Joseph W. Lovibond

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 ”

“ 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 [76] 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Apart from any theoretical explanation it is an experimental fact, that the abnormal rays of direct [15] 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. ”

“ 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; [67] 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. ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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. ”

“ 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.” ”

“ 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 [40] nature each colour may be blended with any value of the overlapping colour. ”

“ 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. ”

“ 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 [16] from the pigments consist of the colour values developed which are associated with the unabsorbed white light.
TABLE II.
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. ”