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. [Illustration: Fig. 4.] The merging of white into grey, and of grey into black is gradual, having no strict lines of demarcation. 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. [Illustration: Fig. 5.] 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 " Yellow. Yellow " Green. Green " Blue. Blue " Violet. Violet " Red. [Illustration: Fig. 6.] 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.