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

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.

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.

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.

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.

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 with Yellow.
Yellow with Green.
Green with Blue.
Blue with Violet.
Violet with Red.

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.

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.

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.

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 5s. 9d. and No. 2 at 5s. 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.

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?

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

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.

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
• [90]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