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].
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.
