Light and Colour Theories, and their relation to light and colour standardization

CHAPTER XII.
The Physiological Light Unit.

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.

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:—

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

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

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.
No. 2. 10 min. exposure to bright sunlight.
No. 3. 20 min. exposure to bright sunlight.
No. 4. 30 min. exposure to bright sunlight.

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.

TABLE VII.
BRIGHT SUNLIGHT (South), 18—9—12.
10 Minutes’ Exposure on White Sensitized Papers.

TABLE VII.
BRIGHT SUNLIGHT (South), 18—9—12.
10 Minutes’ Exposure on White Sensitized Papers.

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.

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

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.

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

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.

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.

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      { No. 3 Window for Yellow.
{ No. 5 Window for Blue.
{ No. 2 Window for Orange.
Subordinates   { No. 4 Window for Green.
{ No. 6 Window for 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.

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).

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.