We saw that in the case of hearing the auditory organ is fitted to respond to air-borne vibrations varying from about thirty to thirty thousand per second. And though the details of the process are at present not well understood, it is believed that certain parts of the recipient surface are fitted to respond to low tones, other parts to intermediate tones, and yet others to high tones. Thus the reception is serial. If there be two pianos near each other, accurately in tune, any note struck on one will set the corresponding note vibrating in the other.[FC] The auditory organ may be likened to this second piano. Special parts respond to special tones. Now, in the case of vision, the conditions are different. The reception cannot be serial. As I range my eye over a flower-bed, I bring the area of distinct vision on to a number of different colours, and these are seen to be distinct, though they are received on the same part of the retinal surface. It might, perhaps, be suggested that special cones were set apart for each shade of colour. But there are only some two thousand cones in the central area of most acute vision, and Lyons silk-manufacturers prepare pattern cards containing as many shades of coloured silks. So that there would be only one cone to each colour. And Herschel thought that the workers on the mosaics of the Vatican could distinguish at least thirty thousand different shades of colour! There are also many phenomena of colour-blending which show that colour-reception cannot in any sense be serial. How, then, are we to account for our wide range of colour-sensation? Just as the blending by the artist on his palette of a limited number of pigments gives him the wide range of colour seen on his canvas, so the blending of a few colour-tones may give us the many shades we are able to distinguish. The smallest number of fundamental colour-tones which will fairly well account for the phenomena of colour-vision, is three. And these three are red, green, and blue or violet. These are the three so-called primary colours. All others are produced from these elements by blending. To explain our ability to appreciate differences of colour, then, it is supposed, on the hypothesis of Young and Von Helmholtz, that three kinds of nerve-fibres exist in the retina, the stimulation of which gives respectively, red, green, and violet in consciousness. Professor McKendrick, interpreting Von Helmholtz, gives[FD] the following scheme:-- "1. Red excites strongly the fibres sensitive to red, and feebly the other two. "2. Yellow excites moderately the fibres sensitive to red and green, feebly the violet. "3. Green excites strongly the fibres sensitive to green, feebly the other two. "4. Blue excites moderately the fibres sensitive to green and violet, feebly the red. "5. Violet excites strongly the fibres sensitive to violet, feebly the other two. "6. When the excitation is nearly equal for the three kinds of fibres, the sensation is white."