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.