It has been said that the seven different colours of the spectrum, painted in certain pro portions on a wheel which is turned with great rapidity, will produce the impression of white. It has been found that these seven separate colours are not required, but that the impres sion of white can be produced by three colours only, viz. :— Red.
Green. Violet.
These three painted in proper proportion on a wheel will give the impression of white. More over, all the seven colours of the rainbow can be produced by varying the proportion of these three colours. On this account they are called the fundamental colours.
Complementary Colours are not to be con founded with the fundamental. There are certain pairs of colours which when blended produce a sensation of white. Thus:— That is to say, given red, the other colour required to produced white is bluish-green; or given bluish-green, the other colour required is red, and so on. These colours are therefore said to be " complementary " to each other, because they together produce white.
Here a mistake must be guarded against. It must not be supposed that it is meant that a mixture of paints of these colours will pro duce a white paint. A red powder and a bluish-green powder will not produce a white one, as everyone knows, nor the mixture of red and bluish-green liquids. But if, at the same instant, the eye be affected by red and bluish-green light, the sensation is not of either colour, but of white. It is the sensation that must be mixed, so to speak, and the mixed sensation is not produced by a mixture of the differently-coloured powders or liquids.
The Perception of Colour by the eye is explained by a theory first proposed by Thomas Young, and afterwards more fully worked out by the German professor, Helmholtz. Accord ing to that theory there are in the eye three sets of nerve-fibres capable of being excited by the fundamental colours (see above). One set is excited by red light, another by green, and another by violet. Just as the different colours of the rainbow may be produced by various proportions of these three colours, so may dif ferent sensations of colour be produced by the excitement of these three sets of nerve-fibres in different amounts. Thus, when all the three sets of fibres are nearly equally excited there is a sensation of white. Red light will strongly arouse the nerve-fibres sensitive to red, and will barely affect the other two. Yellow light will moderately excite the fibres sensitive to red and green, and not those sensitive to yellow, and the result is not a sensation of red or green, but of yellow. Blue light excites moderately the fibres sensitive to green and violet, and barely affects those sensitive to red, and there is a sensation of blue. This theory would
account for colour - blindness. Thus, if the fibres that ought to be sensitive to red for some reason or other did not respond at all, the person would be unable. to perceive red. (Colour-blindness is considered at length in the second part of this section.) are also explained by the theory of colour and its relation to the per ception of colour. If, on awaking in the morning, we look for an instant towards a window through which bright sunlight is streaming, and then turn away the head and shut the eyes, we are aware of an image of the window, in which the panes appear white and the sashes, &c., dark, as they appeared when actually looking at them. This is a positive after-image, and is due to the fact that, the sensitive coat of the eye being highly excitable by the long rest of the night, the effect of the stimulus of the light lasts even after the excit ing cause has ceased to operate. If, however, we gaze for a time at the window and then look away and shut the eyes, or look towards a dark part of the room, we see an image in which the light and dark parts are reversed, the panes being dark and the sashes white. This is a negative after-image. It is due to the sensitive coat of the eye being fatigued in certain parts. The parts corresponding to the panes, on which the strong light fell, are ex hausted and appear dark, while the parts cor responding to the sashes, on which the light did not fall, are still unexhausted and there fore appear light. In a short time the retina recovers and the experiment may be repeated.
Now, suppose we look fixedly for a short time at a white sheet of paper on which is a red spot, a bright light falling on the paper, and then turn the eyes to a plain white sheet of paper or to a white wall, an image of the spot will appear to float before the eyes, but it will not be a red image but bluish-green. The explanation of this is similar to that of after-images. The sensitive coat of the eye has been exhausted, but not to all the con stituents of white light, only to the red. The result is that an after-image floats before the eyes, whose colour is that of white light less the red, in other words, the colour which with red goes to make up the sensation of white, the complementary colour of red, namely bluish green. Similarly if the spot gazed at has been bluish-green, the after-image will be red. If the spot be orange the after-image will be blue, &c. The experiment may also be varied. Thus a large red spot may have a name written across it in another colour, in which case the after-image would show a bluish-green ground and the name would be in a different comple mentary colour.