CHAPTER IV.

Derivation of Colour from White Light.

The method of analysing white light into its colour constituents by means of coloured glass absorbents of known intensity and purity, is illustrated by the set of nine circles in Plate II, which demonstrate that colour is developed by the absorption of the complementary colour rays. The ratios of transmission are equal.

In this set of illustrations the circles represent light of 20 units luminous intensity, and the absorptive value of the three glass colours is each of 20 units, therefore the whole of the light and colour energies are presumed to be dealt with.

In the first set of three circles, _A_ represents a beam of normal white light. _B_ a similar beam as divided into the six colour rays, Red, Orange, Yellow, Green, Blue and Violet in equal proportions, _C_ as wholly absorbed by Red, Yellow, and Blue glasses, each of 20 units colour intensity.

Figures 1, 2 and 3 represent the specific action of Red, Yellow, and Blue glass on the white light.

_Red_ absorbs Yellow, Green and Blue, transmitting Violet, Red and Orange, developing Red only.

_Yellow_ absorbs Blue, Violet, and Red, transmitting Orange, Yellow and Green, developing Yellow only.

_Blue_ absorbs Red, Orange and Yellow, transmitting Green, Blue, and Violet, developing Blue only.

By this method of development, Red, Yellow or Blue, when seen alone are visually monochromatic, although composite in structure, each containing a group of three rays, the middle ray alone exciting the colour sensation.

Circles 4, 5, and 6 illustrate the development of Orange, Green and Violet from the triad groups, by intercepting the light with two glass colours.

Circle 4, Red on Yellow, develops Orange by absorbing Yellow, Green, Blue, Violet and Red.

Circle 5, Yellow on Blue, develops Green by absorbing Blue, Violet, Red, Orange and Yellow.

Circle 6, Blue on Red, develops Violet by absorbing Red, Orange, Yellow, Green and Blue.

By this method of demonstration the six colours fall naturally into two groups. The first group includes Red, Yellow, and Blue, whilst the second group includes Orange, Green, and Violet. The colours of the second group, Orange, Green, and Violet, are true monochromes, each being isolated from the light, by the absorption of the five other rays.

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.

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.