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.

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.

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 -------+------+----------------+----------------+------------