These illustrations deal with light and colour of 20 units intensity;[2] as the intensity of the light here is exactly equal to the absorptive power of the standards, no free light remains; where the absorptive power of the colour standards is less than the light, associated white light remains; for instance, if only one unit of colour was developed, 19 units associated white light would remain.
[2] For description of the light and colour units, refer to chap. III, page 9.
[Illustration: PLATE III _To face page 13._] [_Lovibond, Colour Theories._ ]
This method of colour development by analytical absorption is further illustrated by Plate III, showing the effect of superimposition of the three colours in their several combinations as intercepting a beam of white light.
Not all lights which appear white to the vision are truly normal white; colour may be masked by excess of luminosity, and only become evident when the luminosity has been reduced, by placing neutral tint standards between the light and the observer. Direct sunlight, and some artificial lights, are instances. (Law 6 (_a_) page 8.)
On the other hand, an abnormal light may be too low for the vision to discriminate colour. This may be observed in nature by the gradual loss of colour in flowers, etc., in the waning intensities of evening light. The order of their disappearance is shown in Chart I.
The colour of a substance is determined by the ray composition of the light it reflects, or transmits to the vision, the colour would therefore vary with every change in the ray proportions of the incident light; it follows that constancy in colour measurement can only be obtained by a colourless light. Up to the present diffused daylight is the only light which complies with the condition of ray equality.
The absolute equality of the six spectrum colours may be difficult to establish in any light, and their constancy in equivalence under varying light intensities may be open to argument. But, as everyday work is carried on mainly under daylight conditions, and as the vision is the final arbiter for colour work, theoretical questions outside the discriminating power of the vision, need be no bar to the establishment of a working standard white light; and in saying that diffused daylight is normal white, it is only intended to mean: In so far as a normal vision can determine.