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 Rays. | Primary Colours. | |||||||
| Newton (later) | 7 | Red, | Orange, | Yellow, | Green, | Blue, | Indigo, | Violet |
| Werner | 6 | Red, | Orange, | Yellow, | Green, | Blue, | Violet | |
| Newton and Helmholtz (early) | 5 | Red, | Yellow, | Green, | Blue, | Violet | ||
| 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.
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.
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]
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.
PLATE II
NINE CIRCLES ILLUSTRATING THE ANALYSIS OF A BEAM OF WHITE LIGHT INTO THE SIX COMPOSING COLOURS BY THE ABSORPTIVE METHOD
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.
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.