Sea fog, 10

  Specific colour, 32

  Spectrum colours, 4, 18, 36


  Three colours, 10, 57

  Time, appreciation of colour by, 42


  Ultra violet, 32, 40

  Unit, checking of, 9

  Unit, neutral tint, 18, 24, 26


  Wave length position, 37, 38

  White light, 11, 13


Butler & Tanner Frome and London




Transcriber’s notes:


In the text version, italics are represented by _underscores_, and bold
and black letter text by =equals= symbols. Superscripts are represented
by ^{} and subscripts by _{}

Missing or incorrect punctuation has been repaired. Inconsistent
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In the html version, dittos have been replaced by the repeated text so
that text aligns for easier reading.

Very wide tables (V and VI and VII)have been split to fit on portrait
oriented pages.

The following mistakes have been noted:

  p. 21. NiSO_{4}7H_{2}O, tem." is formatted inconsistently. Left as
    printed.
  p. 40. Fraunhoper is almost certainly a typo for Fraunhofer.
    Left as printed.
  p. 46. interpretated changed to interpreted.
  p. 55. In Paper 6 of table, Red screen, yellow entry, from alignment and
    logic, the 6 should be .6. Left as printed
  p. 61 conditonschanged to conditions.





End of Project Gutenberg's Light and Colour Theories, by Joseph W. Lovibond