The order of the association of simple colours to form complex, is governed by two factors. The first is a physiological limitation of the vision, which is unable to simultaneously distinguish more than two colours, in the same beam of light, this limits the most complex colour to two colour names. The second limitation is one of association, based on the experimental fact, that the particular two must be adjacent in their spectrum order, spectrum red and violet being considered adjacent for this purpose. Under these conditions, any given colour must be either a monochrome, or a bichrome, and all complex colours must be bichromes. Therefore the only possible combinations are as follows:-- Red and Orange Orange " Yellow Yellow " Blue Blue " Green Green " Violet Violet " Red The classified order of associating symbols for describing the components of the whole range of distinguishable colours is set out in the following tables:-- -------------+-------------+------------ Monochromes | Monochromes |Monochromes of a Standard|Brighter than|Duller than Brightness. | Standards. |Standards. -------------+-------------+------------ R. | R. L. | R. Bk. O. | O. L. | O. Bk. Y. | Y. L. | Y. Bk. G. | G. L. | G. Bk. B. | B. L. | B. Bk. V. | V. L. | V. Bk. -------------+-------------+------------ [Illustration] The separation of the six monochromatic sensations from a point of white light, and the formation of binary sensations by the combination of adjacent colours, is graphically illustrated in the above diagram. In order to make the qualitative symbols quantitative it is only necessary to add the numerical unit value to each factor as found by direct experiment. CHAPTER VII. Quantitative Colour Nomenclature. THE GLASS STANDARD SCALES. At an early stage of the investigation, it was found that coloured glass gave better results than coloured solutions, and that Red, Yellow, and Blue, were the only colours suitable for systematic work; it was also found that any colour could be produced by their combination. As already described arbitrary scales were first used in many colours, but were superseded by these three, which, when graded into scales of equivalent value, were found to cover all daylight colours.